Global Navigation Satellite Systems and Their Applications (SpringerBriefs in Space Development) [2 ed.] 303174487X, 9783031744877

Table of contents :
Preface to the Second Edition
Contents
Key Terms and Acronyms
About this Book and the Internaional Space University
Chapter 1: Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems
What Is in a Name?
Why Write a ‘Short Book” on Positioning, Navigation, and Timing Satellites?
What Is the Best Way to Grasp the Field of Positioning, Navigation, and Timing Systems?
Who Is Involved and What are Their Various Perspectives?
Introduction
History
Transit, the First Satellite-Based Navigation System
The Concept of Doppler Tracking
Tsikada/Parus
Pre-GPS and the Decision to Integrate Multiple Research Programs
Chapter 2: Doppler Satellite Positioning, Telemetry and Data Systems
Argos
System Design
Argos Applications
The Argo Program
Cospas-Sarsat
Origins
System Design
Chapter 3: Positioning, Navigation and Timing (PNT) and Current National PNT Systems
GPS
The Space Segment
The Control Segment
The User Segment
How GPS Works
Code-Based GPS
Satellite Ranging
GPS Precision and Accuracy
GPS Error Budget
GPS upgrades
Current Capabilities
DASS
Russian GLONASS
Glonass Modernization
China’s Compass/Biedou System 北斗卫星导航系统
The European Galileo System
Japan’s Quasi-Zenith System (QZSS) or Michibiki, or ‘Showing the Way’
The Current Landscape
New Additions
Korea Positioning System (KPS)
Pakistan
Chapter 4: Aided and Augmentation Systems: Differential GPS, Pseudolites, Ground-Based Augmentation Systems, and Space-Based Augmentation Systems
How Differential GPS (DGPS) Works
Ground-Based Augmentation Systems, The Nationwide Differential GPS System (NDGPS)
International DGPS Networks
International GNSS Service
Assisted PNT
Wide Area Augmentation System (WAAS) (USA)
Kinematic vs. Carrier Phase GPS
Chapter 5: Applications of PNT Systems
Military and National Security Applications
Terrestrial Vehicle Navigation Systems
Precision Agriculture
Ground-Based Augmentation Systems (GBAS) or Local-Based Augmentation Systems (LBAS)
Drones
Marine
Rail Applications
Timing- Banks, Stock Markets, and Precise Scientific Time
Surveying and Mapping
Field Surveys
Atmospheric and Ionospheric Science
LBS Location Based Services
Civil Engineering and Intelligent Construction
Disaster Management and Response
GPS In-situ Networks
PNT Art
Space Navigation and Virtual Launch Range Management and Range Safety
Space Applications
PNT in Geostationary Orbit
NOAA GOES-R Example of Using GPS in GEO
Chapter 6: PNT Markets, Market Trends, and Major Players
Introduction
2020 European PNT Market Report
Chapter 7: National and International Governmental Policy Issues PNT Frequencies, Overlap, and Spectrum Issues
Introduction
Legal Aspects of PNT
PNT Frequencies: Overlap and Spectrum Issues
US GPS and PNT Policies and Issues
European Galileo Policy Issues
Russian Policy Issues
China Policy Issues
Indian Policy Issues
Japanese Policy Issues
British Policy Issues
South Korea Policy Issues
International Organizations and PNT Coordination
PNT Jamming, Spoofing, and Denial of Service Attacks
Cosmic Hazards
Legal, Ethical, and Moral Issues
Chapter 8: A Curious Narrative of the Origins and the Current State of PNT
Introduction
Where Are We Now?
The Current Status
Chapter 9: PNT on Other Worlds, and Our World Beyond PNT
Introduction
Beyond PNT
Back to the Past
Chapter 10: Top Ten Things to Know About Positioning, Navigation, and Timing
Selected Bibliography
Selected Global PNT Web Sites and Resources

Citation preview

SpringerBriefs in Space Development Scott Madry

Global Navigation Satellite Systems and Their Applications Second Edition

SpringerBriefs in Space Development Series Editor Michael K. Simpson, Institute of Space Commerce ISU Space Policy and Law Boulder, CO, USA

The SpringerBriefs in Space Development series unpacks trends in space exploration and its impact on society. Under the auspices of the International Space University, it features interdisciplinary contributions from space experts and rising professionals about this rapidly expanding field. The Briefs are concise and forward-looking, ranging from 50 to 125 pages (25,000-45,000 words). They broach topics such as: • • • • •

Space technology design and optimization Astrodynamics, spaceflight dynamics, and astronautics Resource management and mission planning Human factors and life support systems Space-related law, politics, economics, and culture

Readers will find herein reviews of a hot or emerging field; introductions to core concepts; extended research reports; manuals describing an experimental technique or technology; and multidisciplinary essays. The Briefs are published as part of Springer's eBook collections, and in addition are available for individual print and electronic purchase. They are characterized by fast, global electronic dissemination, straightforward publishing agreements, easy-­ to-­use manuscript preparation and formatting guidelines, and expedited production schedules.

Scott Madry

Global Navigation Satellite Systems and Their Applications Second Edition

Scott Madry International Space University Chapel Hill, NC, USA

ISSN 2191-8171     ISSN 2191-818X (electronic) SpringerBriefs in Space Development ISBN 978-3-031-74487-7    ISBN 978-3-031-74488-4 (eBook) https://doi.org/10.1007/978-3-031-74488-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland If disposing of this product, please recycle the paper.

This book is dedicated to the many people around the world who have used their ingenuity, insight, and skill to advance our understanding of how our access to space can serve all of humankind.

Preface to the Second Edition

I am very pleased to present this newly revised, second edition of this work. The first edition was published in 2015 and was partly written the year before. It is now nine years later, and the situation has changed dramatically, and it was time for a “refresh.” It was needed to accurately reflect the continuing advances in the current status and exciting future potentials that are on the horizon of these useful and fascinating technologies that are behind so much of our modern, interconnected world. The thing about books is that they are automatically out of date as soon as they go to press, but I hope that this much-needed update will allow this work to continue to be useful as a reference, as an introductory window into the subject, and as a course textbook. I hope you find it both useful and interesting, and I am always happy to hear from readers. Any errors are my own, and I wish to thank the many people who have provided me with useful information and ideas. I also thank the staff at Springer Nature for their support. … three satellites in the 24-hour orbit could provide not only an interference and censorship-­ free global TV service for the same power as a single modern transmitter, but could also make possible a position-finding grid whereby anyone could locate himself by means of a couple of dials on an instrument the size of a watch. (Arthur C. Clarke, Wireless World, October 1945)

Chapel Hill, NC, USA September 2024

Scott Madry

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Contents

  1 Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems ��������������������������������������������������������������������������������������    1   2 Doppler Satellite Positioning, Telemetry and Data Systems����������������   11   3 Positioning, Navigation and Timing (PNT) and Current National PNT Systems ����������������������������������������������������������������������������   29   4 Aided and Augmentation Systems: Differential GPS, Pseudolites, Ground-­Based Augmentation Systems, and Space-­Based Augmentation Systems����������������������������������������������������������������������������   59   5 Applications of PNT Systems������������������������������������������������������������������   71   6 PNT Markets, Market Trends, and Major Players������������������������������   87   7 National and International Governmental Policy Issues PNT Frequencies, Overlap, and Spectrum Issues������������������������������������������   95   8 A Curious Narrative of the Origins and the Current State of PNT ������������������������������������������������������������������������������������������������������  109   9 PNT on Other Worlds, and Our World Beyond PNT��������������������������  117 10 Top Ten Things to Know About Positioning, Navigation, and Timing������������������������������������������������������������������������������������������������  125 Selected Bibliography��������������������������������������������������������������������������������������  127

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Key Terms and Acronyms

AK Authentication key A-GPS Augmented GPS ADS-B  The US Automatic Dependent Surveillance and Broadcast system APL The Applied Physics Laboratory of Johns Hopkins University ARGOS Environmental satellite telemetry system AS Anti-spoofing ATC Ancillary Terrestrial Component Biedou Chinese satellite navigation system (Big Dipper) BIH Bureau international de l’Heure BPS Bits per second C/A Code the civilian GPS Course Acquisition Code CD Clock Drift CDGPS Canada-wide Differential Global Positioning System CDMA Code Division Multiple Access CEP Circular Error Probability CISPR International Space Committee on Radio Interference CL Long Code CM Moderate Length Code CME Coronal Mass Ejection CNES Centre National d’Etudes Spatiales, the French Space Agency Compass Chinese second-generation satellite navigation system, also called Biedou II COPUOS  The United Nations Conference on the Exploration and Peaceful Uses of Outer Space CORS Continuously Operating Reference Stations COSPAS  Cosmitscheskaja Sistema Poiska Awarinitsch Sudow (Russian: space system for search of vessels in distress) COSPAS-SarSAT International satellite distress system DAGR  Defense Advanced GPS Receiver, second-generation US military hand-­held GPS receiver xi

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Key Terms and Acronyms

DASS The US Distress Alerting Satellite System DGNSS Differential Global Navigation Satellite System DGPS Differential Global Positioning System DNSS Defense Navigation Satellite System precursor to GPS DoD US Department of Defense DOP Dilution of Precision Doppler Shift The change in frequency when a device transmitting either moves toward or away from you Dual Use The use of a system, network, or satellite infrastructure to support both military and national security and civilian and commercial applications and services EDM Electronic Distance Measuring EEPROM Electrically Erasable Programmable Read Only Memory EEZ Exclusive Economic Zone EGNOS European Geo-Stationary Navigation Overlay System Electro-L Russian satellite ELT Cospas-Sarsat Aviation Emergency Locator Transmitter EMP Electro Magnetic Pulse EPIRB  Cospas-Sarsat Marine Emergency Position Indicating Radio Beacon ESA European Space Agency EU European Union EUMETSAT European Meteorological Satellite Organization FAA Federal Aviation Administration FCC Federal Communication Commission FDMA Frequency Division Multiple Access FTP File Transfer Protocol GAGAN GPS Aided GEO Augmented Navigation (Indian WAAS) GaIn Galileo Industries GDOP Geometric Dilution of Precision GEO Geostationary Satellite Orbit GEOSAR Geostationary Satellite Search and Rescue GIOVE Galileo In-Orbit Validation Element, Galileo test satellite GLONASS  Russian Global Navigation Satellite System (“GLObalnaya NAvigatsionnaya Sputnikovaya Sistema”) GNSS Global Navigation Satellite System GPS US NAVSTAR Global Positioning System GSA European GNSS Agency GUS Ground Uplink Station HAPS Hybrid and Autonomous Positioning Systems HDOP Horizontal Dilution of Precision HEO Highly Elliptical Orbit ICAO International Civil Aviation Organization ICD  Interface Control Documents of the International Committee on GNSS

Key Terms and Acronyms

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ICG International Committee on GNSS IJPS Initial Joint Polar System of METSAT and NOAA IMO International Maritime Organization INRSS Indian Navigation Regional Satellite System INS Inertial Navigation System INSAT Indian GEO satellite ISRO Indian Space Research Organization ITAR International Traffic in Arms Regulations ITU International Telecommunications Union KGPS Kinematic GPS L1 The 1575.42 MHz GPS carrier frequency including C/A and P Code L2 The 1227.60 MHz secondary GPS carrier frequency (P Code only) L2C The L2 civilian code transmitted at the L2 frequency (1227.6 MHz) L5 3rd civil GPS frequency that tracks at low signal-to-noise ratios (1176.45 MHz) LAAS Local Area Augmentation System LBS Location Based Services LEO Low Earth Orbit LEOSAR Low Earth Orbit Search and Rescue LORAN LOng RAnge Navigation ground based radio navigation system LUT Cospas-Sarsat Local User Terminal MCC Cospas-Sarsat Mission Control Center METOP EUMETSAT satellites MEO Medium Earth Orbit MEOSAR Medium Earth Orbit Search and Rescue MHz Megahertz MSGPS USCG Maritime Differential GPS System MTSAT Japanese Multi-functional Transport Satellite Augmentation System Nadejda Soviet polar orbiting environmental satellites NASA National Aeronautics and Space Administration NAVSTAR NAVigation Satellite Timing And Ranging (GPS) NDGPS US Nationwide Differential GPS System NGS US National Geodetic Survey NOAA  National Oceanographic and Atmospheric Administration, U.S. Department of Commerce NUDET Nuclear detonation sensors on GPS satellites P-Code The GPS military Precise code P(Y) Code The encrypted GPS military code Parus Early Soviet military satellite navigation system similar to Transit PDOP Position Dilution of Precision PLB Cospas-Sarsat Personal Locator Beacon PMT Argos II Platform Messaging Transceiver PNT Precision Navigation and Timing POES Polar Orbiting Environmental Satellites of NOAA PPS Precise Positioning Service

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Key Terms and Acronyms

PPT Argos Platform Transmitter Terminals PRN Pseudo Random Noise PRN# Pseudo Random Noise number Project 621B  US Air Force satellite navigation system in development before GPS PRS Public Regulated Service (Galileo) Pseudolite A false, ground-based GNSS satellite, used for local systems QZSS Japanese Quasi-Zenith Satellite Augmentation System MSAS Japanese regional space-based augmentation system RCC Cospas-Sarsat Rescue Coordination Center RF Radio Frequency RIMS European EGNOS Ranging and Integrity Monitoring Stations RMS Root Mean Square RTK Real Time Kinematic GPS SA Selective Availability, the intentional degradation of the civilian GPS signal SARSAT Search and Rescue Satellite Aided Tracking SBAS Satellite-Based Augmentation System SDCM Russian regional space-based augmentation system SGS-90 Soviet Geodetic System 1990 SNAS Satellite Navigation Augmentation System (China) SPS Standard Positioning System, the GPS civilian C/A code signal Sputnik The first Soviet satellite SSAS Argos-based Ship Security Alert System SV Space vehicle TAI Temps Atomique International, or International Atomic Time, the basis of UTC Coordinated Universal Time TDOP Time Dilution of Precision TEC Ionospheric Total Electron Count Timation US Navy satellite system developed before GPS TOA Time of Almanac TOE Time of Ephemeris TTFF Time to first fix Transit First US satellite positioning system Tsikada The first Soviet satellite positioning system USCG United States Coast Guard USGS United States Geological Survey UTC Coordinated Universal Time UTM Universal Transverse Mercator map projection VDOP Vertical Dilution Of Precision VMS Vehicle Monitoring Systems VRA Variable Rate Application systems for Precision Agriculture VTS Vehicle Tracking Systems WAAS Wide Area Augmentation System for GPS in the USA

Key Terms and Acronyms

WADGPS WAGE WGS WGS84 WCRP

Wide Area Differential GPS US military Wide Area GPS Enhancement System (encoded) World Geodetic System World Geodetic System version 1984 World Climate Research Program

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About this Book and the Internaional Space University

This Springer Nature book is published in collaboration with the International Space University (ISU). At its central campus in Strasbourg, France, and at various locations around the world, the ISU provides graduate-level training to the future leaders of the global space community. The university offers a two-month Space Studies Program, a five-week Southern Hemisphere Program, a one-year Executive MBA, and a one-year master’s program related to space science, space engineering, systems engineering, space policy and law, business and management, and space and society. These programs give international graduate students and young space professionals the opportunity to learn while solving complex problems in an intercultural environment. Since its founding in 1987, the International Space University has

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About this Book and the Internaional Space University

graduated more than 5200 students from 110 countries, creating an international network of professionals and leaders. ISU faculty and lecturers from around the world have published hundreds of books and articles on space exploration, applications, science, and development.

Chapter 1

Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

What Is in a Name? First of all, let us consider the many names for what this is. At first, from the 1970’s up until quite recently, there was only the U.S.  NAVSTAR Global Positioning System, or GPS. GPS became a generic term for the capability of satellite-provided navigation and timing and was used broadly. As new systems such as the European Galileo were envisioned and later built and launched, their developers did not like that the U.S. system’s name also came to be the name for the generic technology, and so they proposed and began using the term Global Navigation Satellite Systems, or GNSS, as a more neutral and generic term for this class of systems. These have also been called PNSS, for Precise Navigation Satellite Systems as well. Recently, this has been supplanted by yet another acronym, PNT, for Positioning, Navigation, and Timing, to reflect the importance of these three aspects, especially the vital timing component. Part of this process is that people, organizations, and political and economic interests want to have some control over what things are called, as it gives them power and authority, and this is a natural aspect of the development of new technologies. Today, both GNSS and PNT are used interchangeably. In the book, we will use the more recent PNT term unless GNSS is part of a program or project name, but a rose by any other name…

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_1

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1  Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

 hy Write a ‘Short Book” on Positioning, Navigation, W and Timing Satellites? Our modern world is intertwined with technology. All aspects of our lives are connected with our ability to measure space and time every day. “Where am I, and what time is it”? These two basic questions connect us to all aspects of our modern, technologically driven lives. Earth orbiting satellites, in this case, Position, Navigation and Timing satellites, have quickly become an integral part of our daily lives, and are now an unseen but vital utility that is used throughout the world by nearly everyone. As we shall see, a day without PNT satellites would be a very bad day, and most people in the world use PNT systems every day, without even knowing it. Therefore, it is important that we all, students, entrepreneurs, end users, and policy makers, have some basic understanding of what these satellite systems are, how they work, and what they are used for. We also should consider the growing and inevitable political, economic, and social impact that these have on all of us.

 hat Is the Best Way to Grasp the Field of Positioning, W Navigation, and Timing Systems? The best way to grasp this field is to consider what would happen if our navigation and timing satellites were to suddenly be turned off or be disabled. While there would not be panic in the streets, the effects would be quick, and the impact would be major. In the ancient days, keeping track of the phases of the moon and counting the days, and either morning or afternoon was about as close as things could be measured, but today, almost all aspects of our lives are driven by an accurate and coordinated time reference down to thousandths of a second. PNT satellites provide us with this time reference to the accuracy of state-of-the-art atomic clocks anywhere on or above the planet. They are the global, synchronized timekeeper for every aspect of our electronic lives. Our internet communications, the world wide web, stock market transactions and electrical power grids, to name just some of the most visible ones, are all timed using, primarily, the U.S. GPS constellation today. Car, rail, ship and aircraft navigation systems all rely on these, and even the International Space Station and other satellites high above our heads determine their positions using these satellites. Many new and innovative businesses such as Uber, Lyft, and major distribution and database-driven businesses such as Amazon all rely on GPS-provided time stamps, navigation, and route planning. It is also important to remember that all of these systems are dual use and have vital military and national security uses that are closely guarded and key components of national defense. These PNT satellite constellations provide global and seamless timing and positioning in ways that are often hidden, but which are vital to the operation of our modern, technologically driven civilization, and this is growing quickly. But that is not all.

Who Is Involved and What are Their Various Perspectives?

3

There are currently over three million jobs worldwide that depend on the PNT technologies. Over 130,000 manufacturing jobs and over three million people are involved in some aspect of all the downstream PNT markets, and this will only continue to grow. The commercial value of this market is difficult to precisely define but is in the range of US$100 Billion per year in the US alone. Worldwide it exceeds $175 Billion and growing. And this trend is accelerating. The move from expensive, dedicated GPS receivers to smartphones has created a new and huge growth in new applications such as Location Based Services (LBS) and emergency vehicle routing.

Who Is Involved and What are Their Various Perspectives? The various national (and European Union acting as the same) governments are the primary players in the development, launch and operation of these systems. There are (as of yet) no private PNT systems, but this may happen in the future more quickly than most would expect, although it is complicated in the immediate term, due to the largely dual use (civilian and military) nature of these systems. The United States government has pioneered the developed and operation of the GPS system, and this has been followed by Russia, China, India, and the European Union. Others, including Japan, South Korea, The United Kingdom, and more are on the way. There are both upstream (providers of the enabling technologies) and downstream (markets that exploit the technologies) aspects to the PNT Markets. In the upstream markets, these are dominated by large aerospace corporations that provide the ground segment (control systems) and space segments (satellites and launch vehicles), and support equipment. These include Raytheon, Honeywell, Northrop Grumman, Boeing, Lockheed-Martin, Thales, Astrium and NEC. In the downstream markets, key vendors are Garmin, Trimble, Leica, TOPCON, TomTom, Magellan and Navteq. Google, Apple and Samsung are major users of GPS and other chips for the billions of smart phones that now have satellite PNT receivers built into their capabilities. Literally billions of people around the planet have their lives connected by these systems every day. The PNT industry is a high technology field, and it is inherently dual use, where both national security and military systems coexist with a much larger number of commercial vendors, products and uses in the commercial marketplace. The original U.S. and Russian systems were designed for purely military purposes, but these have been far outpaced by commercial uses and users. The low-price chipset manufacturers actually produce the largest numbers of units sold each year, but these are a relatively small part of the overall commercial market. Included in these are Texas Instruments, Qualcomm, Broadcomm, Infineon and ST Micro, companies most people have never heard of. An unusual aspect of the PNT market is that, other than the producers of ground equipment and satellites and launchers, the entire market assumes that there will be free-of-charge signals provided by complex, billion dollar government developed and operated satellite systems. This is unique in space technologies to this PNT

1  Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

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market, where all aspects of the commercial market receive the basic capability for free from government designed and operated (and funded) systems. There are also a very wide range of PNT receivers, ranging from very inexpensive GPS chipsets, to navigation receivers to car navigation systems to very complex and high-end scientific receivers. These scientific receivers include centimetric surveying equipment, and dedicated ultra-precise timing devices which are not concerned with positioning at all, only the precise time. All of these rely on accessing free, government-provided signals, something unique to this market.

Introduction PNT systems are a relatively recent development in satellite applications, which has traditionally consisted of satellite telecommunications and space remote sensing. The enormous growth in the use of PNT systems such as the US NAVSTAR Global Positioning System has been extraordinary. There are now several similar systems, and these all share a common overall design and architecture and use a constellation of complex satellites to provide very precise timing, navigation and positioning data globally, and they have tremendous application for a wide range of users around the globe.

History The concept of satellite navigation is not really as new as many may believe. But first, let us consider what a satellite is. The term comes from the ancient Latin, satellus- a servant or slave of a powerful lord or master in ancient Rome, or an attendant to an important person. It was Johannes Kepler who first used the term in our context, to describe the moons of Jupiter and planets he observed. Since the planets were named after the ancient Greek and Roman gods, he coined the term satellites (the plural of satellus) to describe these small bodies, scurrying around the great planet, as servants bustle around their great masters. It is a fitting term, as artificial satellites, since the launch of Sputnik, have served us well. Satellites are orbiting servants of humanity, doing our bidding in communications, imaging, science, and now, positioning, timing and navigation. We have always used the stars and the sun to navigate our way across the land and seas, but clouds were always a problem. The first proposed use of an ‘artificial’ navigation satellite was a short story called “The Brick Moon” by Edward Everett Hale, published in 1869. In this, he proposed: “…he [Q] suggested the Brick Moon. The plan was this: If from the surface of the earth, by a gigantic peashooter, you could shoot a pea upward from Greenwich, aimed northward as well as upward; if you drove it so fast and far that when its power of ascent was exhausted, and it began to fall, it should clear the earth, and pass outside the North Pole; if you had

History

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given it sufficient power to get it half round the earth without touching, that pea would clear the earth forever… If only we could see that pea as it revolved in that convenient orbit, then we could measure the longitude from that, as soon as we knew how high the orbit was, as well as if it were the ring of Saturn. But a pea is so small!” “Yes,” said Q., “but we must make a large pea.” Then we fell to work on plans for making the pea very large and very light…. It must stand fire well, very well. Iron will not answer. It must be brick; we must have a [hollow] Brick Moon. Edward Everett Hale, “The Brick Moon”, Atlantic Monthly, Oct. Nov. Dec. 1869.

The brick moon was to be 90 m in diameter, and in a polar orbit 6000 km high over the Greenwich observatory in London, England. Mariners could determine their Longitude (East-West) from this satellite in the same way that they use the North Star to find their Latitude (North-South) in the northern hemisphere. Once the system was operational and proved beneficial, a second was to be launched over New Orleans in the USA, to cover the western Atlantic Ocean area. This was all to be paid for by subscription, and by the maritime insurance companies such as Lloyds of London, in order to reduce navigational errors and ship losses at sea. It was a very visionary concept, if the book was quite fanciful. It is available online (https://www. brickmoon.net). Arthur C. Clarke, the science fiction writer and space visionary, who first proposed the practical use of the geostationary satellite orbit for satellite telecommunications, also proposed a satellite navigation system in his famous 1945 article that first described the use of telecommunications satellites. In this, he said: ,,,three satellites in the 24-hour orbit could provide not only an interference and censorship-­ free global tv service for the same power as a single modern transmitter, but could also make possible a position-finding grid whereby anyone could locate himself by means of a couple of dials on an instrument the size of a watch. — Arthur C. Clarke, Wireless World, October 1945

He was right, and today we have both telecommunications satellites and navigation and timing systems that are an integrated part of our modern world. There were several early ground-based radio navigation systems, including LORAN and OMEGA, which paved the way for our current satellite systems. LORAN, meaning LOng RAge Navigation, was a radio navigation system first developed in World War II by the U.S. as it fought a global war. It used powerful low frequency radio towers along the coastlines to broadcast radio signals and proved to be accurate for ships and aircraft from tens of km to a few hundred meters and was useful up to 2400 km from the broadcast tower. This was a major improvement in global navigation and was very useful. The U.S. Coast Guard took over LORAN operation in 1958, and the system remained operational, with significant improvements, until the 1980s when it was replaced by the development of the GPS system. Some of the old LORAN towers were then converted for use to broadcast differential correction GPS services, as discussed below. The launch of Sputnik in 1957 by the Soviet Union was the first ‘artificial’ satellite, and it broadcast a beep that could be received around the world. Many people listened to the tone and were able to determine the orbital characteristics of the satellite by measuring the Doppler shift of the tone as it passed overhead. Two

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1  Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

Fig. 1.1  The Transit 1-A prototype satellite being prepared for launch. (Reprinted with permission from https://www.darpa.mil/about-­us/timeline/transit-­satellite. © 2024, DARPA.  All rights reserved)

U.S. researchers, William Guier and George Wiefenback, were able to mentally reverse this concept. They realized that if you had a satellite in a known orbit that was broadcasting a signal on a known frequency, then you could use the same Doppler analysis of that signal to determine your position on the ground. A brilliant concept. At this time, another Applied Physics Lab researcher, Frank McClure was working with the U.S.  Navy to develop a navigation system for the new Polaris nuclear ballistic submarine fleet, and he needed a reliable and robust ability for these submarines to determine their position quickly and anywhere in the oceans, even after spending months under the sea. McClure proposed the concept developed by Guier and Wiefenbach to the Navy, and it was quickly approved, funded by ARPA, and developed as the TRANSIT satellite system, as shown below in Fig. 1.1 below. The first launch was in 1959 (less than 2 years after Sputnik) and the system became operational in 1964. The system was only retired in December of 1996, with the completion of the more modern NAVSTAR GPS system. Transit was the first operational satellite navigation system. An example of the AN-BRN-3 receiver is shown in Fig. 1.2 below. It consisted of two 5-foot tall racks, capable of being lowered through a small submarine hatch.

Transit, the First Satellite-Based Navigation System Transit worked by measuring the Doppler shift of the broadcast signal from six satellites in offset polar orbits at an altitude of 1000 km. The ship’s receiver had to lock onto the satellite signal for a period of 10–15  min in order to measure the

History

7

Fig. 1.2  An AN-BRN-3 receiver for Transit navigational satellites, as used on US submarines. (Reprinted from https://commons.wikimedia.org/wiki/File:AN-­BRN-­3_receiver_for_Transit_ navigational_satellite,_Westinghouse,_used_on_US_submarines_-­_ National_Electronics_ Museum_-­_DSC00558.JPG. Public domain)

frequency change and compute their position. As the satellites in the constellation were only visible intermittently, the system only was available several times per day, more near the poles and less around the equator, and it was not a portable system by any means. It required large racks of equipment, which was no problem for ships, but it was a real issue for submarines, requiring specially designed racks capable of being lowered down a small hatch.

The Concept of Doppler Tracking A position on the ground (or at sea) can be determined by tracking the Doppler effect on frequencies broadcast from a satellite. If the broadcast frequency is constant and known, and the orbital ephemeris is also known, the relative motion of the satellite towards or away from the receiver on the ground raises the frequency as the satellite comes towards you and lowers the frequency as it moves away, as shown in Fig. 1.3 below. If we know the precise orbit of the satellite and frequency of the signal that it broadcasts, then we can determine our position on the ground or at sea. Transit and other Doppler systems do not work for aircraft, only on the surface.

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1  Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

Fig. 1.3  left and right Doppler shift positioning. (Reprinted with permission from https://d9-­wret. s3.us-­west-­2.amazonaws.com/assets/palladium/production/s3fs-­public/atoms/files/Argos_users_ manual_en_2016_v1.6.6-­r363_9.pdf. © 2024, Argos. All rights reserved)

The actual computation done is: cos  A  

 fr  fe   C  fe



V



where: C = speed of light V = satellite speed relative to platform fe = transmit frequency (401.646 to 401.654 MHz for the Argos system) fr = receive frequency on the ground You will see that this gives two positions as the answer, as shown in 1.3b above, where the cone intersects the surface of the Earth. Your correct location is determined by knowing the previous location, or through repeated measurements on multiple passes.

Tsikada/Parus The Soviet Union quickly developed a very similar system, called Tsikada/Parus that was based on the same Doppler approach. Tsikada (Russian for cicada, a chirping cricket, as the satellite signal chirped) was the civilian system and Parus was the military one, but they shared a common design and satellite bus. A military constellation of 6 satellites became available in 1974, and a civil navigation program followed in 1978. The civil system was used by thousands of Soviet bloc ships until just recently, when it was superseded by the Glonass system.

History

9

The U.S. Navy Transit system worked very well and demonstrated that satellite navigation was possible and could be done in a practical manner. It was a huge leap forward, providing global and accurate navigation for the U.S. Navy anywhere on the surface of the Earth and in any weather. But it still had several major limitations for its intended use by U.S. submarines. You had to track the satellite for 10–15 min on the surface to measure the Doppler shift, leaving the submarine vulnerable and open to detection. There were only 6 satellites, so you had to wait until a satellite came into view, and so the system could not be used at any time but only on a fixed schedule. The position was only accurate to some 300 m at first, and it required a large amount of equipment in the ship (which got much smaller as the program went on) that took up valuable space and power on a cramped submarine or warship. In fact, when Transit was developed, no computer was small enough to fit in the hatch of a submarine, and so a new computer had to be developed that could be lifted through a submarine’s small hatch. It was over 5  feet (1.54  m) tall and weighed some 250 kg, as shown below in Fig. 1.2 above. Hardly a portable system. The Transit system was also not useable except on the surface of the Earth, so there was not possible future use by aircraft or in space. So a decision was made to develop a second-generation system that used improved technologies to address all of these limitations. This was the origin of the now-common NAVSTAR Global Positioning System, or GPS.

 re-GPS and the Decision to Integrate Multiple P Research Programs There were several research programs that developed technologies that led to GPS.  The U.S.  Navy did research on a program called the Timation system. Researchers at the Naval Research Laboratory (NRL) developed a new concept in 1964 for satellite ranging, or measuring the range, or distance to the satellites, rather than using the Doppler technique. The technique of satellite ranging provided not only better positional accuracy and precise time as well but could also be available on a continuous basis to an unlimited number of users. The first proof-of-concept Timation satellite was launched in 1967 into a polar orbit, and it proved the viability of the concept. The third Timation satellite, launched in 1974, was the first satellite ever launched into orbit with an atomic clock. These, together, demonstrated the benefits of atomic clock-based satellite ranging, the basis of GPS. At the same time, the U.S. Air Force was working on its own satellite navigation system, called Project 621B, that would work with aircraft. This was also based on a satellite ranging concept, but one that would allow determination of altitude as well, vital for use in aircraft and rockets. This started in 1963, and developed many of the technologies that were incorporated into the GPS design. It relied on the use of satellite ranging and used a pseudo-random code that was broadcast by the satellite to a receiver on the ground. It provided altitude as well as positioning, clearly important for the Air Force.

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1  Introduction to Positioning, Navigation, and Timing (PNT) Satellite Systems

There was considerable rivalry and competition between the different military services at that time, as today, but in 1973 the decision was made to combine aspects of both Timation and Project 621B into a single Defense Navigation Satellite System (DNSS) that would be used by all the U.S. military services. This system became the NAVSTAR Global Positioning System (GPS). The decision was made for the new system to be developed and operated by the U.S. Air Force, but that it would be suitable for the navy and army as well. It was originally intended to be a purely military system, with no civilian use allowed or, at best, that civilian use would be limited and of very poor precision, on the order of hundreds of meters like Transit. The new, integrated concept took the signal design of Project 621B, the orbits and orbital prediction techniques from Transit, and the atomic clocks from Timation, and combined them into a single, operational design called DNSS, the Defense Navigation Satellite System. This emerged as the NAVSTAR Global Positioning System (GPS) that we use today, and whose design is the basis for all other current PNT systems.

Chapter 2

Doppler Satellite Positioning, Telemetry and Data Systems

Argos The Argos positioning and data telemetry system is a satellite-based timing, positioning and data collection system that is dedicated to environmental research and applications. Unlike PNT systems, it is a two-way telemetry system that is designed so that Argos terminals on the ground, in the air, or at sea can collect and store data and then broadcast the data up to a satellite, which passes it along to a ground station and then on to the end user. This is sometimes referred to as a ‘store-dump’ satellite data system. It can be used on both fixed and mobile platforms and was first established in 1978 as a joint project between NASA, NOAA, and the French space agency CNES. Argos was originally developed by the U.S. and France to use French hardware on U.S. NOAA polar-orbiting satellites. This is referred to as a parasite payload, one that is not the primary purpose of the satellite, and which uses its power and systems for a secondary purpose. While Argos has low data rates, it provides very cost-effective access to remote data collection devices around the world and is a part of many important global scientific research programs. The system has been operated since 1986 under contract to CLS/Argos, based in Toulouse, France, with a U.S. subsidiary, CLSAmerica operating in Maryland. The European Meteorological Satellite Organization (EUMETSAT) has recently joined the partnership, as has the Indian Space Research Organization (ISRO), expanding the capability of the overall system. There are over 20,000 active receivers today, with over 8,000 used for animal tracking around the world. Systems are currently on US, European, and Indian polar-orbiting satellites, as shown below in Fig. 2.1.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_2

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2  Doppler Satellite Positioning, Telemetry and Data Systems

Fig. 2.1  The Argos data system, showing transmitters receiving location from GPS, transmitting data to the Argos satellite, which transmits the data to a processing center and on to the user. (Reprinted with permission from https://d9-­wret.s3.us-­west-­2.amazonaws.com/assets/palladium/ production/s3fs-­public/atoms/files/Argos_users_manual_en_2016_v1.6.6-­r363_9.pdf. © 2024, USGS. All rights reserved)

System Design The system includes the space segment (NOAA and other polar satellites), the control segment (ground receiving stations and command and control centers), and the user segment (individual Argos devices). Argos devices are programmed to collect data, store the data onboard, and then transmit very small bandwidth signals to the satellites at periodic intervals, set by the user or when interrogated by a satellite passing overhead. There are many kinds of Argos transmitters available on the commercial market. The space segment now includes seven polar-orbiting satellites flying in an 850  km orbit (3 NOAA polar orbiting satellites, 3 METOP European satellites operated by EUMETSAT, the EUMETSAT/NOAA Initial Joint Polar System (IJPS), and 1 Indian SARAL satellite). The satellites receive the data and store them onboard until they can download the data to a receiving station or

System Design

13

Fig. 2.2  showing the location of Argos ground receiving stations. Satellites receiving data while in view of a ground station can download the data in near real time. If over remote ocean areas, the satellites store the data until in view of a ground station, rarely over 2 h. (Reprinted with permission from https://d9-­wret.s3.us-­west-­2.amazonaws.com/assets/palladium/production/s3fs-­public/ atoms/files/Argos_users_manual_en_2016_v1.6.6-­r363_9.pdf. © 2024, Argos. All rights reserved)

transmit in real time if in view of one of over 40 automated ground-receiving stations located around the world, as shown above in Fig. 2.2 There are three main receiving stations that also collect all data on each pass, located at Fairbanks, Alaska, Wallops Island, Virginia, and Svalbard, Norway. The receiving stations automatically pass the data to the French and U.S. global processing centers, which then process the data from the various receiving stations and distribute the data on to the end users, often as an automated email message. Secure access is provided by the ArgosWeb website (http://www.Argos-­system.org) that can display data on world maps, or as tables and charts as needed by the user. Data can also be sent by the ArgosDirect automatic distribution system via email, ftp, or on CD-ROM. Data are archived for 12 months. Users purchase Argos platforms, as the transmitters are called, either mobile or fixed, from one of several commercial vendors. These are fitted with various sensors to measure a wide variety of parameters, including sea surface temperature and salinity, wildlife parameters such as heartbeat, river or tide gages, earthquakes, weather, and more. Each platform has a unique identifier and transmits the platform identification number and all collected data on 401.650 MHz. Each platform has a fixed repetition period, variable between 90 and 200  s, depending on the user requirements. Each platform transmission lasts less than 1  s, so the system can accommodate multiple platforms in the same area. The first generation of the system had platforms blindly transmitting the data, in hopes that a satellite would receive its signals. The newer generation devices are smarter, as described in Fig. 2.2 above.

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2  Doppler Satellite Positioning, Telemetry and Data Systems

The Argos systems fly as small ‘parasite payloads’ on 7 NOAA POES, Eurmetsat MetOp, and Indian satellites. These satellites are in 100-min, near polar, Sun-­ synchronous orbits that are each offset from the other satellite orbits, so that they cover as much ground as possible in each day. Areas at the poles see the transmitter on each pass, acquiring 14 data takes per day, while areas at the equator get around 8–10 per day, day or night, rain or shine. Each orbit overlaps 25% at the equator with the last pass, as shown in Fig. 2.3 below, as the Earth rotates under the satellites in their polar orbits. Each satellite covers an area 5000 km across as it orbits the Earth and can receive data from all Argos transmitters within this area, due to the small data volumes. The satellite will be in view of a platform for approximately 10 min, but it only takes a few seconds to transmit and receive the data. The two processing centers offer redundant capabilities, with one near Washington, D.C. and one in Toulouse, France. These centers process and check the data, and then make it available to the end users, usually within minutes of receiving the data from the satellite. Transmissions within sight of one of 40 ground receiving stations are downloaded in near real time, while remote ocean data, for example, have the data stored on the satellite until it is in view of a ground receiving station. The data are processed using the Doppler shift technique described above, but modern platforms can, and almost always now are, also fitted with a GPS receiver and the more precise GPS position and time can be automatically included in the data uplinked. All data are distributed in latitude/longitude using the WGS84 (World Geodetic System 1984) reference system. Estimated positioning error is processed for each reception. With GPS, the true position is well within 30 m, and with the Doppler technique it can range from 1500 to 250  m, depending on several

Fig. 2.3.  Left: Argos visibility area. Right: the orginal overlaps. (Reprinted with permission from https://d9-­wret.s3.us-­west-­2.amazonaws.com/assets/palladium/production/s3fs-­public/atoms/ files/Argos_users_manual_en_2016_v1.6.6-­r363_9.pdf. © 2024, Argos. All rights reserved)

Argos Applications

15

parameters. Due to technical issues, some passes cannot compute the Doppler position, and so may sometimes transmit the data parameters without coordinates. For terrestrial readings, a digital elevation model (DEM) is automatically used in all ground mobile data to improve the precision of the location by including the altitude above sea level. Each platform message has an ID, a type, the date, latitude and longitude coordinates, the precision level, and then the various data parameters. In the figure below, the platform was measuring sea water salinity and water temperature, each measured at 11 different sea depths. Argos readings are dedicated to studying and protecting our natural environment and must be non-commercial in nature, although a new, commercial system has been developed. New users simply fill out an online request, which is reviewed by the Argos Operations Committee (NASA, NOAA, Eumetsat, CNES, and ISRO). Once approved, the user fills out a Program Overview, describing their data parameters, ID numbers of transmitters, and other information. Before deploying, the devices are tested under conditions similar to their operational conditions, to ensure proper operations. Once deployed, the data are accessed via the ArgosWeb, ArgosServer, or ArgosDirect, ArgosShare, ArgosMonitor or WebServices methods as described on the Argos website (www.Argos-­system.org). Argos transmitters (originally called Platform Transmitter Terminals, or PTTs) can be purchased from several vendors and the prices range between US$ 2500 and US$4500, depending on size, battery life, data storage, and other factors. Some are tiny, for attaching to small birds or other animals, while some are quite large and designed for use on permanent marine buoys, remote pumping stations, or other fixed locations. Others are designed for deep-diving whales and can pop off and float to the surface after a set amount of time to transmit their data. Fees are based on PTT transmissions. Standard service costs US$15 per day per PTT, with data only (no position) costing $7.50 per day. To access data via email, internet, ftp or by fax, the cost is US$0.12 per kilobyte. A very modest cost for such a valuable, global environmental data service.

Argos Applications There are over 20,000 Argos transmitters in use today, and the system can accommodate many more (so think up a novel application!). The number of applications is very broad, including marine, land, wildlife, hazardous materials tracking, remote facility monitoring, ocean racing, and disaster applications. Self-contained Argos systems are used to transmit housekeeping data from remote dams, pipelines, power stations and other sites, many located in inaccessible regions that do not have permanent staff, power, or infrastructure. Over 6000 drifting and moored ocean buoys transmit global ocean data on a continual basis. Information including wind speed and direction, air pressure, air temperature, water temperature and salinity are collected throughout the world’s oceans, as shown below in Fig. 2.4. These provide valuable ‘ground truth’ data to

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2  Doppler Satellite Positioning, Telemetry and Data Systems

Fig. 2.4  A USGS/Argos marine buoy. The drogue can be set at different depths to track underwater ocean currents. (Reprinted with permission from https://www.aoml.noaa.gov/global-­drifter-­ program/. © 2024, USGS. All rights reserved)

calibrate satellite data and are important parts of the World Climate Research Program. Over 70% of all Argos data are voluntarily exchanged globally for use in ocean and weather prediction models. This is a sample ocean buoy transmission: Marine station WMO 12345 on 29 January 2024 at 06:00 UTC, Latitude = 21 deg. 22 min. North, Longitude = 43 deg. 52 min. West, Wind Direction = 70 Degrees, Wind Speed = 23 m/s, Air Temperature = 12.5 Celsius, Temperatures at selected depths: Surface:12.4 C, 10 meters: 8.2 C, 150 meters: 2.2 C. Salinity at 10 meters: 35 ppm.

A global marine drifter array operates around the world, with drogues set at different depths to map ocean currents and collect other data. There is a global, real-time online display of these available at https://www.aoml.noaa.gov/phod/gdp/interactive/drifter_array.html, as shown below in Fig. 2.5 Fixed marine buoy networks are also located around the world’s coastlines, the arctic sea, and across the Pacific Ocean, providing vital real-time wind and wave height data. These data are available in real time through the NOAA National Data Buoy Center’s website (http://www.ndbc.noaa.gov), as shown below in Fig. 2.6.

The Argo Program

17

Fig. 2.5  Real-time status of the Global Drifter Array. Colors indicate the nationality of the 1165 different buoys. Individual buoys can be queried, as shown at right. Reprinted from https://www. aoml.noaa.gov/phod/gdp/interactive/drifter_array.html. © 2024, OpenStreetMap distributed under the Open Database License.

Fig. 2.6  NOAA fixed ocean buoy array locations and a marine anchored buoy. (Reprinted from https://www.ndbc.noaa.gov. © 2024, NOAA)

The Argo Program Over 3000 Argo program (different from Argos) devices called floats are in the world’s oceans. These collect water temperature, salinity, and current data for the upper 2000 m of the world’s oceans each day. They drift at 1000 m depth for days, and then dive to 2000 m every 10 days and rise to the surface, collecting water profile data. On their return to the surface, they broadcast the data collected using the Argos system, and then start the process again. These are operated by many nations around the world and the data are all freely shared with the international scientific

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Fig. 2.7  Left and Right: Argo float network showing drifting floats in the world’s oceans. (Reprinted from https://argo.ucsd.edu/about/status/ and https://globalocean.noaa.gov/research/ argo-­program/. © 2024, NOAA)

community, providing vital global data and ‘ground truthing’ for satellite systems. Over 2  million data measurements have been collected. This is shown below in Fig. 2.7 above. One of the most interesting applications is the use of Argos transmitters on fishing vessels to monitor Exclusive Economic Zone (EEZ) compliance. These Vessel Monitoring Systems (VMS) provide data that are admissible in court regarding illegal fishing operations and are used around the world to monitor fish catch data and EEZ compliance. The ArgoNet system provides a turnkey commercial package for monitoring fishing fleet operations. Over 8000 fishing vessels around the world are equipped with these systems (www.argonet-­vms.net), and several nations have successfully prosecuted illegal fishing operations within their EEZ limits. Since 2005, the United Kingdom requires all fishing vessels over 15 m to have installed onboard a working satellite tracking system, and they have successfully prosecuted offenders using this system and Argos data in British courts. Vessels can upload their position and catch information (by zone and species) automatically to the appropriate regulatory agency. The International Maritime Organization has mandated that all vessels over 500 gross tons must install an Argos-based Ship Security Alert System (SSAS) that allows crews to send an alert message in case of acts of piracy, and to allow fleet managers to track their vessels world-wide. Icebergs in the North Atlantic Ocean are routinely tracked using specially designed Argos buoys that are dropped from U.S. Coast Guard C-130 aircraft onto large icebergs. This is a part of the International Ice Patrol program created after the sinking of the Titanic. The location of these dangerous icebergs can then be broadcast in real time. One of the most important uses of Argos is for wildlife tracking. Thousands of animals, ranging from tiny migrating birds to deep diving whales and land animals are tracked across the globe using Argos devices. These have revolutionized our understanding of the migration patterns of many species of animals, feeding and breeding patterns, and more. Some of these miniaturized Argos transmitters can fit on the smallest birds and can track movements for over a year. Lesser Spotted Eagles migrate some 4000 km each year in an annual migration between central

The Argo Program

19

Fig. 2.8  Left: A Lesser Spotted Eagle’s 2015 migration track using Argos. Right: The eagle with its ARGOS device on its back. (Reprinted with permission from https://www.argos-­system.org/ the-­autumn-­migration-­2015-­of-­lesser-­spotted-­eagles-­has-­started-­in-­september/. © 2024, Argos. All rights reserved)

Africa and northern Europe, as shown below in Fig. 2.8 above. Their migration patterns were tracked using tiny Argos receivers weighing only a few grams. Marine species are also tracked, including deep-diving species of whales, sharks, marine turtles and more. Information on depth of dives, temperature, duration of dives and more can be automatically collected and received as email by the researchers. Land species such as artic caribou are routinely tracked on their annual migration routes. Important public health applications include transmitting information about hospital and clinic admissions, food status, and even school attendance levels. Argos systems can provide vital early warning for human and environmental disasters in the most remote locations of the world by transmitting flood, earthquake, and other data from the most remote areas. The Argos design has been in operation since 1978, about the same time that the GPS system was introduced, and technology has changed significantly since then. The new Argos-3 next generation system is now in the process of being implemented with the launch of Eumetsat’s MetOp satellite. This new capability includes many improvements, including two-way communications (down to the Argos device, now called Platform Messaging Transceivers or PMTs). The updated system has greater data volume transmitted, more efficient data collection and battery savings, and remote control and re-programming of the PMT while in operation. The original system allowed for data ranging from 32 to 256 bits, which is now increased to 4608 bits per satellite pass, 10 times the previous limit. While the original system

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had transmitters constantly broadcasting, waiting for a satellite to come overhead, the new design allows receivers to know when satellites will be overhead and to only broadcast in these windows, and then the satellite can broadcast down to the PMT that it has successfully received a transmission of data, so that the PMT can stop broadcasting, thus saving battery life and extending PMT lifetime. System users can also now send short messages to their transmitters, to alter the transmission schedule, change data collection parameters, or re-program their platforms. The new system is fully compatible with the existing Argos 1 and 2 systems, but significantly increase the utility and efficiency of the overall system. In 2018 a commercial French subsidiary called Kinéis was created to operate the existing Argos system, and they have received significant funding to develop and operate a new constellation of 25 16-U cubesats to meet the commercial Internet of Things (IOT) market needs with a system based on Argos. It will provide 20-byte messages from IOT ground sensors with 15-min data delivery. (https://www.kineis. com/en/connectivity/#message).

Cospas-Sarsat The second Doppler-based satellite system still in use is the Cospas-Sarsat Satellite search and rescue system (http://www.cospas-­sarsat.int). The logo is shown below in Fig. 2.9. This is another excellent example of cold-war era cooperation in space applications. The system is designed to take the search out of search and rescue operations, and was originally a collaborative project of the U.S., Canada, France, and the USSR. Founded in 1979, There are now 43 other nations and entities who are involved in this vital service, as shown below in Fig. 2.10. These include the four original parties to the Cospas-Sarsat agreement, some 65 satellites, 26 Ground segment providers, 11 user states, and 2 participating international organizations. Cospas is translated as Cosmitscheskaja Sistema Poiska Awarinitsch Sudow (Russian for ‘space system for search of vessels in distress’) and Sarsat is ‘Search and Rescue Satellite Aided Tracking’.

Fig. 2.9  The CospasSarsat logo. (Reprinted with permission. © 2024, Cospas-Sarsat. All rights reserved)

System Design

21

Fig. 2.10  A map showing the current Cospas-Sarsat participating nations. (Reprinted with permission from https://www.sarsat.noaa.gov/wp-­content/uploads/cospas-­sarsat-­countries-­1200px-­ spg.jpg. © 2024, Cospas-Sarsat. All rights reserved)

Origins Search and rescue operations are inherently dangerous, and studies show that aircraft accident survivors have a greater than 50% survival rate if they are rescued within 8 h, but this drops to less than 10% after 2 days. Search and rescue is very costly and dangerous, and is often conducted in bad weather and inaccessible terrain. Satellites can and do offer an excellent way to improve this process. The Cospas-Sarsat system originated in the October 16, 1972 air crash in Alaska of two powerful members of the U.S. Congress, House Majority Leader Hale Boggs and Alaska congressman Nick Begich. A massive search program was initiated, with thousands of soldiers, aircraft and even military reconnaissance satellites used in the search, but no trace of them or their aircraft has ever been found. The U.S. Congress requested that NASA investigate the use of satellites for search and rescue, and eventually mandated all U.S. commercial aircraft to use satellite distress beacons. The Cospas-Sarsat program was created as an international cooperative program between France, Canada, and the Soviet Union at the height of the cold war.

System Design The Cospas-Sarsat system is very similar in design to the Argos system, which was developed around the same time. Both were designed as small parasite payloads to be mounted upon NOAA polar orbiting environmental satellites. The Cospas-Sarsat

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Fig. 2.11  The Cospas-Sarsat system. A signal is sent to a satellite from a crashed plane or sinking ship, and the signal is forwarded to an automated ground station, and from there to a mission control center and rescue coordination center, which manages the rescue. (Reprinted with permission from https://www.cospas-­sarsat.int/en/search-­and-­rescue/system-­graphics-­en. © 2024, Cospas-­ Sarsat. All rights reserved)

program added similar payloads onto two Russian satellites for additional coverage, as shown above in Fig. 2.11. The space segment originally consisted of satellite payloads on 2 U.S. NOAA and 2 Russian Nadejda (meaning Hope in English) polar orbiting environmental satellites that received the distress signal on 243 MHz (military) and 121.5 MHz (civilian) from a distress beacon. The satellite then passes the message along to one of several automated ground receiving stations (local user terminals) around the world on 1,433.3 MHz, as shown below in Fig. 2.12. The message is then transmitted to the appropriate national search and rescue coordination center, which manages the search and rescue operation. All rescue operations are conducted by national search and rescue entities. Search aircraft can then home in on the distress beacon broadcast. The current LEO constellation includes five (3 NOAA and 2 EUMETSAT) satellites in sun synchronous orbits at 850 km altitude and an orbital period of 100 min. New generation MEO PNT satellites and several GEO satellites are also used, as presented below. The original 243 MHz (military) and 121.5 MHz (civilian) frequencies have been updated to a single, dedicated 406 MHz frequency in 2009. The LEO satellites can receive alerts in a 6000 km wide area, and the satellites are in view for ~15 min from the ground on each pass. Any point on the Earth will be within view of one of the satellites within a maximum of 2 h. The ground segment consists of a network of automated Local User Terminals (LUTs) around the world, some of which work with the LEO satellites (LEOLUTs) and some work with the new Geostationary satellites discussed below (GEOLUTs).

System Design

23

Fig. 2.12  The global network of Cospas-Sarsat ground stations and their coverage. Areas in gray have messages stored on the satellite until they come into range of a ground station. (Reprinted from https://www.sarsat.noaa.gov/search-­and-­rescue-­satellites/. © 2024, NOAA)

These take the message from the satellite and pass it along to the appropriate national Mission Control Centers (MCCs) for action and to direct the search effort. There are currently 31 Mission Control Centers around the world, 58 LEO automated ground stations and 22 GEO automated ground stations, and a total of 6 LEO, 6 MEO, and 9 GEO satellites provided by the U.S. (GOES), Russia (Electro-L and Louch), Eumetsat (MSG), and India (INSAT). These are all paid for and operated by individual national governments. The user segment consists of portable and low-cost distress beacons for aviation, marine and, recently, individual use. There are over 1.2 million distress beacons in use worldwide. EPIRBs (Emergency Position Indicating Radio Beacons) are marine systems and are widely used on commercial and pleasure vessels. They can be triggered manually or be automatically activated when exposed to water, and can automatically detach, activate, and float if a vessel sinks. The message can be pre-programmed with the name and owner of the vessel, number of people on board, cargo, etc. Specialized Ship Security Alert Devices (SSAS) are used to report incidents of marine piracy. Aviation ELTs (Emergency Locator Transmitters) are located in all commercial and most private aircraft, and can be manually activated, or are automatically triggered if a sufficient G-force is applied in a crash. These originally broadcast on the international aviation distress frequency of 121.5 MHz, in addition to 406 MHz, but the 121.5 frequency has been phased out in 2009. One of the many mysteries of the Malaysian Airline flight 370 disappearance in 2014 was why the ELTs were not activated and failed to alert the Cospas-Sarsat system. Recently, Personal Locator Beacons (PLBs) have been offered to back packers and others, originally in a pilot project in Alaska. This brings satellite search and rescue to individuals, campers and explorers. The various transmitters are shown below in Fig. 2.13.

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2  Doppler Satellite Positioning, Telemetry and Data Systems

Fig. 2.13  Cospas-Sarsat marine, aviation, personal and safety at sea beacons. (Reprinted with permission from https://www.cospas-­sarsat.int/en/search-­and-­rescue/image-­gallery. © 2024, Cospas-Sarsat. All rights reserved)

The first satellite launch was in 1982, and the system was operational in 1984 using two U.S. NOAA polar orbiting satellites and two Soviet Nadejda (Hope) satellites. Using Doppler techniques, it provided apx. 1  km positioning, which was sufficient for global search and rescue use. Worldwide, over 60,000 people have been rescued in over 18,000 individual searches over the lifetime of the program, as shown below in Figs. 2.14 and 2.15 below. In 2022 3,223 people were rescued in 1144 events. Figure 2.15 shows the number and type of Cospas-Sarsat operations from 1982 to 2021. Some 48% were maritime, 22% were aviation, and 30% were on land. Many current generation systems also include GPS receivers, thus significantly improving the location provided from 1 km to less than 20 m. Recently, the system has been expanded to include receivers in Geostationary orbit, as it was determined that existing distress beacon messages could be received at that distance. If the beacon is equipped with a GPS, and most modern ones are, the GEOSAR constellation provides near instant alerts using USA GOES, Indian INSAT and EUMETSAT MSG satellites. Because the GEO satellites are not moving relative to the ground, they cannot use Doppler tracking, and only work with GPS-equipped beacons, but this provides a significant new capability, as shown below in Fig. 2.16. The next generation of capability is now being provided by the addition of a MEOSAR system, with Cospas-Sarsat receivers being included on all

System Design

25

Fig. 2.14  Cospas-Sarsat rescues in 2008, where some 1981 people were saved in 502 individual operations. (Reprinted with permission from http://www.cospas-­sarsat.int/images/content/articles/ Cospas-­Sarsat-­Report_ReducedSize_Jan-­2019.pdf. © 2024, Cospas-Sarsat. All rights reserved)

Fig. 2.15.  Number and types of SAR events between 1982 and 2021. (Reprinted with permission from https://www.cospas-­sarsat.int/images/content/articles/Quick_Stat_SAR.pdf. © 2024, Cospas-Sarsat. All rights reserved)

next-generation US GPS-3, Russian GLONASS, and ESA Galileo satellites. China has recently joined in 2022 with six of its BeiDou constellation satellites. The U.S. component is called DASS, the Distress Alerting Satellite System, designed by NASA, as shown below in Fig. 2.17. These four MEOSAR constellations with (as of this writing) 48 satellites with COSPAS-Sarsat capability, provide near-immediate GPS accuracy using GPS-­ enhanced beacons, but will also provide improved location for traditional Doppler-­ only systems as well, as shown below in Fig. 2.17.

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2  Doppler Satellite Positioning, Telemetry and Data Systems

Fig. 2.16  Showing the overlapping GEOSAR coverage provided by U.S., Indian and EUMETSAT geostationary satellites. (Reprinted from https://www.sarsat.noaa.gov/search-­and-­rescue-­ satellites/. © 2024, NOAA)

Fig. 2.17  The integrated Cospas-SarSAR system. (Reprinted from https://www.sarsat.noaa.gov/ search-­and-­rescue-­satellites/. © 2024, NOAA)

These three overlapping systems, LEOSAR, MEOSAR and GEOSAR, are a single, complimentary system, and each provides enhanced capabilities and redundancy. For example, the LEOSAR satellites provide superior detection near the poles where GEOSAR signals are weak and GEOSAR systems provide near instant response for GPS-equipped transmitters. Argos and Cospas-Sarsat are excellent examples of the powerful benefits of space technology for practical applications and are excellent examples of international cooperation in space for the benefit of humankind.

System Design

27

While these Doppler based systems, all dating back to the 1970’s, have excellent capability, and have been significantly updated, there are also significant limitations to the Doppler approach. Receivers need to maintain a track on a satellite for several minutes, and the coverage is not equal globally. Doppler also does not allow for elevation differences, and thus are not useful for aviation or space users. These limitations led to the development of the next generation of space positioning systems, the U.S. NAVSTAR global positioning system, or GPS.

Chapter 3

Positioning, Navigation and Timing (PNT) and Current National PNT Systems The US NAVSTAR GPS System

GPS The US NAVSTAR GPS system was the first and is still by far the most used satellite navigation system. It is made up of three segments: the space segment (a constellation of satellites in orbit) the control segment (ground stations and control centers that operate and oversee the system) and the user segment (billions of smart phones and hand-held GPS receivers). A brief timeline of the GPS systems shows that development began in 1973 (to replace TRANSIT), with the first satellite launch in 1978 and full, global operational capability achieved in 1993. This global capability continues today and is assured, free of user cost, into the foreseeable future. The intention of the NAVSTAR design was to provide a major improvement over the TRANSIT system in all areas; by providing a global system that would put all of the expensive and large equipment either in the space or control segments, allowing an unlimited number of passive, small and portable receivers that could be used 24/7, anywhere on the Earth and up to Earth orbit, and in all weather conditions. It is a passive user system, and the user cannot transmit, thus allowing an unlimited number of concurrent users. The system was initially designed to be a primarily military capability with a very degraded civilian component, but this all changed in 1983, when President Ronald Reagan, in response to the Soviet shooting down of Korean Airlines flight 007 on September 1, 1983, ordered free civilian access to what had originally been designed as a military capability. In 1996 US policy established joint civil and military management of the system, and in 1997 the US Congress passed a law requiring that the civilian GPS signal shall be provided globally without cost or user fees. President Bill Clinton turned off Selective Availability (SA), which significantly degraded the civilian system, in 2000, and in 2007 it was announced that SA capability would no longer be built into the next generation of GPS III satellites. A major upgrade of both the satellites and ground infrastructure was started in 2003, which © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_3

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3  Positioning, Navigation and Timing (PNT) and Current National PNT Systems

Fig. 3.1  A GPS Block llR(M) satellite in orbit 20,200 km above the Earth. It is actually much farther away from the Earth than depicted here. (Reprinted from https://oceanservice.noaa.gov/ facts/gps.html. © 2024, NOAA)

significantly improved the operation, precision, and management of the overall system. There are three segments that make up the GPS (and all other PNT systems): the space segment, the control segment and the user segment. One of the satellites is above in Fig. 3.1.

The Space Segment The GPS space segment consists of a minimum of 24 satellites in six, equally spaced orbital planes inclined 55 degrees to the equator, at an altitude of 20,200  km (12,550 miles), so that each satellite orbits the Earth exactly two times per day, a semi-synchronous orbit. We refer to this as the GPS “birdcage”, as shown below in Fig. 3.2. There are a total of 4–6 satellites in each plane, and the systems is designed so that a minimum of 4 satellites will be in view anywhere on the Earth (normally there are far more, but it takes 4 for the system to compute your position, as described below). This minimum number of satellites in view is basic to how the system works. The constellation requires a minimum of 24 satellites for global coverage, but often has up to 32. The satellites are constantly rising and setting above you wherever you are, in an ever-changing pattern and geometry, unlike geostationary satellites.

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Fig. 3.2  The GPS Constellation of satellites. (Reprinted with permission from https://geoxc-­apps. bd.esri.com/space/ satellite-­explorer/#. © 2024 Esri and its data contributors. All rights reserved)

Satellites are launched one at a time from Cape Canaveral, Florida, and Atlas, Delta (previously) and SpaceX Falcon rockets (currently) have been used. The satellites are bid for and built in batches, due to the constant need to replenish the constellation, and have been constructed by Rockwell International, Boeing and Lockheed-Martin. The system is managed for the US government by the U.S. Space Force (previously the Air Force) and has been operational since 1978, with constant, global coverage since 1994. The satellites are in these very high (20, 200 km or 12,550 miles) orbits for several reasons, including a larger viewing area on the ground (we need 4 in view, to compute our position), more precise orbits (due to the distance from the Earth and lack of its gravitational variations), and survivability from attack (as this is a military-­use system). These are all important factors in terms of how the system was designed and operates and for providing the required level of precision. Each satellite also contains a sensitive nuclear burst detector (NUDET) system, so the constellation also provides instantaneous reporting of any nuclear detonation on the Earth, at sea, or in the atmosphere.

The Control Segment The GPS control segment includes the master control station at Schriever Space Force Base in Colorado Springs, Colorado, an alternate control station at Vandenburg Space Force Base in California, 12 command and control sites that are used to communicate with the satellites, and 16 remote monitoring stations located around the world, as shown below in Fig.  3.3. This gives the US.  Space Force 2nd Space Operations Squadron, which operates the system for the U.S. government, daily

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Fig. 3.3  The GPS control segment. (Reprinted from https://www.gps.gov/multimedia/images/ GPS-­control-­segment-­map.pdf. © 2024, GPS/NOAA)

operational control over the space segment. The automated monitoring stations track the satellites as they pass overhead and pass the data back to the master control station. Four control stations, located in Kwajalein Atoll, Ascension Island, Diego Garcia Island, and Cape Canaveral, are used to communicate with the satellites, to update atomic clock synchronization, navigation, and other data to them. These four sites are equally spaced around the equator and are located in highly secure military facilities.

The User Segment The user segment is your smart phone or dedicated GPS receiver, as shown below in Fig.  3.8. There are many types of dedicated GPS receivers available, ranging from GPS-on-a-chip, circuit boards, smart phones, inexpensive hand-held units, and GPS watches, to dedicated car navigation systems with moving map displays, to high-end aviation and marine systems that are linked to inertial navigation systems, to high end surveying systems with sub-centimeter accuracy costing tens of thousands of dollars. The design of the system allows for an inexpensive and small receiver, because all of the expensive and complex parts (such as the atomic clocks) are in either the space or control segments. The original GPS receiver was the AN/PSN-8 military system, as shown below in Fig.  3.4. It was a one-channel backpack receiver that weighed some 8  kg (17 pounds). Some 1400 were manufactured between 1988 and 1993 for a cost of

The User Segment

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Fig. 3.4  The AN/PSN-8 military GPS receiver, the first system. (Reprinted with permission from https://timeandnavigation. si.edu/multimedia-­asset/ psn-­8-­manpack-­gps-­ receiver. © 2012, Jaclyn Nash/National Museum of American History, Smithsonian Institution. All rights reserved)

US$45,000 each. The original requirement was a total of 700 for all of the US Army. Compare this initial requirement with the fact that over 720,000 commercial GPS units were sold in 2022 alone, demonstrating the tremendous commercial utility of GPS beyond its original military roots. There are over 7.5 billion GPS receivers in the world as of 2024 and growing every month. The first commercial GPS receiver was the Magellan Nav 1000, first made available in 1988 for just under US$3,000, as shown below in Fig. 3.5. It was a single channel receiver, could store 100 waypoints, and, since the constellation was not completed yet, had a feature that would allow it to sleep and wake up when the constellation had sufficient satellites in view to be used. The current U.S. military hand-held mobile GPS receiver is the AN/PSN-13 Defense Advanced GPS Receiver, called the DAGR (pronounced dagger), as shown below in Fig. 3.6. This is a dual frequency military receiver that can automatically decode the military encrypted P-code GPS signal that is not available to civilians. It also has anti-jamming and anti-spoofing features. Over 125,000 of these have been ordered for the U.S. military and allies, a purchase worth over US$300 million. These contain sensitive military encryption components, and it is illegal for civilians to purchase or be in possession of one of these military-grade receivers. There are also multiple military aviation, marine and space systems, many of which are integrated with Inertial Navigation Systems. These include rack-mounted units, circuit boards, and individual computer chips. In 2023 it was announced that this would be replaced by a BAE Systems NavGuide Assured PNT device which has a color screen.

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Fig. 3.5  The Magellan NAV 1000 GPS Receiver, 1988. (Reprinted with permission from https:// timeandnavigation.si.edu/ multimedia-­asset/ magellan-­nav-­1000-­gps-­ receiver-­1988. © 2012, Jaclyn Nash/National Museum of American History, Smithsonian Institution. All rights reserved)

GPS can also be very useful for civilian field data collection, as shown below in Fig. 3.7. There are many types and designs, including wristwatches, hand-held, flat panel, and even GPS-on-a-chip. The IkeGPS, as one example, is an innovative, integrated GPS field data collection system that consists of a single, hand-held, waterproof and ruggedized device with a sub-meter GPS, 5-megapixel digital camera, digital compass, laser range finder, and mobile computer. This system can be used for very rapid and accurate data collection and disaster damage assessment in the field. The laser range finder allows the user to remotely collect position data of features that cannot be safely accessed, up to 1000 m away (Ikegps.com). As we now have several PNT constellations, manufacturers are developing multi-Constellation receivers. Trimble, a major manufacturer of PNT receivers, has in 2014 announced a new model, the BD-930, that has a multiple-frequency GPS/ Glonass/BeiDou/Galileo receiver with a built-in UHF module to allow high-speed data transfer. This will provide centimetric real time kinematic positioning and was available as of July 2014. Your iPhone (in the U.S.) is also a dual GPS and Glonass receiver. Other systems sold around the world have different combinations of systems.

The User Segment

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Fig. 3.6  The AN/PSN-13 Defense Advanced GPS Receiver (DAGR) used by the U.S. military. (Reprinted with permission from https:// timeandnavigation.si.edu/ multimedia-­asset/ defense-­advanced-­gps-­ receiver-­dagr. © 2012, Jaclyn Nash/National Museum of American History, Smithsonian Institution. All rights reserved)

Fig. 3.7  A variety of GPS receivers from Garmin. (Reprinted with permission from https://www. garmin.com/en-­US/aboutgps/. © 2024, Garmin. All rights reserved)

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How GPS Works The GPS system is actually a very precise timing system that allows us to determine our position and to navigate. Each satellite has multiple atomic clocks and constantly broadcasts several coded messages on coded frequencies that are received by your GPS unit. GPS is all about time. Atomic clocks are the most precise means of measuring time yet devised. A cesium atomic clock, as used in GPS satellites, operates by exposing cesium atoms to microwave energy until they vibrate, and then measuring the frequency of these vibrations as a measurement of time. Modern cesium clocks measure this vibration to an extraordinary standard, to 2 or 3 parts in 1014. Another way to consider this is that this is an accuracy of 2 nanoseconds per day, or two billionths of a second per 24 h. The second itself was defined by the cesium atomic clock method in 1967, and it was in 1971 that International Atomic Time, or TAI (the French version) was defined. This is the basis of Coordinated Universal Time, or UTC, which is the basis for all terrestrial time today. In order to maintain the most accurate time possible, TAI is actually a running average of over 200 atomic clocks in multiple facilities around the globe, because minute variations in gravity affects this level of measurement. The actual measurement of time and its terrestrial and astronomical applications is very complex, but this should allow a basic understanding of how GPS time is computed. One interesting aspect of how precise these systems are is that, because of the relativity of time with regard to velocity and gravity, as discovered by Einstein, the atomic clocks aboard the satellites have to be set a tiny bit slower when built, to account for their orbital velocity and altered gravity in orbit relative to that of users on the Earth. In order to produce the correct fundamental frequency of 10.23 MHz in space, their frequencies are set to 10.22999999543 MHz before they are launched. GPS is all about time.

Code-Based GPS The civilian, or Coarse Acquisition (C/A code) and military GPS code (Precision or P code) work in related, but slightly different ways that are referred to as code-based GPS. The way the civilian system works is as follows. The civilian system, or C/A code, broadcasts a repeating stream of microwave signals consisting of 1023 bits, called the pseudorandom binary code. It is transmitted at 1.023 Mb per second and is repeated every millisecond. Each satellite broadcasts a unique code, and the receiver uses the Code Division Multiple Access (CDMA) telecommunications processing protocol to recognize multiple satellites on the same frequency. Each satellite also broadcasts a navigation message that contains the satellite ephemeris data, which allows the receiver to calculate the precise position of the satellites in orbit, as well as data about the health and status of the satellite, which is referred to as the almanac. These navigation messages are modulated on top of the C/A and P(Y)

Satellite Ranging

37

codes at 50 bits/second. The data stream is broadcast at very low power, barely above the random background noise level. This is referred to as the pseudorandom code that is received by your GPS device. Civilian GPS receivers have limitations set for speed and altitude, to prevent their use for military purposes. Any civilian GPS traveling over 1000 knots or over 65,000 feet altitude will cease operating. The GPS military P-code is also a pseudorandom code, but it is much longer, on the order of 720 Gb, and only repeats each week. It is transmitted at a higher rate of 10.23 Mb/sec. The P-code is actually a short segment of a master code, of some 26.7  Tb length, so each satellite repeatedly broadcasts its segment of this longer master code. In order to prevent unauthorized use or interference with the P-code, it is encrypted, and is modulated with a W-code to generate a classified Y-code, which is what is actually transmitted. This is referred to as the P(Y) code, which can be processed only by military GPS receivers. It provides higher precision location and additional security. The navigation message has three individual parts: the GPS date, time and satellite status is the first, the second part is the satellite ephemeris data, and the third is the almanac with information on the health of each satellite. The ephemeris data are updated and valid for only 4 h, while the almanac data are good for up to 180 days, but this is actually updated daily. The almanac data helps the receiver to decide which four satellites to search for and use, as their geometry affects the precision of the location fix. Once a satellite signal is received, it then downloads the ephemeris data for that individual satellite. The ephemeris data is required before the receiver can use that satellite for navigation. The almanac data includes status and orbit data for that satellite, ionospheric information (a major source of GPS error), and information about GPS time. The almanac is useful when you power on your receiver, so that you can know which satellites will be visible. With older, single channel receivers, the almanac data allowed for some correction of ionospheric delay. Satellites update their ephemeris every 2 h through the control segment, and the ephemeris is valid for up to 4 h at a time. Inexpensive GPS receivers can take up to 36 seconds to download these housekeeping data, due to the very low data rate. When you turn your GPS on after moving a long distance, it will take some time for the system to ‘reinitialize’ using this procedure before it can begin operating. A-GPS, or Augmented GPS uses a cellular or web network to more quickly go through this process, primarily for smart phones, and is discussed below. These data are then modulated onto the GPS carrier frequency for broadcast down to the Earth. Two original frequencies, 1575.42  MHz (the L1 frequency) and1227.60 MHz (L2) were used. The C/A code is transmitted on the L1 frequency, while the P(Y) code is broadcast on both L1 and L2.\

Satellite Ranging Your civilian GPS receiver also has a firmware copy of the same 1023 PRN code, and software in your GPS receiver slides its copy of the signal sequence to match the signal received from a GPS satellite, as each satellite signal is received. It

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processes the difference in the timing of the signals to determine your local position and time by comparing this signal being broadcast by each satellite and the same code on the receiver. To do this, we need to know when the signal left the satellite and what time it is at your receiver with a very high precision. This technique is referred to as satellite ranging, or determining your distance from a satellite, by measuring the time lag between when the coded signal left the satellite and when it arrived at your receiver. To do this calculation in both our satellites and our receiver would be simple if both had atomic clocks, but the cost of atomic clocks precludes their use in our inexpensive receivers (~US$3,000–$100,000 per clock). Thus, the GPS system uses a very clever trick to ‘set’ our inexpensive quartz clock in our receiver to the correct atomic time. This is one of the brilliant innovations that makes GPS work. The receiver listens to the signal from four or more satellites, and it can, using software, measure its own clock error by shifting through all the codes. There is only one correct clock time that will cause the satellite times to align correctly, and the receiver then resets its clock to that time, getting an accurate ‘atomic time’ without the need of the expensive atomic clock, just by constantly resetting its inexpensive quarts clock. This is done on a continuous basis. Once this is done, it has a sufficiently precise approximation of GPS time, and then can compute your correct position as if it had an atomic clock in your hand. Very clever. GPS has nothing to do with angles or triangulation like celestial navigation. The system works with satellite ranging or measuring our distance from each GPS satellite. If we turn on our GPS receiver and get a signal from one satellite, we can slide through the code and determine the lag between when a signal left that satellite and when it arrived at our receiver. This is your range from the satellite, and it allows your GPS to compute that you are somewhere on a sphere that is a given distance from that satellite, as shown below in Fig. 3.8, all by measuring the time delay from

Fig. 3.8  How GPS trilateration works. (Reprinted with permission from https://gisgeography. com/trilateration-­triangulation-­gps/. © 2024, GIS Geography. All rights reserved)

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when the PRN signal departed the satellite and arrived at your receiver. The time for a GPS signal to reach a receiver on the ground is ~200 milliseconds. The delay between when a signal left the satellite and when it arrives at your receiver is something between 65 and 85 milliseconds. Thus, the need for extremely accurate atomic clocks to make the system work. With one measurement from one GPS satellite, we know that we are somewhere on a sphere that is the given time delay distance from that satellite, but we have no idea where. With two satellite signals, we know we are somewhere on two spheres, and the intersection of these two spheres is somewhere on the surface of an ellipse, as shown below in Fig. 3.9. Therefore, with two measurements, we are somewhere on an ellipse. A third signal reduces our position to one of two points, which is the intersection of the three spheres. Often, one of these two points can be eliminated using software because it is deep under the ocean or up in space, or is moving at a very fast speed, and our position can often be determined with only three satellites using software in our receiver. But the system is designed to always have a minimum of four satellites in view, and this provides us with our final position solution. There is only one place in the universe that is at the intersection of these four spheres, and that is your position. You also have your precise time information as well. This allows us to use GPS to navigate anywhere on the Earth, at sea, in the air, and up to and beyond Low Earth Orbit (LEO). If we know the distance between the satellites and the receiver, and we know the speed of light in a vacuum, we can determine our precise location. The speed of light through a vacuum is 299,792,458 m per second (~186,000 miles per second).

Fig. 3.9  Showing GPS precision before and after the ending of selective availability. (Reprinted from https://www.gps.gov/systems/gps/modernization/sa/data/. © 2024, NOAA)

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This means that light moves ~30 cm in one single nanosecond. Of course, we know the speed of light in a vacuum, but the signal must propagate through the Earth’s atmosphere, including the ionosphere, and these slow the signal and cause positioning error on the ground. These, in reality, are not perfect, intersecting spheres, and there are several sources of error, mentioned in detail below, that cause uncertainty and lack of precision in determining our true location. A one hundredth of a second error would give us a 3000 km error on the ground, and even just an error of one nanosecond gives us a 30 cm positioning error, thus the requirement for multiple, very precise atomic clocks in the system. The GPS system is a marvel of modern technology and has revolutionized our ability to routinely measure time and our location anywhere on and beyond our planet. The U.S. has spent over US$25 billion on the GPS system since 1973, including the construction and launch of over 120 satellites, over 120 launch vehicles, 162,000 GPS receivers for U.S. military and other government and authorized users, and other development and other costs. This does not include the cost of operation of the system, including staffing and payroll by the US Space Force, which is accounted for separately. This averages out to ~US$750 million per year (data from Dec. ‘94 Selected Acquisition Report -GPS JPO) to manage and operate the GPS system. A tremendous amount of cost for a public benefit that is provided freely without cost to the world. This free access, and the power of knowing exactly where we are and what time it is, is what has powered the enormous growth in the use of GPS technology. We must also remember that military uses, for targeting weapons and navigating military forces, is what has allowed these vast investments to be made.

GPS Precision and Accuracy The GPS system’s Standard Positioning System (SPS) is the civilian signal, and the accuracy of the system is determined by several factors. We refer to this as the GPS error budget. This consists of several variables, including the accuracy of the atomic clocks, ephemeris error (how certain we are of the exact position of the satellite), atmospheric errors, relativistic timing adjustments, and also receiver error (how the receiver software computes the location, software rounding errors, etc.). The atmosphere, both the ionosphere and troposphere, can bend GPS signals, making the satellites appear farther away than they are, thus causing error. This is the largest natural source of error. Knowing the precise location of the satellites is important, and the broadcast ephemeris data may not always accurately reflect their true position at a given time. Multi-path interference is caused in urban areas, where the GPS signals can be reflected or bounce off of large buildings, again creating signal delays and errors in the position. Selective Availability (SA) was the early use of intentional time and ephemeris data errors, in order to reduce the precision of the civilian GPS signals. This was the largest source of error but has now been discontinued. There are others, orbital error, receiver and software, and more. If we add all of these up, we can see the total GPS civilian code error budget, as shown below.

GPS Precision and Accuracy

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GPS Error Budget Selective availability (no longer used) Ionospheric errors Ephemeris errors Satellite clock errors Multipath errors Tropospheric errors Signal measurement noise errors Total errors

~50–100 m ~5 m ~2.5 m ~1.5 m ~0.5 m ~0.5 m ~0.25 m ~10 m

It should be noted that nearly all of these sources of error can be largely removed using Differential GPS techniques (DGPS) and other techniques, which are discussed below. The removal of Selective availability, as shown below in Fig. 3.10, made a radical improvement. The geometry of the satellites used in computing our position is also very important. The effect of the geometry of satellites is referred to as the Dilution of Precision or DOP, which originated with the LORAN system. Your receiver constantly computes the DOP, which is composed of several

Fig. 3.10  GPS Standard Positioning Service Performance, as measured by the FAA. (Reprinted with permission from https://www.nstb.tc.faa.gov/reports/2020_Q4_SPS_PAN_v2.0.pdf#page=38. © 2024, FAA. All rights reserved)

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components, including the horizontal (HDOP), Vertical (VDOP), Position (PDOP), Time (TDOP) and Geometric (GDOP) components. In short, you want your satellites to be evenly spaced across the sky for the best DOP, and if you are in a canyon, for example, and your 4 satellites are all bunched together in the same area of the sky, you will have a poor DOP and this reduces the precision of your computed location. Your DOP is a total measure, and ranges from 1 up. Generally, a DOP of 1–2 is excellent, 3–4 is good, 5–7 is fair and above 8 is considered to be poor. A DOP of greater than 20 can mean a position error of over 300 m. But it depends on your application and many users disregard the DOP entirely, while surveyors and geodesists pay very close attention. The computed error budget must be multiplied by your DOP to determine the actual precision of your position at a given time and place. In the early days, DOPs varied greatly, but today, with so many satellites and improvements in all aspects of the system, very low DOPs are the norm.

GPS upgrades The US GPS system provides excellent capabilities, but is still essentially an early 1970’s design, so a major upgrade was undertaken to provide significant improvements into the future. In 2014, Lockheed Martin won the contract for the new Block III satellites that have a second civilian code, improved anti-jamming capabilities, a stronger signal, more power, a new military code, and a completely upgraded ground control segment. The second civilian signal “L2C” is designed to meet future commercial GPS user needs and is located at 1227 MHz. This allows automatic ionospheric correction between it and the L1 signal, which will significantly improve positioning accuracy as the state of the ionosphere is the largest source of error in our GPS error budget. It also broadcasts with higher power, improving reception under tree canopies and in restricted areas, even somewhat indoors. The U.S.  Commerce department estimates this alone could generate an additional US$5.8 billion in economic benefit through 2030. A third civil signal, L5 is designed to meet vital safety-of-life transportation and related needs. It is broadcast in a band reserved for aviation safety services, and features higher power, greater bandwidth, and advanced signal structure. Users can use L5 along with L1 to improve aviation, rail, water and highway safety and signal access. Recent research shows that using a new technique called ‘trilaning’, the use of the three civil signals together, may provide real-time sub-meter precision without the use of external augmentation systems. This new system is referred to as GPS III, and the plan is to acquire at least 32 new Block IIIA satellites. The satellites will contain laser reflectors for improved orbital determination, already a feature with Glonass, and a feature being included in Galileo. Currently, only 17 modern satellites are in service, with full coverage by ~2027. A new military GPS code, called the M-code, has been available since 2016. The new M-code is completely autonomous, where the previous P-code required

Current Capabilities

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acquisition of the C/A code first. New Block IIR-M satellites also have a spot beam that can saturate a given area several hundred km wide with a much stronger signal to overcome jamming and spoofing for national security needs. In 2008 the U.S. Air Force awarded a contract to Raytheon to create a Next Generation Operational Control System (OCX) for GPS. This updates and adds new capabilities to the control segment, including controlling the new satellite signals and other features. This program has seen significant delays, and only came online in 2022, a decade after its planned operation, but is now operational.

Current Capabilities There has been significant and continuing improvement in the Standard Positioning Service since 2001, which currently provides 95% 1.82 m global horizontal error, 95% 4.52  m global vertical error, and 95% 12.70 Nanoseconds time error, an extraordinary level of capability. These are shown in Fig.  3.10 above. This was ­measured between 1 October and 31 December of 2020 at 27 sites from across the U.S., including Hawaii, Alaska and Puerto Rico by the U.S.  Federal Aviation Administration. Your position and navigation using GPS is determined using a standard reference model of the Earth called The World Geodetic System, version 1984 (WGS84). This is a standardized, global coordinate system used in geodesy, mapping, GPS, and navigation. It includes a global coordinate system, a standard spheroid reference surface (the datum or reference ellipsoid), and a gravitational surface (the geoid) that defines sea level. The current version is called version 1984, but it was actually last updated on 7 January 2024 to use reference frame G2296, which is aligned with the IGb14 International Geodetic Reference Frame of 2014. This a global datum, where the origin is the center of the Earth’s mass, computed to less than 2 cm precision. The datum surface is an oblate spheroid, and this unified, global datum replaces numerous regional and national datums that were mutually incompatible. The WGS84 datum is accurate worldwide and is globally consistent to within + − one meter. Your GPS receiver actually works in WGS84, but can, using software, display your position in a variety of coordinate systems such as Universal Transverse Mercator (UTM), and others, depending on your needs and the maps you are using, but it works in WGS84. The U.S. government has committed to operating the GPS system for the foreseeable future and broadcasting the Civilian code in the clear without cost. One possible major change will be to eventually adopt the three orbital plane configuration of Glonass and Galileo instead of six, allowing multiple satellites to be launched at a time, reducing launch costs and speeding replenishment if needed.

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DASS The next generation GPS system also includes the Distress Alerting Satellite System (DASS) to provide improved and redundant Cospas-Sarsat capability to all MEO navigation satellites, as mentioned above. The United States today operates an integrated national PNT architecture. This includes all aspects of the U.S. government, systems, and applications representing the complex and interrelated nature of positioning, timing, and navigation services at a national level.

Russian GLONASS The Russian Glonass system, GLObal NAvigation Satellite System or “GLObalnaya NAvigatsionnaya Sputnikovaya Sistema”) was begun in 1976, with the first launch in 1982. It was declared operational in 1990 but did not have a complete constellation until 1996, as shown below in Fig. 3.11. Today, it is operated by the Ministry of Defense of the Russian Federation and is very similar in design and operation to the U.S. GPS system, but with some minor design differences. It is the largest project of the Russian Federal Space Agency and has consumed a large portion of its budget over recent years. The satellites are launched 3 at a time on Proton launchers into one of three orbital planes, instead of the 6 planes in GPS. The 24 satellites are in three 19,130 km orbits spaced 120 degrees apart, with a 64.8 inclination to the equator, as shown below in Fig.  3.14, The inclination is higher than GPS due to the northern location of the Russian land mass. One difference from GPS is that each

Fig. 3.11  Left: The Russian Glonass space segment with three orbital planes and a Glonass-M satellite. (Reprinted with permission from https://geoxc-­apps.bd.esri.com/space/satellite-­ explorer/# © 2024 Esri and its data contributors. All rights reserved. Right: A Russian stamp with a Glonass satellite. Reprinted from https://en.wikipedia.org/wiki/GLONASS#/media/File:Stamp-­ russia2016-­glonass.png. Public domain)

Russian GLONASS

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satellite transmits on a different frequency, where all GPS satellites broadcast on a shared frequency, and each satellite has a 17-day ground repeat pattern. Glonass transmits a C/A on L1 and P-code on L1 and L2, like GPS, but uses a Frequency Division Multiple Access (FDMA) technology, unlike GPS, which uses a Code Division Multiple Access (CDMA) technology. Another difference is the Glonass uses a Russian-developed PZ-90 global reference system, which is slightly different from the WGS84 used by GPS and the West, and Russia also has its own time reference system. Glonass is a dual use system, with free access to the Civilian code and mandatory use for several Russian critical infrastructure and military users. The Glonass system suffered with the demise of the Soviet Union, and the constellation was down to only 6 working satellites in 2000, making the system no longer operational. Russian President Putin put an emphasis on the restoration of the system, and by 2011 the system was fully restored. In 2007 all restrictions on the system were lifted, and the system was made available for civilian use, while still maintaining a separate military component. There have been two Proton launch failures, each with 3 satellites, but the constellation is currently fully maintained. In 2014 there were two, unexpected Glonass system-wide outages. On April 2, 2014 there was a system-wide failure of the Glonass system, which lasted more than 10 h. This was reportedly caused by erroneous data being uploaded to the satellite ephemeris data. The constellation was slowly restored, as controllers had to wait until each satellite came into view of a control station on Russian territory for the new data to be uploaded. The same thing occurred again on April 15, with 8 satellites going offline. There are a variety of user receivers, including ones that use both Glonass and GPS, as shown below in Fig. 3.12. The Control segment, shown below in Fig. 3.13, is entirely located within the Russian landmass, with the ground control and time reference centers located in

Fig. 3.12  A Russian military dual Glonass/GPS receiver. (Reprinted from https://commons.wikimedia.org/wiki/File:Glonass-­receiver.jpg. Public domain)

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3  Positioning, Navigation and Timing (PNT) and Current National PNT Systems

Fig. 3.13  The Russian Glonass ground segment. (Reprinted from https://commons.wikimedia. org/wiki/File:GLONASS_GroundSegment.jpg. Public domain)

Moscow. Four tracking and telemetry stations are spread across Russia, and there are several newer external Glonass control stations in Brazil, China, Indonesia, India, and Angola ­ (https://www.gpsworld.com/roscosmos-­to-­deploy-­glonassmonitoring-­stations-­in-­five-­countries/). The US Congress in 2014 denied a Russian request to place a ground station in the US, so Russia responded by ordering all PNT reference stations in Russia to no longer provide data internationally. All U.S. market Apple iPhones and many other smartphones contain dual GPS and Glonass receivers, providing increased accuracy, quicker position fixes, and better satellite geometry.

Glonass Modernization Russia has undertaken a major upgrade of the system, similar to the GPS modernization mentioned above. This includes additional frequencies and increased power. This will make the system more compatible with GPS and Galileo and provide better precision and also a MEOSAR capability. All Glonass satellites also carry nuclear explosion detection sensors and other secret military signals intelligence and other sensors (https://www.thespacereview.com/article/4502/1). In 2028 Russia plans to add six satellites in inclined GEO orbits to the constellation for improved redundancy and signal reception (https://tass.com/science/1439099). There have been four generations of satellites, the original Glonass, the Glonass-M series in 2003, the Glonass-K1 first launched in 2011, and the current Glonass-K2, first launched in 2014. Russia continues to have an operational version of the LORAN-C ground-based navigation and timing system called Chayka, or Seagull. This system operates along

China’s Compass/Biedou System 北斗卫星导航系统

47

the Russian coastal zones (https://en.wikipedia.org/wiki/CHAYKA) and provides a non-space position and time backup around the Russian coastlines.

China’s Compass/Biedou System 北斗卫星导航系统 China has developed its own, independent PNT system since 1999, and it is now the largest system today, as shown below in Fig. 3.14. They were originally invited to participate in the European Galileo system as a partner in 2004 and by 2006 there were multiple joint projects underway. This arrangement did not work out, and China was dropped from Galileo by Europe, causing significant friction. As a result, China decided to rapidly pursue their own system and, in fact, BeiDou became fully operational well before the European Galileo system. China was the third, fully-­ operations system and currently has the largest number of PNT satellites. It is managed by the China National Space Administration. The first Chinese system was called BeiDou (or the big dipper constellation, used for finding the pole star for navigating). This design was different from the GPS, in that it was a two-way, regional configuration like Argos, with four geostationary satellites interrogating the user terminals and the possibility of transmitting SMS data to user terminals. This design limited the total number of concurrent users and only operated between 2000 and 2012 and was not global but only available in the Asian region, as shown in Fig 3.15 below.

Fig. 3.14  The BeiDou orbital configuration of MEO and GEO satellites. (Reprinted with permission from https://geoxc-­apps.bd.esri.com/space/satellite-­explorer/#. © 2024 Esri and its data ­contributors. All rights reserved)

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Fig. 3.15  Chinese BeiDou system early regional satellite coverage. (Reprinted from https://en. wikipedia.org/wiki/BeiDou#/media/File:BeiDou-­coverage.png Public domain)

The second-generation Chinese system, originally called Compass, but now called BeiDou II was begun in 2012 and has achieved initial operational capability in the Chinese/Asia Pacific region in 2013. The current system is made up of 38 satellites: 5 GEO, 30 MEO and 3 in highly inclined orbits (as of 2024). These originally provided regional capability for the China region, as shown in Fig. 3.15, but in 2020 this has now been extended world-wide. This system has the largest number of satellites of any PNT constellation. One unique feature of the BeiDou design is that they have the SBAS augmentation capability (like the U.S. WAAS discussed below) built into the system as an integral part, rather than operating as a separate augmentation capability, but this capability is restricted. It is a CDMA-based system, and BeiDou MEO and Inclined GEO satellites transmit a D1 message, and the GEO component satellites transmit a separate D2 message, which also includes the augmentation service data. There are two levels of service, with a public access and restricted access. The satellites broadcast on the same four frequencies as the European Galileo system, raising the potential of signal interference and jamming. BeiDou is currently the largest system, in terms of the number of satellites. Timing is provided by hydrogen masers, and six satellites are equipped with MEOSAR capabilities. China also, like Russia, continues to operate a land-based LORAN-type navigation and timing system which extends well inland.

The European Galileo System

49

The European Galileo System Ever since the early success of the U.S.  GPS system, both commercially and in terms of a powerful new military space system, Europe has been very uncomfortable with relying on such an American capability, even though the U.S. provided European NATO allies with full access to the military P-Code. As the commercial markets grew, European nations decided that they must be a player in this new domain. Major drivers have been European sovereignty and independence, industrial policy and technology competitiveness, employment, as well as new commercial markets. The system was approached in two phases, first creating the GPS augmentation system EGNOS, discussed below, and then creating their own system comparable to the U.S. GPS, to be called Galileo. The development of the Galileo system has been protracted and difficult. Partly due to the complexity of ESA and EU processes with many internal and competing national interests, and partly due to budgetary limitations. A major launch failure in 2014 also has slowed the process. European nations first authorized some €80 million in 1999 for system definition, and some €3.4 billion to start construction, which was not sufficient to complete and launch the constellation or to build and operate the ground system. These funds were taken from existing agriculture and EU administrative budgets in very contentious budget negotiations. Ongoing operational costs are estimated at €600 million per year, and ongoing replenishment costs are not yet available, so Europe has faced a very difficult political and financial challenge. How do you compete with a U.S. GPS system that is provided free of charge and that meets most civilian and military user requirements? This has been the challenge of Galileo. The program was started originally by ESA, but has now been taken over by the EU, and the EU is now the owner of all assets and leads and funds the program, with ESA playing a technical advisory role. The original plan was that private investors and European companies were to invest up to 2/3 of the system costs in a public private partnership, but this failed to materialize as there was no commercial interest, and in 2006 the EU decided to assume full ownership and control. To date, some €10 billion has invested so far, with overall lifetime costs likely to run to over €20 billion (similar to GPS). China was originally invited to be a partner in 2003, and China was to invest over €230 million, and several other nations also joined the program, including Israel, but this fell apart, and China was un-invited, without a return of their over €2 million invested so far. This caused China to decide to pursue their own BeiDou system, which was fielded before Galileo using the same broadcast frequencies. Galileo is designed to be independent but interoperable with GPS and Glonass, and consists of 30 MEO satellites in three, evenly spaced orbital planes at 56-degree inclination, with an altitude of 23,222 km, as shown below in Fig. 3.16. Each satellite has two rubidium atomic clocks and two passive hydrogen maser atomic clocks, and the system operates in much the same way as GPS and Glonass.

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Fig. 3.16  The Galileo satellites in their three orbital planes. (Reprinted with permission from https://geoxc-­apps.bd.esri.com/space/satellite-­explorer/#. © 2024, Esri and its data contributors. All rights reserved)

The completed system now offers five levels of service: • Galileo Open Service (OS) an open access, Full, Free and Open (FFO) service. • Galileo High Accuracy Service (HAS) is a free access service complimenting the OS, providing a second navigation signal and other ‘added value’ services. This offers higher real-time service with accuracy less than 20 cm in normal operations. • Galileo Public Regulated Service (PRS) is an encrypted security and military code which is restricted to governmental users. It is similar to the GPS P-code and is designed to remain operating if other services are disabled in a crisis. • Commercial Authenticated Service (CAS) This provides users access to an authenticated Galileo PVT (position, velocity, and Time) solution. • Search and Rescue Service (SAR). This is the Galileo contribution to the Cospa-­ Sarsat a MEOSAR search and rescue capability. The first satellite was a placeholder to maintain the ITU-awarded frequency in April of 2008, named GIOVE (Galileo In-Orbit Validation Element). This was the first use in space of a passive hydrogen maser clock, which was to be the heart of the Galileo design. Four operational design satellites were launched in October of 2011 and 2012, launched from ESA’s space port in Guyana on a Soyuz launcher with a Frigat upper stage. These four initial satellites allowed the ground control system to be tested. A 2014 launch of two more satellites failed, after the Frigat upper stage left them in an incorrect orbit. The system was declared partly operational in 2016. The Galileo headquarters is located in Prague, Czech Republic, which is the home of the European PNT Supervisory Authority (GSA). There are two ground

The European Galileo System

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operations centers, located in Munich, Germany and Fucino, Italy. Galileo claims to be the most precise of all the PNT systems, with an accuracy of less than 1 m. By 2017, a total of seven atomic clocks in space had failed, raising general concerns about the system design. With the departure of the U.K. from the EU, all Galileo work there was halted, and the U.K. sought a return of the €1.4 billion already invested in the project, without success. This was a major problem as Britain had been a major contractor and participant with much expertise. There were major systemic outages in 2019 and 2020 for brief periods of time. As of 2018, all new cars and trucks sold in the EU must contain a Galileo-enabled emergency call feature. The Indian Regional Navigation Satellite System (INRSS), also called NavIC for Navigation with Indian Constellation (Navic is also the Hindi word for a sailor or navigator). Once China decided to operate their own PNT system, India’s government approved the development of an independent PNT system in 2006, with a commitment to operate a regional Asian system centered on India, and not a global system, as shown in Fig. 3.17 below. This decision was also partly caused when, in the 1999 war with Pakistan, the U.S. did not allow India full use of military P-code receivers.

Fig. 3.17  The NavIC current service area. (Reprinted with permission from https://www.isro.gov. in/SatelliteNavigationServices.html. © 2023, ISRO. All rights reserved)

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The program was entirely internally developed by ISRO, the Indian Space Research organization. INRSS was designed to consists of a 7-satellite constellation, with three GEO and four Highly Elliptical Orbit (HEO) satellites. This may be expanded in the future. The satellites have rubidium atomic clocks, and in 2019 and 2020 a total of nine of these clocks had failed in orbit, similar to the Galileo problem. India has announced their intention to develop their own atomic clocks in the future. Like GPS, it has an open civilian system and an encrypted restricted service for use by the Indian military. Five satellites had been launched by 2014, and the constellation was completed with the final two in 2016. Part of the design is that all aspects of the system will be designed and constructed in India, and the satellites will also be launched completely on Indian rockets. India also plans to create their own standardized national timing system. The GAGAN wide area augmentation system is an integrated aspect of the design, providing SBAS wide area augmentation throughout the Indian region using the same GEO satellites. Today, all commercial vehicles in India are required to have working NavIC tracking systems, and Indian smart phones have used the service since 2020. The ground segment consists of an ISRO Navigation Center, 21 ranging and monitoring sites located around India, a network timing center, several laser ranging stations for orbit determination, and two master control stations (MCCs), as shown below in Fig. 3.18.

Fig. 3.18  IRNSS system architecture. (Reprinted with permission from https://www.ursc.gov.in/ navigation/irnss.jsp. © 2023, ISRO. All rights reserved)

Japan’s Quasi-Zenith System (QZSS) or Michibiki, or ‘Showing the Way’

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India is currently planning to expand the system to 11 satellites with extended area coverage, all using Indian atomic clocks. There are also studies for expanding to a global system with a total of 24 satellites, and frequency requests have been made to the ITU for this.

J apan’s Quasi-Zenith System (QZSS) or Michibiki, or ‘Showing the Way’ Japan has developed a unique PNT augmentation system called the Japan Quasi-­ Zenith Satellite System (QZSS), or Michibiki, operational since November 2018. This system is designed to address the problems inherent in using GPS in dense urban areas such as Tokyo and other Japanese cities. In urban landscapes, large buildings reflect the incoming GPS signals, and create what is called “multi path interference’. As these reflected signals travel farther, they create errors in the position. Japan is very dependent on GPS car and truck navigation, and a solution to this problem was needed. The QZSS system consists of four satellites, one in GEO and three, equally spaced satellites in a very highly inclined geo-synchronous orbit, called a Tundra orbit, with a maximum altitude of 35,786 km and with a period of 24 h. These satellites trace out a ‘figure 8′ pattern over the ground, with one satellite always being high enough over Japan to be nearly vertical, thus allowing good signal reception in the urban canyons of Tokyo and other major cities, as shown below in Fig. 3.19. The system was started in 2002, with the first launch in 2010 and had a completed system in 2013. The system is designed to be fully interoperable with both the GPS and Galileo signals for redundancy and reliability, and it works by transmitting signals compatible with the GPS L1C/A signal that provide ranging correction for the GPS signal. It also provides system health data. QZSS also has an innovative timekeeping system that does not require atomic clocks on the satellite, reducing the cost and complexity of the satellites. JAXA ran a nickname campaign in 2009 and chose the Michibiki name for the system, meaning ‘showing the way’ out of over 11,000 entries. While QZSS is a GPS augmentation system, Japan has announced in 2023 that they are also in the planning stages of developing their own full PNT system for the Japanese island region. This would initially use seven satellites, including a geostationary satellite, to be operational by 2026, as shown below in Fig. 3.20. Recent documents have mentioned eleven satellites covering a larger regional area in the future. This would provide four-satellites-in-view at all times and would allow the determination of position and timing using the QZSS system without the US GPS or other systems, becoming a fully independent PNT system. It would also expand the area covered, as shown below in Fig. 3.21. This will also enable a sub-meter and centimeter-level augmentation capability in the future over the larger service area. An encrypted Public Regulated Service is also available to authorized users.

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Fig. 3.19  Left: The Japanese QZSS augmentation system. (Reprinted with permission from http:// global.jaxa.jp/countdown/f18/overview/michibiki_e.html. © 2003, Japan Aerospace Exploration Agency. All rights reserved. Right: The ground track of the satellites. Reprinted with permission from https://global.jaxa.jp/countdown/f18/overview/orbit_e.html. © 2003, Japan Aerospace Exploration Agency. All rights reserved)

Fig. 3.20 The proposed new seven-constellation system, providing independent capability. (Reprinted with permission from https://qzss.go.jp/en/overview/services/seven-­satellite.html. © 2024, Cabinet Office, Government of Japan. All Rights Reserved)

New Additions

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Fig. 3.21  The ground track of the satellites and coverage area for the current and expanded constellation. (Reprinted with permission from https://qzss.go.jp/en/overview/services/seven-­satellite. html. © 2024, Cabinet Office, Government of Japan. All Rights Reserved)

The Current Landscape The current situation is that we now have five operational PNT systems, the U.S. GPS III, Russian Glonass, Chinese BeiDou II, European Galileo, and Indian IRSS or NavIC systems, plus the Japanese QZSS.  Four of these are global and India’s and Japan’s are regional in coverage. The civilian components of these provide multiple interoperable signals providing precision positioning, navigation and timing worldwide. Restricted military capabilities are also provided.

New Additions The United Kingdom Space Based Positioning, Navigation and Timing Programme (UK SBPNTP). But the story is not finished. The U.K., as mentioned above in the Galileo section, has left the EU and had spent a great deal of expense and has gained significant expertise in the Galileo program that it has now been frozen out of. The U.K. government has stated that if they cannot remain in the Galileo program, that they will have to pursue their own, national PNT system for both commercial and national security requirements. From 2018 to 2020 there was a program called the United Kingdom Global Navigation Satellite System (UK GNSS), that was managed by the U.K.  Space Agency (UKSA), with the strong support of the U.K.  Ministry of Defense and government. This was then renamed the United Kingdom Space Based Positioning, Navigation and Timing Programme (UK SBPNTP), which, as of this writing, is currently ongoing. Some £90 million has been allocated to date. One

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interesting aspect of this is that the U.K. government has purchased, along with the Bharti Enterprises of India, the bankrupt OneWeb satellite company, and there is potential that this could form the basis of the UK PNT system. At this point, there has been no firm commitment of funds for such a program, which would be a significant cost to the U.K., and we shall see how this develops.

Korea Positioning System (KPS) As both China and Japan have operational satellite PNT capabilities, South Korea has recently announced that it intends to develop an indigenous, regional PNT capability for itself. In 2021 they announced the development of a US$2.3 billion Korean Positioning System (KPS), to be operational by 2034. It will be a regional system, similar to India’s, and will extend from Korea to New Zealand and the surrounding southeast Asian region. The system was presented at a bilateral meeting with U.S. President Joe Biden in 2021, where an agreement on the development of the system was announced (https://spacenews.com/south-­koreas-­gnss-­project-­to-­takeoff-­with-­3-­3-­billion-­budget/). The system will include two central stations, two satellite uplink facilities, and seven reference stations, all in South Korea, and will also include a Korean Augmentation Satellite System (KASS), similar the U.S. WAAS and other SBAS systems discussed below, in order to develop ground station networks and a GEO satellite augmentation signal for signal augmentation capabilities.

Pakistan Once India developed their own PNT system, Pakistan was placed under internal political and military pressure to do the same. But Pakistan does not have the economic or technical ability to develop, launch, and operate such a system. Therefore, Pakistan reached out to their ally (and Indian foe) China, to ask if they could participate in the BeiDou system. China, reports indicate, was cool to the idea, but has establishes one ground control station in Pakistan, and may also provide the Pakistani military limited access to the restricted BeiDou P-code capabilities in the future. Will there be others? It is difficult to say, but at this point, it does not appear that other nations are likely to pursue their own PNT systems, due to the significant technological difficulty, cost, and since so much PNT capability is provided free of cost by the existing systems. But as the cost and size of satellites and launchers continues to fall, and as smaller and smaller satellite systems are being developed, including ever smaller and less expensive atomic clocks, this cannot be discounted. But at this point, we have five operating systems, and it appears that we will likely have three additional systems soon.

Pakistan

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An excellent way to visualize all of these systems, and also to keep updated on their ever-changing status, is to use the free online PNT Planning Software from Trimble (http://www.trimble.com/GNSSPlanningOnline). This simple program allows you to pick your location, date and time, and to visualize many aspects of the GNSS satellite status in that time range, as shown below in Figs. 3.22, 3.23, 3.24, 3.25 and 3.26.

Fig. 3.22  Showing the first page of Trimble’s PNT Planning Online page, with the options to pick your location, height, date and time. I used the map option to choose my building at the University of North Carolina at Chapel Hill. Once these are set, you can choose which PNT constellation (or individual satellites) to view using the Satellite Library tab, which uses the current downloaded satellite data. (Reprinted with permission from https://www.gnssplanning.com/#/settings. © 2024, Trimble, Inc. All rights reserved)

Fig. 3.23  Showing the World View option of all satellites in view, and the relative elevation of all satellites in view. (Reprinted with permission from https://www.gnssplanning.com/#/settings. © 2024, Trimble, Inc. All rights reserved)

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Fig. 3.24  Showing the total number of satellites in each constellation in view, and the sky plot of their locations over the chosen 6-hour period. This can be set into motion. (Reprinted with permission from https://www.gnssplanning.com/#/settings. © 2024, Trimble, Inc. All rights reserved)

Fig. 3.25  Showing the Delusion of Precision (DOP) and individual satellite visibility. (Reprinted with permission from https://www.gnssplanning.com/#/settings. © 2024, Trimble, Inc. All rights reserved)

Fig. 3.26  Showing the state of the Ionosphere in Total Electron Count (TEC) and scintillation. (Reprinted with permission from https://www.gnssplanning.com/#/settings. © 2024, Trimble, Inc. All rights reserved)

Chapter 4

Aided and Augmentation Systems: Differential GPS, Pseudolites, Ground-­Based Augmentation Systems, and Space-­Based Augmentation Systems

How Differential GPS (DGPS) Works DGPS uses a fixed reference station whose position has been surveyed with high precision, as shown below in Fig. 4.1. The GPS signal’s position received is compared with the actual known position, and the difference is computed and transmitted in near real time by low-frequency radio to other GPS receivers in the area every second, thus correcting the signal. Many GPS receivers now have the ability to receive on this second frequency, but many also use a separate, dedicated receiver. The correction is best close to the DGPS reference station and provides less than 1 m precision at the reference station, and can still be used effectively up to ~400 km from the reference station, as shown below in Fig. 4.1.

Ground-Based Augmentation Systems, The Nationwide Differential GPS System (NDGPS) The U.S. government is committed to continuing to provide GPS to the civilian community for the foreseeable future, at a minimum Standard Positioning Service Performance Standard, which is a worst case of 7.8 m 95% of the time. In practice, the actual accuracy depends on many variables, including receiver quality, atmospheric and ionospheric conditions, and blockage due to buildings and terrain, but is often much better than this minimum standard. This accuracy is augmented in the U.S. by the Nationwide Differential GPS System (NDGPS), a ground-based augmentation system that also provides system integrity data. These broadcast a local correction signal around a limited area. Some 50 countries around the world operate similar systems to a common international standard for interoperability. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_4

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Fig. 4.1  How South PAN and other SBAS systems work. (Reprinted under CC-BY-4.0 license from https://www.ga.gov.au/scientific-­topics/positioning-­navigation/positioning-­australia/about-­ the-­program/southpan. © 2021, Commonwealth of Australia)

In the US, the US Coast Guard began establishing a network on differential GPS beacons along the coast and major ports, using existing but obsolete LORAN-C radio beacons in the 275 to 325 kHz band. This system was designed for coastal and port navigation and was so successful that the Department of Transportation and US Coast Guard expanded the USCG Maritime Differential GPS System (MSGPS) into a nationwide differential system called the National Differential Global Positioning System (NDGPS) Program, which has been operational since 1999. This system provided real-time broadcast of differential corrections to improve the overall reliability and accuracy of GPS signals using a network of fixed stations, called reference stations. The sites are owned by the U.S.  DOT, but are managed by the USCG. Over 92% of the lower 48 states had single coverage, and 60% have dual coverage, as shown below in Fig. 4.2. These stations also monitored the GPS satellites for errors and broadcast a warning not to use a given satellite that is not working correctly. As you can see in Fig. 4.2 above, Canada also operates a similar system. The entire NDGPS network also acquires water vapor data for NOAA and is also used for tectonic plate movement analysis and earthquake warnings, in partnership with the University Navstar consortium. This system was terminated in 2020, after 25 years of service, given the advances in SBAS coverage and their lower cost. CORS- the NOAA Continually Operating Reference Stations. There are many other governmental and private sector DGPS networks around the world. While these provide real-time correction, there are also many systems around the world that provide post-processing of GPS data days or months after the data were collected, to even higher precision. The National Geodetic Survey of NOAA coordinates a network of Continually Operating Reference Stations Stations (CORS) that provide both carrier phase and code-based measurements for post-processing of

Ground-Based Augmentation Systems, The Nationwide Differential GPS System…

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Fig. 4.2  shows the US NDGPS network. Pink areas have coverage of at least two reference signals, blue have one. (Reprinted from https://www.gps.gov/cgsic/meetings/2015/jacobsen.pdf. © 2024, GPS/NOAA)

GPS data throughout the US and its territories. A wide range of scientific, surveying and GIS users take advantage of the system at no cost. As of January 2024, there were over 2000 CORS stations, contributed by over 200 governmental, academic, and private organizations, as shown below in Fig. 4.3. The NDGPS system provides five categories of services: –– –– –– ––

1 m code based real-time data 10–15 cm real-time high accuracy data 1–3 cm post-processing of data 2–3  mm stationary scientific measurements for plate tectonics and earthquake warning –– water vapor data for weather forecasting to the National Weather Service of NOAA Future plans include a national, 3-D, dynamic, real-time service with 15 cm accuracy at a 240 km (150 mile) distance on a dynamic platform such as a boat or train, and less than 2 s of warning for satellite errors. The CORS network is accessible at http://www.ngs.noaa.gov/CORS_Map/. GPS correction data can be downloaded remotely around the clock. The NDGPS system was designed to be an international and non-proprietary standard and is free to all users.

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Fig. 4.3 CORS stations. (Reprinted from http://www.ngs.noaa.gov/CORS_Map/ © 2024, NGS/NOAA)

International DGPS Networks Over 50 nations around the world also operate similar systems, creating an international, seamless precision coastal and (partly) land navigation system, as shown below in Fig. 4.4.

International GNSS Service This is a voluntary federation of 350 worldwide agencies and organizations that pool resources and operate 512 permanent GNSS base stations and data to generate precise GNSS products. It provides, voluntarily, the highest-precision global international civilian GPS resources. https://igs.org/about/#at-­glance

Assisted PNT Assisted PNT, or simply A-PNT or A-GNSS is an augmentation system that speeds up the initial time-to-first-fix when a PNT device is first started, as shown below in Fig.  4.5. Relying on the satellite transmission of almanac and ephemeris data to

Assisted PNT

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Fig. 4.4  International DGPS systems. (Reprinted from https://www.gdgps.net © 2024, NASA JPL)

Fig. 4.5  An example of an Assisted GPS system. (Reprinted from https://commons.wikimedia. org/wiki/File:A-­GPS.svg. Public domain)

‘warm up’ and be ready to work can be a slow process, with transmission at only ~50 bits/second. So assisted PNT systems use a radio network to provide this initial information required to begin operation. This is commonly used with PNT-equipped smart phones. An internet or cell service connection provides this initial data much faster. The A-GANSS system, for example, operates throughout Europe for the Galileo constellation. (https://en.wikipedia.org/wiki/Assisted_GNSS). These are primarily used in Smartphones, where quick initialization is required, particularly in emergency calling situations.

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Space Based Augmentation Systems (SBAS), also known as WADGPS, for Wide Area Differential GPS. While ground-based correction systems using DGPS are very useful, they cannot be used in most of the world, are expensive to maintain, and are not useful for international aviation and marine requirements. For this, Space Based Augmentation Systems (SGAS) have been developed.

Wide Area Augmentation System (WAAS) (USA) The first satellite-based augmentation system was the U.S. WAAS, as shown below in Fig.  4.6. Over broader areas of North America, the satellite-based Wide Area Augmentation System (WAAS) provides continental-scale and real time augmentation. WAAS was developed to provide highly accurate navigation primarily for commercial aircraft meeting FAA standards, but the system also works well for hand-held receivers on the ground and at sea. This was developed because the standard civilian GPS does not meet FAA aviation requirements for accuracy, integrity, and availability. This system was activated in 2003 and provides differential correction over broad areas with less than 3 m precision 95% of the time.

Fig. 4.6  The U.S. WAAS system uses commercial telecommunication satellites to broadcast augmentation services for the GPS constellation. (Reprinted from https://commons.wikimedia.org/ wiki/File:FAA_WAAS_System_Overview.jpg. © 2024, FAA)

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There are 38 ground reference stations, with 20  in the continental U.S., 5  in Mexico, 4  in Canada, 7  in Alaska, one in Hawaii and one in Puerto Rico. Three master control stations determine the regional offset, which is then uplinked through six Ground Uplink Stations (GUS) and broadcast through three commercial geostationary satellites to aircraft (or receivers) throughout North America. The cost of the system is some US$50 million per year, but the current legacy ground Instrument Landing System costs over US$80 million in annual system maintenance alone. The systems is interoperable with the other regional augmentation systems presented below. The WAAS system operates in North America, as shown in Fig. 4.7 below, displaying the system in early 2024. The coverage area extends well into South America and the Atlantic and Pacific Oceans as well. It extends into the Pacific due to the station in Hawaii. There are several other compatible international space-based systems, including the European Geostationary Navigation Overlay Service (EGNOS), the Japanese Multi-functional Transport Satellite (MTSAT)-based Satellite Augmentation System (MSAS), the Russian SDCM system, and the Indian GPS And Geo-­ Augmented Navigation (GAGAN) system, as shown below in Fig. 4.8. Australia and New Zealand have the new SouthPAN system (not shown in the graphic below), and South Korea has joined the group in 2024, with initial operating capability of the Korea Augmentation Satellite System (KASS). Many inexpensive hand-held GPS receivers are WAAS enabled for increased accuracy. China provides this

Fig. 4.7  The footprint of the WAAS augmentation signal. (Reprinted from https://www.nstb.tc. faa.gov/RT_NPACoverage.htm. © 2024, FAA)

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Fig. 4.8  Showing the coverage of WAAS, EGNOS, GAGAN, MSAS, etc. Not shown are the new Korea and Australia and New Zealand systems. Note the lack of coverage of South America and Africa. (Reprinted from https://www.gps.gov/cgsic/meetings/2017/amos.pdf. © 2024, GPS/NOAA)

capability as a component of BeiDou and so does not have a separate augmentation capability. EGNOS (Europe) The European counterpart of the US WAAS system is the European Geostationary Navigation Overlay Service (EGNOS) It was jointly developed by the European Space Agency, ESA, the European Union, and EUROCONTROL, the European air traffic control system, and is operational since 2009. It is designed to augment and compliment GPS and other systems, and consists of 4 geostationary satellites, 34 ground control stations called Ranging and Integrity Monitoring Stations (RIMS), and six Navigation Land Earth Stations (NLES) that broadcast the data up to the satellites, as shown below in Fig. 4.9. Four Mission Control Centers (MCCs) manage the system in Spain, Italy, England, and Germany. The system is operated by the private company, European Satellite Services Provider. The EGNOS coverage almost meets the WAAS coverage area over the north Atlantic and extends to cover the Mediterranean and extreme northern Africa. There were initial plans to extend coverage into Africa, but this is not yet available. There is reduced ability to use the system on the ground and in urban areas, due to the high northern latitude and viewing angles to the GEO satellites. The EGNOS system offers three core capabilities. The Open Service is free and used by low-cost and mass market type receivers. The Safety of Life Service (SoL) is designed for commercial aviation users and provides enhanced and guaranteed performance and integrity capabilities for aviation, rail, and maritime services. Finally, the EGNOS Data Access Service (EDAS) is a restricted service providing ground-based access to EGNOS data via the internet for enhanced security and performance guarantees. GAGAN is the GPS-aided GEO Augmented Navigation system from India. This system has 15 reference stations, 3 control stations, 3 uplink stations, and 3 GEO satellites, all developed, launched, and operated by India. The project was overseen by the Airport Authority of India and ISRO, the Indian Space Research Agency, and

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Fig. 4.9  EGNOS ground network stations. (Reprinted from https://commons.wikimedia.org/wiki/ File:EGNOS_map.svg. Public domain)

was first operational in 2008. Raytheon Corporation designed and built the system. All aircraft registered in India after July 1, 2021 must be equipped with GAGAN receivers. http://india.gnss.asia/node/67 MSAS (Japan) The Japanese Multi-Functional Satellite Augmentation System (MSAS) was commissioned in 2007 using two GEO satellites. It was designed and is operated by the Ministry of Land, Infrastructure and Transport and the Japan Civil Aviation Bureau to work with both GPS and follow-on systems and improves the position down to about 2 m in both vertical and horizontal. It uses the MTSAT-1R and 2 GEO satellites located at 140 and 145  °. The system became operational in 2007. SDCM (Russia) Russia has developed the System for Differential Corrections and Monitoring (SDCM) as a component of its GLONASS system. This is somewhat different from the others, in that it provides integrity monitoring for both the GLONASS and US GPS system, where the others, at present, only work for GPS or their own systems. Due to the size of Russia, there are a total of 18 reference sites in Russia as well as three international ones, and three GEO satellites for broadcast. It achieved partial operational status in 2011, but, reportedly, still has not been certified for civil aviation use as of 2023. China has taken a different approach, in that the augmentation coverage is an inherent part of the BeiDou design and is broadcast from the GEO satellites that are a part of the BeiDou constellation. So there is no separate BeiDou augmentation system, it is designed into their overall system.

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In early 2024, South Korea has reached initial operating capability with their Korea Augmentation Satellite System, or KASS (https://www.gpsworld.com/ koreas-­kass-­satellite-­navigation-­system-­now-­certified-­and-­operational/). This covers the immediate area surrounding South Korea, an area that is already covered by the Japanese MSAS system. And finally, Australia and New Zealand have joined together to create the Southern Positioning Augmentation Network (SouthPAN), with initial capability achieved in 2022, thus becoming the first augmentation system in the southern hemisphere. It provides real-time positioning to 10 cm accuracy. The areas of coverage are shown below in Fig.  4.10. An extensive economic impact document was produced in 2017 (https://frontiersi.com.au/wp-­content/uploads/2018/08/SBAS-­ Economic-­Benefits-­Report.pdf) detailing the potential benefits. Figure 4.11 below shows how the system works. Together, these systems provide continental-scale GPS correction to less than 3  m, as well as integrity information, but the southern hemisphere is largely not covered, and this is a major problem to be addressed in the future. Aviation safety would be significantly improved across the Global South with a systematic addition of these regions. There are several initiatives in Africa, but no operational capability.

Fig. 4.10  The extent of the new SouthPAN SBAS system. (Reprinted under CC-BY-4.0 license from https://www.ga.gov.au/scientific-­topics/positioning-­navigation/positioning-­australia/about-­ the-­program/southpan. © 2021, Commonwealth of Australia)

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Fig. 4.11  How SouthPAN works. (Reprinted under CC-BY-4.0 license from https://www.ga.gov. au/scientific-­topics/positioning-­navigation/positioning-­australia/about-­the-­program/southpan. © 2021, Commonwealth of Australia)

WAGE. There is also an encrypted U.S. military SBAS system, the Wide Area GPS Enhancement (WAGE) system. This is only available to encrypted P(Y) code receivers, and functions like the other systems. Finally, there is now a commercial SBAS system, created by the John Deere tractor company, designed to support its precision agriculture systems. This uses reference stations and commercial satellites covering North America, Europe, Australia, and South America to broadcast to their own, proprietary receivers, working on their GPS-equipped tractors. The system provides 4.5 cm 65% of the time and 10 cm 95% of time precision in real-time for a monthly fee. (https://web.archive.org/ web/20060711194114/http://www.navcomtech.com/docs/StarFireSystem.pdf). Elon Musk has announced an agreement with John Deere to provide Starlink capabilities for their GPS tractors worldwide, significantly broadening this capability.

Kinematic vs. Carrier Phase GPS All previous discussion on how GPS works relates to code-based GPS.  There is another technique called Carrier Phase or Kinematic GPS, which works in a fundamentally different way, and which also provides the highest precision location data. This does not use the code-based process described above, but works as an electronic distance measurement (EDM) system that measures the exact number of 17 cm carrier cycles between the satellite and receiver on the ground. This provides a location within 1 mm plus 1 part per million of the base length. This gives a fixed base station and multiple mobile units a position accurate to millimeters on the ground.

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In Conclusion, there are multiple national and regional augmentation systems, both ground and space-based, which provide significant improvements to the precision, integrity, and availability of PNT systems. New, commercial systems are also now becoming available as well. These systems are now a key component of the global PNT capability, and are vital for most of the applications, which are presented in the next chapter.

Chapter 5

Applications of PNT Systems

Military and National Security Applications We cannot deny that GPS and the other PNT systems were developed for, and are vital to, national security needs, and GPS is an aspect of almost every U.S. military activity. It is the same for Glonass and the Russian Federation, and for China, India, South Korea, and the EU. First and foremost, before all of the other, amazing applications, PNT was designed as a military timing, positioning, and navigation system, and it is used for this purpose every hour of every day around the world. Even artillery shells now have GPS receivers, and every military vehicle, ship, drone, guided missile, and airplane now relies on GPS or their own national system. In fact, the dependence of U.S. military operations on GPS has led to concern about what would happen if access to the system was somehow denied intentionally or by a major solar storm or coronal mass ejection that could disable the satellites. The adversaries of the U.S. know that disabling GPS would cause significant problems for the U.S. and their allies’ military activities. These systems are, at their hearts, military and national security tools and their impact on how military operations are conducted around the globe has been profound (Fig. 5.1). The graphic above shows many of the major civilian GPS application areas, including support of power grids, banking and finance, transportation, agriculture, and communications systems. Many of the current PNT-based business and other applications integrate the PNT capabilities with closely related Geographic Information Systems (GIS), mobile and cloud computing, big data, data analytics, AI and machine learning, and more, to weave an integrated suite of capabilities that is far greater than the individual parts. It is not just getting a coordinate pair and time stamp, that is only the beginning. Weaving these into an integrated computerized context is where the action, and the money, is.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_5

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Fig. 5.1  The various applications of PNT for timing and navigation. (Reprinted from https:// media.defense.gov/2010/Feb/25/2000391197/-­1/-­1/0/100225-­F-­JZ027-­158.JPG. © 2024, U.S. Department of Defense)

Terrestrial Vehicle Navigation Systems Today, all package delivery providers, from Amazon to FedEx, rely extensively on PNT to manage their businesses. Where to pick up and drop off packages, timing, and how to compute the least cost path for each driver is a classic GPS/GIS

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problem. Amazon and similar multibillion-dollar businesses are driven by massive, real-time databases and PNT capabilities all integrated together. Uber, Lyft, and similar rideshare businesses. These recent location-based rideshare businesses are innovations that did not exist before 2010. These are GPS and cloud-based database engines that that offer on-demand rides using a network of drivers who are equipped with GPS, with large-cloud based computers driving the system’s real-time matching of drivers’ location and the location of those seeking a ride. Uber has over 150 million users and provides an average of 28 million trips per day in 70 countries. In 2024 it had a market capitalization of some US$146.35 Billion. It is a business entirely built on GPS. It, and the several others that followed its model, are businesses entirely based on GPS, networking, integrated with a large real-time cloud database system behind the scenes. Many traditional cab companies around the world also use GPS or other systems to monitor their cab locations, and car rental companies also routinely monitor the GPS-equipped cars for speeding and in accident investigations. Many local and regional bus systems today use PNT systems to track their bus fleet’s location in real time and to display waiting times at bus stops and stations. Major trucking companies do the same.

Precision Agriculture Precision agriculture is the integration of PNT, remote sensing, GIS, and ancillary weather and historical crop production data to increase productivity and lower costs. It uses high resolution PNT in the cab of the tractor with a GIS screen, which automatically drives the vehicle, and delivers different amounts of water, fertilizer, seed, and pesticides for each square meter of a field, rather than for an entire field, using a computerized Variable Rate Application (VRA) software, as shown below in Fig. 5.2. This can be based on analysis of previous yield and recent satellite or drone imagery. Such data can be collected by Precision Agriculture devices at harvest, to determine the yield per square meter, as well as soil analysis, rainfall, and remote sensing imagery acquired during the growing season. This allows reduced application amounts, lower cost and higher yield. Automatic tractor guidance allows for working at night or in low visibility conditions, and reduces driver fatigue. There is great potential for improving the environmental impact and sustainability of agriculture worldwide by using precision agriculture, despite its high initial cost. Given the growing global population and changing climate, such systems may become a key to feeding the planet in the future. Several commercial systems are available and in use today. In early 2024 Elon Musk announced an agreement with John Deere tractors, to incorporate his Starlink satellite data system in GPS-equipped tractors, making precision agriculture possible in remote areas where existing cell service is not available. ­ (https://www.businessinsider.com/ john-­deere-­tractors-­will-­soon-­come-­with-­starlink-­satellite-­internet-­2024-­1).

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Fig. 5.2  GPS being used for precision agriculture. (Reprinted from https://www.ers.usda.gov/ amber-­waves/2016/december/precision-­agriculture-­technologies-­and-­factors-­affecting-­their-­ adoption/. © 2024, U.S. Department of Agriculture)

 round-Based Augmentation Systems (GBAS) or Local-Based G Augmentation Systems (LBAS) For precision aviation approach and landing, Local or Ground-Based Augmentation Systems have been developed. These use the GPS constellation, but augment the precision and reliability by using local, ground-based pseudolites, or signals broadcast from the airport that simulate additional, local GPS satellites. These provide for precision approach, departure, taxi, and CAT I, II and III approaches in any weather. It also allows for an unlimited number of customized approaches through difficult terrain, avoiding noise sensitive areas, mountains, and other hazards. GPS instrument approaches are now common throughout the U.S. and provide additional safety, flexibility, and are customized for unusual contexts such as in high mountains or locations with other restrictions, as shown below in Fig. 5.3. There are over 4,100 runway approaches in use today in the U.S. with minimums of only 200 feet, some in places where traditional ILS approaches are not available. There are now twice the number of WAAS GPS approaches today than traditional Instrument Landing System approaches, or ILS. The future of aviation GPS in the U.S. is called ADS-B, the Automatic Dependent Surveillance and Broadcast system. It is a part of the Next Generation Air Transportation system which is now in place. The design has each aircraft broadcasting automatically its position and time based solely on differentially corrected WAAS GPS, as shown below in Fig. 5.4. This is used for traffic surveillance both on the ground and in the cockpit. This system can operate at 10% of the cost of the national network of ground radars, which require frequent maintenance and calibration, and are often located in remote locations. It is far more accurate and operates

Ground-Based Augmentation Systems (GBAS) or Local-Based Augmentation Systems…

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Fig. 5.3  Ground-Based Augmentation System (GBAS). (Reprinted from https://www.faa.gov/ about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/laas/howitworks © 2024, FAA)

Fig. 5.4  The US ADS-B system design. (Reprinted from https://www.faa.gov/air_traffic/technology/equipadsb/capabilities/ins_outs. © 2024, FAA)

better over oceans and mountains. It also provides improved pilot situational awareness and warning of real-time traffic conflicts. One concern is if, through intentional disruption or a natural event like a solar storm or terrorism, such a system would become unusable and the impact this would have could be enormous.

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Drones All current commercial and recreational drones today have GPS receivers, and their use is integrated into the software that controls these devices, allowing detailed flight planning, accurate repeat coverage, and data acquisition. GPS is a key component of all drone operations.

Marine Standard marine navigation has been revolutionized by the incorporation of GPS navigation and marine moving chart displays, but there are many other applications from shipping container tracking, tracking of hazardous materials, and piracy alerts and search and rescue. The largest marine application by far is managing the massive container shipping ports around the world. The moving of thousands of ships and millions of individual shipping containers in major ports like Rotterdam are now entirely managed automatically using robotic container movers based on high quality GPS and other data, integrated into an Automated Port Management System (APMS). There are now 40 major container ports around the world that are largely or completely automated using GPS (https://www.mckinsey.com/industries/travel-­ logistics-­and-­infrastructure/our-­insights/the-­future-­of-­automated-­ports_). This is a massive economic activity, driving the global economy, and such automation provides significant time and cost efficiency improvements. Another interesting use is the use of GPS by the U.S. Coast Guard to precisely place navigation buoys. This provides for both more precise and more efficient work, saving millions of dollars each year and improving safety.

Rail Applications Many railroad systems, including both freight and passenger services, are beginning to use a dedicated GPS system called Positive Train Control (PTC). This includes PNT location and time, along with two-way radio and telemetry to automatically detect and avoid collisions, misdirected trains, speed violations, and other issues. PNT systems are also used routinely to track valuable or hazardous shipments via rail, road, and sea. The U.S. DOT has invested several billion US$ in this, and today most trains and over 50,000  miles of track in the U.S. are covered with PTC capability.

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Timing- Banks, Stock Markets, and Precise Scientific Time GPS and other PNT systems are, at their heart, very precise timing systems that allow us to determine our position and to navigate through our ability to precisely measure time. The current generation of GPS provides timing on the order of ~40 nanoseconds, and the future architecture will provide ~1 nanosecond timing. PNT provides atomic clock precision anywhere on the planet without the need to have expensive atomic clocks at each location, and to synchronize all of these. Synchronizing complex communications systems, email, financial networks, and power grids is now a standard GPS application in the U.S. and other countries. Any transaction that benefits from a time stamp can use GPS, and various scientific activates, where divergent locations need to time their observations precisely, all use PNT for coordinating astronomical observations. There are many applications that rely upon the precise timing signal from space for a variety of important uses, and there are currently over 1 million dedicated GPS timing receivers in use around the world that are not used for navigation at all. PNT provides the global connection to a synchronized, atomic clock-based timing system. There are over 500,000 cell-­ based telecommunications stations around the world that use GPS or other PNT systems to synchronize their networks. Many science experiments, including the Very Large Baseline Array and many experiments in astronomy and particle physics rely on GPS for precision timing. Electrical power grids synchronize the current across the grid using GPS, and the global banking and stock markets also use GPS to time stamp millions of transactions each day. The variation of these applications is constantly growing, and even Hollywood movie cameras use GPS timing to edit film acquired by several cameras at the same time. Weather RADAR is synchronized using GPS, and wireless telephone and data networks all manage their base station networks using GPS time. The list continues to grow, and there will be more and more timing applications in the future. GPS is all about time.

Surveying and Mapping GPS has completely changed the way that properties are surveyed around the world. Traditional methods were quickly replaced with high precision dedicated survey GPS systems using Real Time Kinematic (RTK) technologies with precisions down to a few cm, as shown above in Fig. 5.5. This is now the standard means of conducting property surveying today throughout the world. It is faster and more precise, and easily integrates into Geographic Information Systems (GIS) systems.

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Fig. 5.5  A survey-grade GPS. (Reprinted under CC-BY-4.0 license from https://www.eftf.ga.gov. au/about. © 2021, Commonwealth of Australia (Geoscience Australia))

Field Surveys Scientific field surveys, for applications as varied as archaeology to zoology, are now routinely done with PNT, and the ability to geolocated photographs, visualize imagery and maps in real-time, and navigate in the field to specific locations are standard parts of scientific field work and are fully integrated with project Geographic Information Systems (GIS).

Atmospheric and Ionospheric Science GPS has revolutionized our understanding of the ionosphere. Because the ionosphere alters the GPS signal, we can use GPS to accurately measure the state of the ionosphere on a global and continuous basis. This has been a totally unexpected

Civil Engineering and Intelligent Construction

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Fig. 5.6  Total electron count. (Reprinted from https://www.swpc.noaa. gov/products/us-­total-­ electron-­content. © 2024, NOAA/SWPC)

byproduct of these technologies. What was originally a major source of error in GPS has become a significant global scientific resource. The NOAA Space Weather Prediction Center provides near-real time Total Electron Count (TEC) data on their website at https://www.swpc.noaa.gov/products/us-­total-­electron-­content, as shown below in Fig. 5.6.

LBS Location Based Services Location Based Services is a new concept where smartphones are used to track where people are, and to actively offer then deals and discounts or other enticements from businesses as they pass by. Many people see this as an intrusion and question the practice, but it is steadily growing.

Civil Engineering and Intelligent Construction GPS has now been incorporated into a variety of civil engineering applications, including continuous analysis of structural deformation of bridges and buildings, and the incorporation of CAD data directly into construction equipment. All large bridges have precision GPS receivers located on the top of their towers that constantly monitor the deformation of the structure, and GPS-based CAD systems are now fully integrated with civil engineering projects in what is called intelligent Construction. PNT receivers on the blades of earth movers can be loaded with CAD data and then sculpt the landscape to within a few cm of the desired contour, as shown below in Fig. 5.7. This is now standard practice around the world and was unthinkable before satellite PNT systems.

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Fig. 5.7  Intelligent Construction using high precision PNT. (Reprinted with permission from https://qzss.go.jp/en/usage/useimage/us06_construct.html. © 2024, Cabinet Office, Government of Japan. All Rights Reserved)

Disaster Management and Response PNT has quickly become a critical component of disaster work, providing both timing and location data for search and rescue teams, damage assessment, infrastructure mapping, fire mapping of hot spots, storm tracking and flood prediction, asset tracking, among other uses. A key practical use of PNT in disasters is emergency vehicle fleet management, often referred to as Vehicle Tracking Systems (VTS). These are where PNT can provide real-time tracking of all vehicles, boats, and aircraft and even relief supplies involved in the relief effort. These allow the disaster management teams to view their resources and watch the disaster response unfold. They can more efficiently track and dispatch their resources and more effectively manage the overall response. These systems can also provide additional information such as engine RPM, emergency lights on or off, fuel and water amount available, etc. There are a wide variety of commercial GPS tracking systems available, and several new, Open Source systems as well. These generally consist of a GPS/PNT device, a transmission device (radio or cell phone), a tracking server that receives the data, and a user interface, which is often a web-based GIS browser, as shown below in Fig. 5.8. Knowing where your assets are and what their capabilities are in real time provides disaster managers improved situational awareness and gets the right things where they need to be in time to make a difference.

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Fig. 5.8  An Open-Source real time emergency vehicle tracking system developed by the author. The image on the left shows the recorded path of an emergency vehicle, and the image on the right shows its path in a 3-D view, with information on speed and vehicle configuration showed at right.

GPS In-situ Networks We are seeing a growing number of in-situ weather, river height, and other remote monitoring systems, and there will be many more of these in the future. Large networks of distributed data collection platforms can serve multiple functions, including early warning and real-time, broad data collection before, during, and after an event. In 2013, NASA’s Jet Propulsion Laboratory (JPL) and the Scripps Institution of Oceanography at UC San Diego announced the new GPS-based Real-Time Earthquake Analysis for Disaster Mitigation Network, to provide enhanced warning for earthquakes, tsunamis, and other extreme events in the Western U.S. The system uses real-time data from existing GPS monitoring stations that have been upgraded with low-cost seismic and weather data sensors, as shown below in Fig. 5.9. This system can warn of earthquakes, track flash floods, and assist in damage assessment for critical infrastructure in near real time. The system is based on integrating GPS, accelerometers, pressure, and weather data; all collected in real time throughout a large geographical region. The data are collected and analyzed in real time. Data generated can include ground motion, for example, which could provide early estimation of earthquake damage. Buildings could automatically disable elevators in earthquakes as well, as soon as a warning is received. Bridges could also detect deformations and automatically stop traffic. The integration of Internet of Things (IOT) systems is coming soon. Hundreds of high-quality GPS units are already located throughout California, which can provide both earthquake intensity as well as water vapor data, which is now constantly collected through the atmospheric distortion that water vapor causes in the GPS data, rather than through twice daily balloon launches, as shown below in Fig. 5.10. The recently upgraded system can detect the very fast moving ‘P’ waves of an earthquake, making possible automatic warnings for the later arrive of the slower, but more intense ‘S’ waves, which do the most damage. Such systems could provide warning between several seconds to a few minutes, depending on the distance to the

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Fig. 5.9  NASA JPL real-time earthquake analysis for disaster mitigation network GPS station. (Reprinted from https://www.nasa.gov/ technology/tech-­transfer-­ spinoffs/gps-­sensor-­web-­ helps-­forecasters-­warn-­of-­ monsoon-­flash-­floods/ © 2024, NASA)

Fig. 5.10  A portion of the over 500 real-time GPS monitoring stations that make up the Real-Time Earthquake Analysis for Disaster Mitigation Network. Red areas have the largest probability of earthquakes. (Reprinted from https://www.jpl.nasa.gov/news/nasa-­tests-­gps-­monitoring-­system-­ for-­big-­us-­quakes © 2024, NASA JPL)

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epicenter of the earthquake, and could automatically trigger warnings, notify emergency personnel, disable elevators, close bridge access, start generators, close airports, etc. throughout a large area. Existing rain gauges can now easily be equipped with internet or microwave broadcasting, to provide real time flood alerts in the same way.

PNT Art Cuban-born American artist Jorge Rodriguez Gerada has done several large landscape portraits using GPS.  In October 2014, the National Portrait Gallery in Washington, D.C. commissioned him to create a huge portrait on the national mall between the Lincoln memorial and the World War II Memorial. He has done similar projects before in Belfast and Barcelona, and in Washington he created a huge digital portrait that was designed on a computer and then created using 10,000 pegs and eight miles of string placed in the ground using DGPS. The pegs were then shaded with different colors using all organic material, much like a gigantic paint by the numbers. You can see several of his works at his website https://www.gerada-­art. com and https://npg.si.edu/exhibition/out-­many-­one-­jorge-­rodriquez-­gerada. These are only a few of the many PNT applications here on Earth today. There are many others, and the number will certainly continue to grow in the future as new improvements are made, new systems come online, and new ideas emerge. What is your new GPS idea? But the benefits of PNT also extend out into space as well.

 pace Navigation and Virtual Launch Range Management S and Range Safety One very interesting application is for satellite launch facilities to replace expensive and complex ground-based telemetry and tracking systems for launch operations and range safety. This is similar to the ADB-S approach in aviation mentioned above, and provides reduced cost, increased availability and improved safety margins. A 2014 Delta 4 launch of a US GPS satellite was tracked using GPS rather than the traditional network of RADAR systems along the launch path, the first use of such a system. The Range Safety and Telemetry System (RSTS) is a GPS-based system built by Honeywell International which uses two 5.4-m antennas for S-band telemetry and a UHF vehicle destruction system. This is now used at the Kodiak Launch Center in Alaska. At Cape Canaveral, the Ballistic Missile Range Safety Technology System (BMRST), also built by Honeywell, is a mobile system with a control center in a van and two mobile tracking antennas that provides the same capability. The system can be loaded onto U.S. Air Force transports for remote use, as shown below in Fig. 5.11. All SpaceX Falcon 9 s are able to provide GPS metric

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Fig. 5.11 The BMRST trailer. (Reprinted File:BMRST_Truck.jpg. Public domain)

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from

https://commons.wikimedia.org/wiki/

tracking for range safety operations, rather than traditional C-band radar transponders on the vehicle, and large amounts of ground equipment and staff are eliminated. Two GPS receivers on the second stage use an S-band radio to send the vehicle’s position and velocity to the range safety officers in the trailer at the launch site. Launches have in the past often had to be delayed due to ground infrastructure not being available, so this is a part of improving launch range ‘responsiveness’, and GPS actually provides better position and velocity data. It does require military P code GPS, due to the speed and altitude limitations on civilian GPS systems.

Space Applications Spacecraft and satellites in Low Earth Orbit (LEO) can and do use GPS on a routine basis. The International Space Station has several redundant P-code GPS systems, and these are used for navigation and attitude control, the first use of GPS for attitude control of a space vehicle. Four GPS antennas are located in a 1.5 by 3-m rectangle on the S-Zero truss, a configuration precisely designed for determining vehicle attitude. The four antennas feed information through cables to two GPS receivers in the U.S. Destiny laboratory. Which are merged with data from two ring laser gyros. This provides continuous ~10-m position and precise attitude data that is far better than traditional ground-based techniques that were only updated daily and with far less precision. GPS systems are used in the Space X Dragon vehicles in the same way, and are used for attitude control, and pre-docking. The NASA Crew Return Vehicle, scheduled to fly in 2024, will be equipped with the same GPS systems as those used on the ISS. (https://ntrs.nasa.gov/citations/20110011438).

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PNT in Geostationary Orbit Satellites and spacecraft in LEO orbits now routinely use PNT signals for positioning and attitude control, but with the PNT satellites in MEO, and all broadcasting their signals down towards the Earth, there is very little signal up in GEO or beyond. But there is a very real need for PNT signals in GEO, with the ever greater crowding of the GEO arc, new interest in satellite servicing and refueling, and more. There have been several papers published on the subject (Ruiz and Frey 2005), and some GEO satellites today actually are using PNT signals for maneuvering and station-­ keeping. NASA has conducted research on this topic and has found that high altitude GPS, as they call it, is indeed possible. GEO satellite users (and beyond) can use what is called weak-signal acquisition. This is where a receiver in GEO, far above the PNT satellites, receive the very weak signals broadcast from a PNT satellite on the opposite side of the planet, where it is broadcasting towards the Earth from the other side. This was never envisioned in the original GPS design. The Earth blocks the main signal, but all transmitters also produce unwanted side lobes (Figs. 5.12 and 5.13 below), which are normally seen as wasted energy. But these side lobes can be used to acquire useful PNT signals in this manner up to GEO and beyond.

NOAA GOES-R Example of Using GPS in GEO NOAA operates the U.S. Geostationary weather satellites, and the current generation uses a state-of-the-art GPS receiver system called the Viceroy GPS Receiver (GPSR), specifically designed for this unusual requirement. These are used for Fig. 5.12  A signal diagram of a GPS transmission received in GEO. Most of the main lobe is blocked by the Earth, the red area. The faint yellow and green side lobes can be used. (Reprinted from https:// www.goes-­r.gov/ downloads/resources/ documents/GOES-­ RSeriesDataBook.pdf © 2024, NOAA)

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Fig. 5.13  An example of how a GPS transmitter’s side lobes can be used to receive useable signals in GEO and beyond. (Reprinted from https://www.nasa.gov/missions/mms/record-­breaking-­ satellite-­advances-­nasas-­exploration-­of-­high-­altitude-­gps/ © 2024, NASA)

onboard autonomous satellite navigation at GEO, which is 15,000  km above the GPS satellites, and the faint GPS signals are pointed down at the Earth, not up at GEO.  This system exploits the extremely weak side lobe signals as shown in Figs. 5.12 and 5.13 above. The GOES-R satellites acquire a position fix within 8 min more than 95% of the time using this method, which updates the onboard attitude control system and providing regular, autonomous GEO station keeping services. NASA has flown in 2019 four research satellites, called the Magnetospheric MultiScale Spacecraft (MMS), which were launched into a very high elliptical orbit of some 187,000 km (116,200 miles) or about half the distance to the Moon. The satellites were able to receive sufficiently strong GPS signals to navigate at that distance, using the extremely weak side lobes from satellites on the opposite side of the Earth, as shown above. (https://www.nasa.gov/missions/mms/record-­breaking-­ satellite-­advances-­nasas-­exploration-­of-­high-­altitude-­gps/). NASA will soon fly GPS receivers into deep cislunar space to determine the potential to use existing GPS signals in and even on the surface Moon for the upcoming Artemis missions, as discussed below. In conclusion, there are many PNT applications, and there will be more in the future. People are very creative and develop new scientific, commercial, and practical uses for technology that were never conceived of by the developers. This is certainly the case with PNT, and there will certainly be many more applications moving forward. What is your great new idea?

Chapter 6

PNT Markets, Market Trends, and Major Players

Introduction The global PNT market has been estimated by several groups using different methods and approaches. One study by Spherical Insights and Consulting valued the 2022 market at US$ 223.64 billion, with the market expected to reach US$ 541.78 by 2032 (https://www.sphericalinsights.com/press-­release/navigation-­satellite-­ system-­gnss-­market). This is a larger commercial market than space remote sensing, with a compound annual growth rate (CAGR) of about 9% per year. They report that North America remains the largest current market, but that the Asia Pacific region is expected the have the fastest growth in the future. (https://www.globenewswire.com/en/news-­r elease/2024/01/19/2812169/0/en/Global-­N avigation-­ Satellite-­System-­GNSS-­Market-­Size-­To-­Exceed-­USD-­541-­78-­Billion-­By-­2032-­ CAGR-­of-­9-­25.html). Their full report is available here: https://www. sphericalinsights.com/checkout/3027 Another recent study by Precedence Research estimates the 2023 GNSS market to be some US$231 billion, growing to around US$596 billion by 2032 with a CAGR of 9.95%. For the U.S., the study as a market of US64.7 billion in 2022 growing to $167 billion by 2032. Significant commercial players, including manufacturers, include major electronics firms like Qualcomm, Texas Instruments, Trimble, Rockwell Collins, L3Harris, Hexagon AB (Sweden), and Topcon (Japan) (https://www.precedenceresearch.com/navigation-­satellite-­system-­market). A study by Straights Research found the global GNSS chip market (just the chips) to be some US$168 million in 2022, estimated to grown to US$716 million in 2031 with a CAGR of 15.9%. https://straitsresearch.com/report/global-­navigationsatellite-­system-­chip-­market. And a study by Zion Market Research in 2022 found that the global PNT receivers market in 2019 was around US$5.9 billion, expected to reach over US$13 billion by 2025, with a CAGR of almost 10% (https://www.linkedin.com/pulse/ © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_6

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global-­gps-­gnss-­receivers-­market-­hit-­13090-­millions-­2028-­ram-­rupnur/). Major players in the market study include Trimble, Topcon, JAVAD GNSS, CHCNAV, John Deere, EOS Positioning Systems, Broadcom, Garmin, MediaTek, and others. One study in 2021 estimated the total number of PNT devices, including smart phones, was ~6.5 billion units, estimating that this will rise to over 10.6 billion by 2031. Overall, the global market was estimated to be some €150.5 billion (https:// w w w. s t a t i s t a . c o m / s t a t i s t i c s / 1 1 7 4 5 4 4 / g n s s -­d e v i c e -­i n s t a l l e d -­b a s e worldwide/#:~:text=In%202021%2C%20the%20installed%20base,150.5%20billion%20euros%20in%202021.). Looking back instead of projecting forward, the Research Triangle Institute in North Carolina, USA did a retrospective study of the economic benefits of the U.S.  GPS system from the beginning of the system to 2019, as shown below in Fig. 6.1. The study came to the following conclusions: The U.S. GPS system alone has generated some US$1.4 trillion in economic benefits to the private sector since 1984  in ten major market sectors, with most benefits occurring since 2010. The individual market sectors that were analyzed included: agriculture, electricity, finance, location-based services, mining, maritime, oil and gas, surveying, telecommunications, and telematics (shipping, transportation, etc.). The graph above shows a dramatic increase around the 2010 timeframe, with significant continuing growth, as shown below in Fig.  6.2. The study estimated that economic benefits in 2017 alone amounted to some US$300 billion worldwide. The distribution within these sectors shows some 51% of benefits to the telecom sector (timing cell towers, etc.),

Fig. 6.1  A chart showing the trend of GPS benefits over time by individual sector (O’Connor et  al). 2019. (Reprinted with permission from https://www.gps.gov/governance/advisory/ meetings/2019-­11/gallaher.pdf. © 2019, RTI. All rights reserved)

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Fig. 6.2  This shows the relative timeline of the adoption of GPS by different market segments. The phasing of introduction of GPS-driven markets is presented. (O’Connor et al. 2019). (Reprinted with permission from https://www.gps.gov/governance/advisory/meetings/2019-­11/gallaher.pdf. © 2019, RTI. All rights reserved)

followed by telematics (shipping, delivery, transportation, etc.), followed by LBS and the rest are in single digits (O’Connor et al. 2019). The figure above shows a timeline of the major milestones of the adoption of the GPS system by the different application areas (O’Connor et al. 2019). The study also conducted an analysis of the potential economic impact of a stoppage of GPS. They estimated that there would be ~$1 billion per day in losses in the event of a complete loss of GPS, as shown below in Fig. 6.3. The relative impacts are shown in the figure below. This is a massive economic and social consequence, and this is the first quantification of the economic impact of such a stoppage, and this is only the U.S. GPS and not all the other similar systems. The telecommunications, shipping, and global distribution network would be the major applications impacted. And this study is only for the U.S. GPS system’s use, and the overall PNT numbers are certainly much larger as Galileo and the other services have come online.

2020 European PNT Market Report The European GNSS Agency (GSA) released a market report in 2020 (its third) which predicted an installed base of over 7 billion GNSS devices by 2022, including smartphones. Global revenues were projected to increase by 9% per year through 2016 and 5% through 2020. Reaching a total market of some €350 Billion (US$478

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B) per year in 2022. Smart phones would make up nearly half of this total market, and with Locations Based Services (LBS) growing from €150 mil to over €800 million over the next 5 years, as shown below in Figs. 6.4, 6.5, 6.6, 6.7 and 6.8 below. https://www.euspa.europa.eu/sites/default/files/uploads/euspa_market_ report_2022.pdf Fig. 6.3  The economic impact of a 30-day GPS outage to different business sectors. O’Connor et al. 2019. (Reprinted with permission from https:// www.gps.gov/governance/ advisory/ meetings/2019-­11/gallaher. pdf. © 2019, RTI. All rights reserved)

Fig. 6.4  PNT growth estimates by region. Growing from 1.8 bil per year to 2.5 bil. In 2031. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 1. © 2022, EU Agency for the Space Programme. All rights reserved)

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Fig. 6.5  Global GNSS projections for devices and services. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 1. © 2022, EU Agency for the Space Programme. All rights reserved)

Fig. 6.6  This shows their prediction of cumulative revenue through 2031, showing each individual market segment. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 1. © 2022, EU Agency for the Space Programme. All rights reserved)

The EUSPA has also produced an updated 2024 report (https://www.euspa. europa.eu/sites/default/files/euspa_market_report_2024.pdf), stating that the overall PNT installed base will grow from some 5.6 billion units in 2023 to almost 9 billion in 2033, as shown in Figs. 6.9, 6.10, and 6.11 below.

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Fig. 6.7  The Geographical split of current GNSS industry. The U.S. remains the largest, followed by Europe, Japan, and China. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 1. © 2022, EU Agency for the Space Programme. All rights reserved)

Fig. 6.8  The market share in components and receivers by individual market segments and regions. This shows that there are clear differences regionally between the different applications. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 1. © 2022, EU Agency for the Space Programme. All rights reserved)

The consumer, tourism and health areas are clearly the largest, followed by automotive uses. Revenue is projected to increase to some €4.6 trillion in the same period, as shown in the Fig. below, broken down by service and device revenue: The cumulative revenue projections are shown below. This shows a total market of €4.6 trillion by 2031. There is also an interesting market study from Earnst & Young for Australia and New Zealand, SBAS Test-bed demonstrator Trial Economic Benefits Report, produced in 2019, which covers the economic benefits of the now completed Australian

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Fig. 6.9  Installed base of GNSS device growth from 2023–2033. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 2. © 2024, EU Agency for the Space Programme. All rights reserved)

Fig. 6.10  GNSS Revenue projections. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 2. © 2024, EU agency for the space programme. All rights reserved)

SBAS system that was then being proposed. This 246-page report conducted analysis of the benefits of SBAS signal availability from 27 demonstration projects conducted between 2017 and 2019. The results showed an Australian $ 7.6 billion benefit over 30 years. Agriculture, natural resources, construction and roads had the largest benefits, but all sectors showed tangible benefits (https://frontiersi.com.au/ wp-­content/uploads/2018/08/SBAS-­Economic-­Benefits-­Report.pdf). This study could serve as a useful guide to others interested in a detailed analysis by different sectors of the benefits of SBAS applications.

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Fig. 6.11  The cumulative GNSS revenue growth projections. (Reprinted with permission from EUSPA EO and GNSS Market Report ISSUE 2. © 2024, EU agency for the space programme. All rights reserved)

There are many other such market analyses available, looking at different aspects of the commercial PNT markets, by region, by application, etc. but they all pretty much tell the same story of continued growth and expansion of the global markets. Some of these studies’ links are included below. Most of these are proprietary commercial reports and cost several thousand dollars to purchase. (https://www.marknteladvisors.com/research-­l ibrary/navigation-­s atellite-­ system-­market.html) https://www.mordorintelligence.com/industry-­reports/gnss-­chip-­market https://www.researchandmarkets.com/reports/5806116/military-­g ps-­g nss-­ devices-­market-­report?utm_source=GNE&utm_medium=PressRelease&utm_ code=bj7kmm&utm_campaign=1862718+-­+Global+Military+GPS+and+GNSS+ Devices+Market+Worth+%242.72+Billion+in+2023+-­+Heavy+R%26D+Investme nt+in+Satellite+Navigation+Solutions+Presents+Lucrative+Opportunities+for+M arket+Players&utm_exec=joca220prd). Continued growth at about 8-% per year is a common thread in these, with significant economic impacts across the board. Clearly, this is a significant and growing global commercial market, all based on multiple nation states operating expensive and complex space systems and making the data available in the clear and globally without cost. There is no comparable business model which is organized in this way, making the global PNT market unique. It would be an interesting exercise to consider what parts of this the marketplace would be willing to pay for, and for how much, but as long as the signals are provided by governments free of charge, we cannot know the answer.

Chapter 7

National and International Governmental Policy Issues PNT Frequencies, Overlap, and Spectrum Issues International PNT Issues

Introduction We now have five operational PNT systems, with several more on the way, and even more regional space-based and ground-based augmentation and differential correction systems, so international cooperation in frequency management, signal interference, orbital locations, and compatibility are of growing importance. The lack of signal interference is vital, partly due to the very limited spectrum allocated and available, the low power of the signals, and also because of the possibility of intentional interference or jamming, due to the dual use nature of the technologies. PNT signal compatibility is important, so that multiple systems can be easily used together for improved results and also for system redundancy in case of failure. Each of the PNT system operators have, through the International Committee on GNSS (discussed below), agreed to mutual policies regarding interference and signal compatibility, in addition there are several bilateral agreements between the operating states. But this is difficult to achieve and maintain in practice, and there have been complex issues between the PNT operating states. In a policy perspective, PNT systems are often grouped together into PNT 1 systems (GPS, WAAS, EGNOS, MTSAT, GAGAN, and the Chinese SNAS). These are publicly owned civilian systems (and augmentation systems) that are freely available and represent the first generation of systems. PNT 2 systems include the new GPS 3, Glonass, Galileo, BeiDou and IRNSS systems. These are also delivered as unencrypted signals and are interoperable, but also can include safety of life, public regulated, and value-added capabilities beyond the original and free signals.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_7

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Legal Aspects of PNT There are a variety of laws and international treaties that cover PNT systems. The Outer Space Treaty, Liability Convention, and the Vienna convention all pertain. When PNT and augmentation systems are used for civil aviation, the Chicago Convention of 1944 and Warsaw Convention of 1929 are relevant, as is the International Maritime Organization Convention and Resolution 860(20) under maritime law (https://www.imo.org/en/About/Conventions/Pages/Convention-­on-­ the-­International-­Maritime-­Organization.aspx). International Telecommunications law governing the World Trade Organization and International Telecommunications Union are involved regarding frequencies, as well as the national laws of each of the system providers. Regarding civil aviation, ICAO document A32–19: Charter on the Rights and Obligations of States Relating to PNT Services states, in part: 2. Every State and aircraft of all States shall have access, on a non-discriminatory basis under uniform conditions, to the use of GNSS services, including regional augmentation systems for aeronautical use within the area of coverage of such systems. And 4. Every State providing GNSS services, including signals, or under whose jurisdiction such services are provided, shall ensure the continuity, availability, integrity, accuracy and reliability of such services, including effective arrangements to minimize the operational impact of system malfunctions or failure, and to achieve expeditious service recovery.

The ICAO Charter on GNSS of 1998 is limited to aerial navigation and has no binding force, except in the court of public opinion. Managing the complex international issues between the various GNSS-operating states, especially given the dual use and economic importance of the applications, has been a difficult process, and this will not change as we move forward.

PNT Frequencies: Overlap and Spectrum Issues PNT satellites are essentially very specialized telecommunications satellites, and thus must work within both the laws of physics for electromagnetic radiation and also the international telecommunications regulatory framework. The electromagnetic spectrum is a very limited and precious resource, and its use is regulated by the International Telecommunications Union (ITU). PNT satellites broadcast in a small part of the L and C bands, as these have been set aside for mobile communications. The competition for frequencies is intense and growing, as new technologies are developed, and new commercial markets are seeking to encroach on the PNT frequencies. Protecting existing frequency allocations is required, and frequency interference, both intentional and accidental, are serious international issues. Frequency overlap is a complex diplomatic issue, partly because jamming of another’s PNT signal that overlaps your own frequency in a time of crisis effectively jams your own signal. In the U.S. in 2013 there were serious concerns about a new mobile satellite system called “Light Squared” that would have used frequencies adjacent to

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Fig. 7.1  The radio spectrum frequency allocation of the U.S. (Reprinted from https://www.transportation.gov/research-­and-­technology/infographic-­59ghz-­and-­us-­frequency-­allocations © 2024, U.S. Department of Transportation)

GPS. The FCC eventually prohibited this system, but this will not likely be the last such case. Each global region has their own frequency allocations, and these are very complex, as shown in Fig. 7.1 above. All of the various PNT systems are bunched together in a very small region of the spectrum, as shown below. They all overlap, or nearly overlap, as shown below in Figs. 7.2 and 7.3.

US GPS and PNT Policies and Issues As the original designer and provider of PNT services, the U.S. government’s policies have evolved over time. As mentioned previously, it is widely reported that the original concept for GPS was for an exclusively military system, and then with an added, but severely degraded civilian signal. The shooting down of the Korean Airlines 747 changed all this, and President Ronald Reagan opened access to GPS to the world, while still maintaining a restricted military capability. The development of other systems like Glonass, Galileo and BieDou has changed the dynamic from a U.S.-only into a multinational and multi-system context. The current U.S. policy perspective is based on the principles of compatibility, interoperability, and transparency with the other PNT systems. The U.S. is committed to the support of the GPS system for the foreseeable future and is committed to a policy of free

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Fig. 7.2  PNT frequency allocations in the several PNT bands. (Reprinted from https://www.transportation.gov/sites/dot.gov/files/docs/pnt/281716/spectrum.pdf © 2024, U.S.  Department of Transportation)

access to the civilian code, without any user fees, on a continuous and world-­ wide basis. One interesting aspect of U.S.  GPS policy is that the U.S. government is not liable for lack of performance of GPS when GPS is used as a means of aerial or marine navigation. Therefore, users do so at their own risk, and there is no legal recourse. This has led to the development of SBAS augmentation systems and has been one of the main justifications for the development of the EU Galileo system. The GPS system is not, as is often presented, a strictly U.S. military system. It was originally developed as such, but it has evolved into a much more generic and vital national asset. It is operated for the nation by the U.S. Space Force’s Space and Missile Systems Center at Los Angeles Air Force Base in California, which acts as the executive agent for the system. The master control facility is located at Schriever Space Force Base in Colorado. The overall system is managed through a joint civil and military body established by the President, and U.S. GPS policy is made at the highest level. The program is largely funded by the Department of Defense, but also receives funding from the Department of Transportation and several other departments and sources, as shown below in Fig. 7.4. Over U.S. $22 billion has been spent on the development and operation of the system so far, and the current annual budget is ~$1.8 billion. The U.S. government is committed to international cooperation in PNT and seeks to ensure that other systems are interoperable but do not interfere with GPS operations. It also is committed to minimizing interference through spectrum regulation, interference detection, and regulatory enforcement. It seeks to

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Fig. 7.3  The several PNT systems and their frequencies. (Reprinted with permission from https:// qzss.go.jp/en/overview/services/superiority.html. © 2024, Cabinet Office, Government of Japan. All rights reserved)

prevent hostile use through illicit jamming, spoofing, or other disruption. The U.S. no longer uses Selective Availability, and current satellites no longer have this capability built into their design. Figure 7.4 below shows the current organizational structure for the management of the GPS system. The President and White House have the ultimate responsibility and authority, but the operations are delegated to a National Executive Committee for Space-Based PNT, and an executive working group, which is jointly chaired by the Departments of Defense and Transportation. NASA chairs the technical advisory board, and representatives of several departments and agencies participate. International issues are managed through the Department of State. The Department of Commerce, through its Office of Space Commercialization, promotes the interests of the commercial GPS users, manufacturers, and service providers, and participates in discussions with other nations to promote international cooperation regarding GPS. The Department of Commerce also plays an important regulatory role, in that the National Telecommunications and Information Administration, along with the FCC, manages the frequency spectrum used by GPS. The Bureau of Industry and Security sets guidelines for export licensing of civilian GPS equipment.

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Fig. 7.4  The US National Space-Based PNT organization structure. (Reprinted from https://www. gps.gov/governance/excom/orgchart.pdf. © 2024, GPS/NOAA)

The U.S. government in 2014 eased export regulations for commercial GPS receivers, moving high-end GPS receivers out from under the stricter control of the State Department to the Commerce Department, which had already overseen export of lower-end GPS systems. This takes civil GPS receivers off the State Department U.S. Munitions list and International Traffic in Arms Regulations (ITAR) and moves them to the Commerce Control List (CCL), which broadens export possibilities. All military-grade receivers capable of receiving the P(Y) Code are still subject to ITAR restrictions and are tightly controlled.

European Galileo Policy Issues The European Union is now the political, financial and program driver for the Galileo system, although it was originally conceived and developed by the European Space Agency (ESA). This was defined in 2008, when the EU assumed all Galileo responsibilities. Navigation is now an EU ‘competence’, and not that of the individual member states. The EU is the overall program manager, with the technical support of ESA and the GNSS Supervisory Authority (GSA). The EU is the owner of all Galileo and EGNOS infrastructure and is the legal custodian of frequency rights. The EU delegates the budget, design and procurement to ESA. There are now

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three functions in Galileo; the EU as the GNSS regulator, ESA as the system designer and agency responsible for procurement, and a GNSS service provider which operates the system. There are multiple functions for European organizations, including Eurocontrol as the aviation requirements agency. One potential issue is that the EU may in the future require the use of Galileo receivers within the EU nations, protecting the European manufacturing and commercial business sector. Several versions of EU GNSS Action Plans include this as an option due to the delays in fielding the system and the entrenched (and free) GPS markets. With Galileo being the fourth system to achieve full operation, they face significant market constraints, and may mandate Galileo equipment (to be built within the EU) for a variety of European activities, including civil aviation, dangerous cargo tracking and eCall (112) emergency cell phone calls. The EU does now require all new cars and trucks sold within the EU to have built-in a Galileo-based emergency eCall capability. This is a complex international issue, as the U.S. and Europe signed a 2004 agreement stating, in part: “The Parties agree to consult with each other before the establishment of any measures … that have the effect, directly or indirectly, of mandating the use of any civil satellite-based navigation and timing signals or services, value-added service, augmentation, or global navigation and timing equipment within its respective territory (unless the mandating of such use is expressly authorized by ICAO, or IMO). (http://www.insidegnss.com/node/3990).

Russian Policy Issues On May 17, 2007, President Vladimir Putin put forth Presidential Decree number 638, ‘On Use of GLONASS Global Navigation Satellite System for the Benefit of Social and Economic Development of the Russian Federation’, which stated that access to GLONASS civil signals is free and unlimited for use by both Russian and international users, and that Russian Federation organizations, executive authorities, local self-governments, and Federal subjects shall use the GLONASS system. It also directed the Federal authorities to approve and adopt a Federal GLONASS use program. The intention is for guaranteed provision of services and the mass introduction of the use of these for both national security and social and economic benefit (https://www.unoosa.org/documents/pdf/psa/activities/2012/un-­latvia/ ppt/1-­2.pdf). The GLONASS system, like GPS, is a dual use program and its use is mandatory for governmental and critical infrastructure users. Recent data show a precision for the civilian signal of ~2.8 m. Russia now mandates the use of Glonass systems for some uses. A decision by the Russian Ministry of Transport in 2012 has mandated that Glonass or Glonass+GPS systems must be used for all civil aircraft and helicopters within the Russian Federation starting in 2018. All Russian registered, as well as commercial international aircraft, will be required to use the Russian systems. Hazardous cargo aircraft had to comply by January 2015. Ground vehicles shipping hazardous cargo now also have to use these systems. Russia has also threatened to block imports of

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cell phones and other electronic devices that do not also include Glonass systems as well as GPS. The Russian government has decreed that all spacecraft, commercial aircraft, commercial river and ocean vessels, railways, hazardous materials shippers, surveying equipment and timing devices in Russia shall use the Glonass system. In 2014 there was a diplomatic issue between the U.S. and Russia over ground stations. Russia wanted to locate a Glonass reference station in the U.S. and had allowed the U.S. to place 11 GPS geodetic ground reference systems within the Russian Federation, and which were operated by the Russian Academy of Sciences’ Geophysical Service as part of a global scientific effort. Due to the crisis in Ukraine, the U.S. refused permission, and Russia has stopped provided data from these 11 sites until further notice. Cuba has since agreed to host the Glonass reference site. Another interesting policy issue with Glonass was that radio astronomers discovered that the system’s L1 band is in the same location as the emission band of extra-­ solar hydroxyl molecules, and that the Glonass constellation was interfering with radio astronomers around the world. Efforts were made to minimize this problem in later satellite designs, and this has been largely resolved, demonstrating that international PNT conflicts can be resolved amicably. In 2015 Russia and China created a Russia-China Committee, with the Russian ROSCOSMOS Space State corporation and the China Navigation Satellite System. This seeks increased cooperation and coordination between the two in areas of compatibility, augmentation, performance monitoring, and applications. Four working groups were created which meet annually (https://glonass-­iac.ru/en/committee/).

China Policy Issues The BeiDou system has recently transition from a regional to a global system. China has pursued its own course with speed, focus, and resources. After its brief involvement with Galileo, China quickly developed their own system as a national priority with both military and commercial goals. A blueprint, approved by the Chinese government in 2012, envisioned BeiDou capturing 60% of the estimated US$ 65 billion PNT market within China by 2020. They also announced that 40% of the system’s use would be for the Chinese military. “The era of China relying on a foreign satellite navigation system is in the past,” a Xinhua press release has stated. “The era of China’s BeiDou has arrived.” China now operates the largest number of PNT satellites, makes all classes of receivers, and provides services worldwide. BeiDou is a vital part of the Chinese ‘Belt and Road’ or ‘New Silk Road’ initiative of providing infrastructure and development funds and technologies around the world. Some US$1 trillion has already been spent, with estimates of up to US$10 trillion possible. BieDou is an integral part of this initiative.

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Indian Policy Issues India has pursued a totally domestic approach, where all satellites, launchers, ground infrastructure, and associated hardware and software are intended to be designed, developed, built, and operated in India. Their system is well-integrated with India’s other space applications in telecommunications, remote sensing, and their practical applications. India also has a required use of their system for all commercial vehicles in the country. Studies have been done for expanding the regional system into a fully global capability, but this decision has not yet been made. Several European atomic clocks used in the first iteration of satellites have failed, and India is pursuing developing their own. India has an ‘all Indian’ policy and they will continue in this.

Japanese Policy Issues Japan is expanding its QZSS system to operate as a full PNT capability. This will only operate over Japan and its surrounding oceanic region, and so it has limited impact on others, but Japan will soon operate both a full NPT and space-based augmentation system.

British Policy Issues Great Britain was a major investor and source of technical expertise in the European Galileo program, but they are now completely frozen out of that program as a result of the very divisive Brexit decision. They feel the need to have an independent system of their own, both for national security and national economic competitiveness perspectives, but the cost will be very large and it is uncertain at this time if they intend to pursue this course alone.

South Korea Policy Issues South Korea has announced the development and funding of their own regional system, and they will face similar frequency and applications issues as the established players. They have moved quickly, have made significant progress, and now have a working system.

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International Organizations and PNT Coordination In 2005, the United Nationals International Meeting for the Establishment of the International Committee on Global Navigation Satellite Systems was held in Vienna, Austria. Their logo is shown above in Fig. 7.4. This grew out of the third UN Conference on the Exploration and Peaceful Uses of Outer Space (UNISPACE III) in 1999. This meeting created the International Committee on GNSS (ICG), a voluntary body established to promote cooperation on matters of mutual interest related to civil satellite-based positioning, navigation, timing, and value-added services. This group considers issues of compatibility and interoperability between the various systems. The GNSS providers, along with other interested member states of the UN and International organizations are either members, associate members, or observers (the lists are on their website). The members have agreed that all GNS providers will openly publish documentation that describes signals, spectra, and performance data. The publication of Interface Control Documents (ICDs) is a key aspect of this process, as this is where each service provider documents the frequencies and technical aspects of their systems. It is assumed the new operators will officially join as their systems come online. (https://www.unoosa.org/oosa/en/ourwork/icg/icg.html). The focus is on compatibility and interoperability, the exchange of detailed information on systems, and the exchange of views of ICG work plans and activities. The providers have developed a working definition of compatibility and interoperability, to include respect for spectral separation between signals and addressing signal, geodetic reference, and time standard considerations. The ITU provides the framework for discussions on radiofrequency compatibility and related issues. Currently, the present situation is defined by a number of bilateral arrangements between the various PNT states that operate PNT satellite constellations in orbit, and there is not an overall or comprehensive agreement structure. There have been suggestions that this group could lead discussions on an international convention on PNT, which would reflect State sovereignty and authority and that would be submitted to international public law. This could address relevant issues relating to the future of PNT services such as the rights and obligations of operators, liability issues, settlement of disputes, and the joint provision of new services. This would be a reasonable approach, but the reality is that the geo-political will simply is not there to do this, and this is not likely in the foreseeable future.

PNT Jamming, Spoofing, and Denial of Service Attacks Jamming of PNT signals is a growing concern. Jamming is the saturation of a frequency to overwhelm the signals and deny the use of that system. Jamming of GPS, Wi-Fi and other signals is illegal in the U.S. (a federal felony) and in most other

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countries. In 2014, the U.S. Federal Communications Commission fined a Chinese electronics vendor, CTS Technology, a record $34.9 million and ordered it to stop selling signal jammers that interfere with authorized wireless communications, including GPS. The FCC also ordered CTS to provide information about those who purchased these systems. Illegal small, local area GPS jammers can be purchased on the internet for ~US$50 and can be powered by a car cigarette lighter. They jam all GPS signals near the vehicle and are often used for avoiding automatic electronic tolls or avoiding tracking by employers. In 2013, a New Jersey engineering company employee purchased an illegal GPS jammer to keep his employer from tracking his GPS-­ equipped company vehicle. Unfortunately, he frequently drove past Newark International airport and disrupted the airport’s and airline’s GPS units on a daily basis. He was arrested and fined $31,875 (and fired). http://www.cnet.com/news/ truck-­driver-­has-­gps-­jammer-­accidentally-­jams-­newark-­airport/. There are now systems available to police that can locate these devices on the highway, as they are often used to avoid electronic tolls. The FCC operates a toll-free number, 1-855-55NOJAM, or 1-855-556-6526 to report the use of illegal jammers. There have also been several reports of disruptions of GPS timing signals at the London Stock Exchange from an unknown source. And these are in the civilian sector. There are multiple reports of Russian GPS jammers being shipped illicitly to Iraq during the Iraq war, for use in jamming U.S. GPS-guided missiles, reportedly with limited success. But jamming is an ever-growing problem, and there is extensive jamming currently throughout the Middle East and also in Ukraine. Russia has had significant success jamming U.S.-provided GPS munitions in the Ukraine war. There are multiple reports that wherever Russian Federation President Vladimir Putin goes, a bubble of GNSS jamming envelops that location. Spoofing of GPS signals is another major problem. Spoofing is where an electronic device is used to mimic the exact signal of a given PNT satellite, which then broadcasts erroneous information. One technique, referred to as a ‘carry-off’ attack, is where the broadcast power is slowly increased, thus gaining precedence over the actual satellite’s signal, and sending incorrect information to all receivers in that local area. There have been several well documented examples of this in Ukraine, where ship GPS locations in the Black Sea all begin to drift away in the same direction many tens of kilometers from their true position. This has also occurred in the Eastern Mediterranean Sea with commercial aircraft (https://www.forbes.com/sites/ erictegler/2023/12/05/gps-­s poofing-­i n-­t he-­m iddle-­e ast-­i s-­n ow-­c apturing-­ avionics/?sh=1838dd8a3a6f). It has been suggested that the Iranian capture of a U.S. drone in 2011 was achieved by spoofing. PNT denial of service attacks are where a service provider has the operation of the ground segment or space segment disrupted, either by electronic or physical means. This would be a most serious action, perhaps considered to be an act of war, but the satellites or ground infrastructure could be attacked and the system either significantly degraded or made unusable. In the event of a major international conflict, it is almost certain that nations have plans for PNT denial of service attacks on the ground and/or space segments of potential adversaries. The explosion of a

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nuclear device in the MEO orbit would cause a massive Electro-Magnetic Pulse (EMP) that could destroy the electronics in all nearby satellites. Jamming, spoofing, and denial of service by various means are a significant problem for all of the operators and users of these systems. Several operators have, in recent years, taken measures to increase signal strength, improve encryption and taken other steps to address this growing problem. The major ground GPS infrastructure facilities are all located in very highly secure locations for this reason. Interestingly, one of the most commonly used reasons to convince national leaders to invest in their own systems is that the U.S. GPS could be jammed, so we need our own. This begs the question that if an adversary does jam or deny use of GPS, what is to stop them from jamming yours at the same time? In times of such events, it is likely that any and all such systems would be jammed. This is one of the major concerns of all PNT operators. How can you secure the use of your system and potentially deny use to your adversaries, when the signals are all crammed in such a tiny portion of the spectrum.

Cosmic Hazards Perhaps the most likely causes of a system-wide disruption would be a massive solar flare that would release a powerful radiation surge that could damage the electronics in the PNT satellites. Even worse, a major coronal mass ejection (CME) from the Sun, if powerful enough and aimed at the Earth, could disable the electronics in satellites, particularly satellites in MEO and GEO that are more exposed. The MEO orbit is a very high radiation environment in the Van Allen belt, and satellites are already radiation hardened, but a major CME could disable or damage many of these.

Legal, Ethical, and Moral Issues There are significant legal and ethical issues relating to these powerful new capabilities that relate to such issues as tracking of individuals without their knowledge, warrantless tracking of individuals by governments and police, storing and selling data on individual persons movements, stalking, etc. These are very new issues, and there are very few legal precedents. Technologies develop much faster than the legal structure, and the decreasing size and cost of GPS devices could lead to 24/7 tracking of individuals without their knowledge. GPS systems can be used to track employees, family members, or those suspected of crimes, and GPS ‘ankle bracelets’ are routinely used to monitor thousands of parolees and sex offenders. The tracking of employees can reasonably address efficiency and compliance with traffic laws and overtime rules for truck and bus drivers. Rental car companies can now monitor the location and use of the vehicles to monitor for speeding, for example,

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in case of an accident and insurance claim. But who owns this data? Who has access to it? Where is it stored? Who is liable if it is wrong or wrongly used? How can you know if there is tracking data of you? What about data breaches? In the U.S., there are no federal laws regulating the use of GPS to monitor employees, but the States of California, Connecticut, Delaware, and Texas have such laws, which generally require employee consent. Delaware outlaws the use of GPS tracking of vehicles except by law enforcement and the parents of minors. Texas allows private investigators to use GPS tracking devices, with the consent of the owner of the vehicle. U.S. courts have held that there is no expectation of privacy in relation to property owned by the employer, including cars, email, and smart phones. In 2012 the U.S. Supreme Court decided that, without a proper warrant, police could not place GPS trackers on vehicles, as this was a warrantless search and thus was unconstitutional, but the question may be raised again.

Chapter 8

A Curious Narrative of the Origins and the Current State of PNT

Introduction The following is a rather different version of how we got where we are in the development of PNT than you will hear at international conferences or see in program brochures or on system provider websites. Based on personal knowledge, extensive reading of Wikileaks, logic, many years of professional experience, and discussions with numerous unnamed sources who cannot speak publicly, let me present the following for your consideration: Once, long ago, the U.S. military needed an improvement to the Transit satellite navigation system, and U.S. researchers developed an extraordinarily robust new idea that would also be very expensive, costing tens of billions of dollars. In order to sell it to the U.S. Congress, the Pentagon claimed that it will have amazing civilian and commercial uses, and that they will provide a civilian signal for this use, along with the primary purpose of a military system. They succeed in getting the funding and built the system and it worked extremely well. No one was more surprised than the Pentagon when what they said actually happened, and the GPS system becomes a vital, global utility with thousands of unforeseen uses and billions of users. This was made possible when President Ronald Reagan opened up the GPS civilian signal to the world after the shooting down of KAL 007. The generals were also shocked when the early and highly degraded civilian component, which they had to include for purely political purposes to get the initial funding, worked almost as well as the military receivers, so they created a “Selective Availability” system to reduce the civilian precision to make it nearly useless, but did not turn it on. Then one day, they finally turned it on without telling anyone, and then only turned it off when the U.S. invaded Grenada and then Panama, and for the first Gulf war. This was because there were not enough military receivers yet available, and individual soldiers were buying every inexpensive GPS unit on the market, and having their

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_8

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families mail them to them in the field. So SA was used ‘in reverse’ of is original purpose, off in war, on in peace. Russia somehow found a way to invent their own exact copy of the GPS design, and claimed they thought of it first. The U.S. was not overly concerned because the Glonass system was less capable, encrypted, and only available to the Russian military, and they were confident that it could be jammed if needed. Both systems are, at their hearts, cold war military systems, but the many uses of GPS began to quickly overtake the military market, and GPS became an amazingly useful scientific, commercial, and business infrastructure. Europe felt left out, even though TomTom and other GPS developers were making millions by selling GPS units using the free GPS signal and without paying any license fees at all. So Europe decided to create their own system as a public-private partnership, with European industry paying the majority of the cost. The European business community, being no fools, quickly declined, as they saw no way to compete with a working system that was provided for free and that met most users’ needs. European space and political leaders continued to lead the charge against the ‘bad U.S. military’ GPS (which by now it was not) and for creating a ‘good international civilian’ Galileo system (which it was never going to be), even though, as members of NATO, they all had access to the P-Code military system for free. But they wanted their own system. The U.S. fought hard at the highest diplomatic levels against the development of Galileo, seeking to keep their advantage as the sole global military and commercial system, as would be expected. Europe also managed to rename the now generic use of the term GPS to GNSS, to differentiate this from the U.S. system. Europe asked China, Israel and others to join in Galileo, party for political cover, claiming it would be a true, international, and civilian alternative to GPS, but largely to get the millions needed to start development, and also to get their votes in the ITU to get frequency allocations for the new system, which they knew the U.S. would try to block. The U.S. fought hard against this, and there was much internal wrangling in Europe over who would get the money and ground stations. Europe decided that they would put their code frequency right on top of the U.S. frequency and the U.S. was furious, as this would stop them from jamming Galileo in time of crisis if it was used against the U.S. Europe said this was a stabilizing and good thing, but eventually, in the face of intense political pressure from the U.S. at the highest level, relented and moved it, just a bit. The U.S. fought hard against Galileo, and warned Europe that China was seeking advanced technologies for their own system and was using them. Europe disregarded this and saw China as too far behind to be a threat. China tried to buy atomic clocks in France and was denied but purchased twenty in Switzerland and also bought (or otherwise acquired) much other advanced technology from European industry while a Galileo partner. It was a field day for Chinese ‘technology transfer’. Europe won their Galileo frequency allocation in the ITU, and then quickly dropped China and the others as partners, keeping the €two million already paid by China (which they never gave back). China was furious and took all the technology it had bought (and otherwise acquired as fast as it could) and decided to create their own global BeiDou system and to deploy it before Galileo, thus gaining frequency rights as the first to put the system in place. The U.S. and Europe

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fought hard against China’s frequency access, sensing a potential threat. China announced that they would put their frequency right on top of Galileo (just as Europe had tried to do to the U.S.), thus preventing Europe from jamming BeiDou in time of crisis, and Europe was furious. China said this was a stabilizing and good thing, and that Europe did this to the U.S., but eventually, in the face of much political pressure, relented and moved it, just a bit. Europe felt that China had wrongfully taken much of their technology and had played unfairly. China felt used by Europe and wanted their €two million back. Meanwhile, GPS had become such a global utility that The U.S. Coast Guard (a military component of the U.S. government) funded and fielded a Differential GPS network around the entire coast of the U.S. to defeat Selective Availability, and to ensure that ships entering U.S. harbors could navigate more safely with GPS. So one branch of the U.S. military (U.S. Air Force) was keeping SA on, while another branch (USCG) was funding and fielding a U.S. government funded system to defeat it. Thousands of other DGPS systems were placed around the world, totally defeating SA, and, finally, President Bill Clinton turned off SA for good, as people have developed a reliable and unjammable way to defeat it with DGPS. The Soviet Union collapsed and Glonass dropped down to only a few satellites; no longer being able to function. President Vladimir Putin assumed power in Russia and made rebuilding Glonass a top priority, and the system was repopulated and enhanced. He even put a Glonass-equipped collar on his dog. He also opened the signals to international use. Once China announced their own system, India felt the need to also have its own, particularly after the U.S did not share its military-grade system in the 1999 war with Pakistan, and so India announced the development of the Indian Regional Navigation Satellite System, later named NavIC. Once India was developing their own system, Pakistan, India’s enemy and China’s ally, then felt the need to approach China, seeking its own BeiDou satellite, and access to military receivers so that they can announce that they too have a satellite navigation system like India was developing. Pakistan was denied this by China after long negotiations, but Pakistan did get a BeiDou ground station located in Pakistan as a consolation prize and could then say that they are a part of the system and thus on a par with India. China did not like India creating their own system but could not block it. Since the Indian system was only regional, the U.S., Europe, and Russia did not try to block it either. Brazil, which manages one of the most effective and useful space programs in the world, was perfectly happy using GPS, and wisely decided not to develop their own system, thus avoiding the development of the inevitable, competing Argentina and Chile regional systems. Vladimir Putin, having rebuilt Glonass, mandated the use of Russian receivers by commercial aircraft and other commercial users, in order to develop and protect Russia’s emerging commercial markets, and Europe, unable to define a viable, long-term funding model, also considered creating protectionist barriers and requiring Galileo receivers in European aircraft and cell phones. European airlines and other industries, quite happy with the free GPS system they had used for decades, quietly fought hard against this, but it was likely that this would happen once the Galileo system was operational. Europe gave their industry a secret sneak peek at the inner workings of the Galileo architecture and

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signals, in order to give them an advantage in building and getting Galileo receivers to market first, and the U.S. quietly fought against this. The U.S. and other scientists and other researchers in industry around the world quickly decoded the first Galileo satellite transmissions (even though they were encrypted), published them openly, and rapidly moved to market receivers that can use GPS, Glonass and Galileo signals, even before Galileo was operational. Europe felt that this was unfair and wanted to protect their industrial base. The U.S. now sought to rename the now generic use of the term GNSS to PNT, thus again regaining ‘ownership’ of the name of the technology back from Europe. Japan, politely, designed and launched their unique QZSS augmentation system and nobody cared, as it was no threat and only worked over Japan. They later, quietly, expanded the system design to a true, stand-­ alone PNT system, and nobody really cared about that either. Australia, sitting in a geographical ‘sweet spot’ where it will have free access to all of these systems being developed, sat back, smiled, and paid not a cent, shaking their heads. They then created the first SBAS system in the Southern Hemisphere along with New Zealand. South Korea decided that if China and Japen had systems, they had to get in the game and developed yet another Pacific regional system which largely overlaps with Japan’s and China's. The U.K., shocked, shocked that the EU would not let them continue in Galileo after the bitter Brexit divorce, was forced to consider the cost of going it alone to keep in the race, so they are looking at creating their own system. The U.K. had extensive investment in Galileo and was a key source of expertise, but they were now frozen out completely. The UN created a forum where the various players were to interact responsibly and share detailed information on their systems to ensure interoperability and compatibility; all with the expected convoluted, tense, and diplomatically complex results, hedging, partial data disclosures, peeking, and finger pointing. The dance continues today, driven by the enormous commercial potential and deep national security roots of the technology. Nation states, private corporations, and individuals act in their own self-interest, and this will not change, on the Earth or in orbit. This is not the version of this you will hear at international conferences, but the international politics behind the PNT curtain are intense and ongoing.

Where Are We Now? The figure below, Fig. 8.1, shows the growth of the several PNT constellations. As this is written, in mid 2024, there are a total of 139 PNT satellites in orbit, as shown in Figs. 8.1 and 8.2. As of mid 2024, these include (Table 8.1):

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Fig. 8.1  The growth of PNT satellites in orbit over time. (Reprinted under CC-BY-4.0 license from https://link.springer.com/chapter/10.1007/978-­981-­32-­9915-­3_4. © 2020, The Author(s))

Fig. 8.2  All 139 PNT satellites in all orbits in 2024. (Reprinted with permission from https:// geoxc-­apps.bd.esri.com/space/satellite-­explorer/#. © 2024 Esri and its data contributors. All rights reserved)

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Table 8.1  This is the total number of PNT satellites in orbit as of early 2024. These satellites are primarily in the MEO orbit, but there are several in GEO and a few in highly elliptical orbits. (Data reprinted with permission from https://www.gnssplanning.com/#/satellites. © 2024, Trimble Inc. All rights reserved) System: active GPS GLONASS Galileo BeiDou QZSS NavIC Total

Satellites Selected 31 24 25 48 4 7 139

Healthy 31 24 25 48 4 7 139

The Current Status Having read this brief ‘history’ of GPS/GNSS/PNT, and looking at the current status, we must ask ourselves ‘what is the future for these systems and their applications’? We now live in a world where there are five operational global PNT systems, soon to be seven or eight, with some 140 satellites, and between 30 and 40 satellites in view at any one time from anywhere around the world, when any four would do. We now get an overall DOP of between 1 and 2 anywhere and at any time, which is excellent. What will this mean to end users? This will certainly provide better access and precision in areas such as urban canyons and under forest canopies. There will be improved ability at the poles as well, where there are virtually no users. We will have more options for software to provide faster and more reliable position fixes and initialization when first turning your receiver on. Accuracy will be enhanced, and the addition of signals from both the L1 and L2 frequencies will provide high precision for almost any imaginable application or user requirement, eventually including global, real time millimeter precision. Dual frequencies will greatly limit the problems currently caused by the ionosphere. If one system actually does go down, as Galileo and Glonass each did twice, such redundancy would ensure continuous operation of vital systems around the world. But at what cost? Why do we pay for so many redundant systems providing the same capability? Dual use, national pride, commercial benefits, industrial base, technology development and more are the answer. Moving forward, technical differences such as the use of CDMA and FSMA need to be resolved, as well as differences in geodetic references and time standards between the various systems, but these are all manageable issues within the existing international structure. From an optimal, end-user perspective, these individual systems need to work together in true interoperability, rather than simply co-exist as separate, redundant, overlapping and competing entities. It would make sense to have a global PNT system of systems, rather than individual national capabilities. But there are also major issues regarding frequency overlap, jamming, and spoofing,

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and these will only grow over time, particularly in times of simmering international tensions and overt armed conflict. These issues will drive the possible business, commercial and scientific applications that will be possible in the future, such as precision agriculture, UAV piloting, and civil engineering, to name just a few. True centimetric precision anywhere and anytime, and even indoor positioning will become widely available, and costs will continue to decline. PNT will become even more of an invisible utility, used behind the scenes for a growing range of practical applications and markets around the world. But we cannot forget that these systems exist as a part of, and cannot be separated from, the global geopolitical context of powerful nation states and their military might, broadly defined national security interests, global economic competitiveness, science, technology development, and national pride. These systems are instruments of national interests and are not commercial or public benefit technologies. We have to remember always that these systems are paid for and operated by national governments (and the EU acting as such) at great cost, and that they then give away the signals for free because they see it to be in their national interest to do so. This is a unique arrangement and is the keystone to the entire system. PNT has become a mass commodity, with some 7 billion receivers, largely smart phones, and there is no disruptive advantage for new, redundant systems that provide the same basic capabilities and services. The U.S. GPS system is considered to be good enough for the vast majority of global uses and users, yet new systems continue to be developed for the above-mentioned dual use, national security, economic competitiveness, and national pride reasons. Multiple systems broadcasting open and interoperable signals do provide important redundancy benefits to all users, but we need to create functioning processes to resolve problems in frequency use and allocation as new technologies are developed. Limited frequency allocations and competing new technologies requiring scarce frequency allocations will continue to be major problems. There is very little frequency allocated for this use and no more will be provided. Legal liability issues are still to be addressed, as the technologies continue to outpace the existing international and national legal frameworks. In the end, we must ask “how many of these do we need and how many can we afford?” While we do gain by having more than one or two systems in terms or redundancy, we do not gain in the same way, as end users, by going from 3 to 6 or 7 to 10 redundant systems. What is driving the desire to spend billions to create and operate 5 or 8 or 10 such competing systems? There are many answers to this, but the primary driver is the dual use nature of these technologies, military and national security requirements, national pride, and international economic competitiveness, and these are powerful forces. PNT systems are, at their hearts, fundamentally national security systems, operated by nation states for their own benefit, and we should recall that the original justification and funding for the GPS system was for strictly military purposes. But the reality of the present is that commercial and public applications have far outpaced the military uses of these, tens of thousands to one, and new applications are developed every month. PNT systems have become a vital invisible utility, enabling

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a wide range of useful purposes, from timing the internet to managing national power and communications grids, and these will only increase in number. We must find better ways to coordinate and cooperate to ensure that this amazing ability to master both geographical space and time continues to provide broad benefits to all peoples on the Earth. All people. We must also avoid international conflicts that could risk the denial or destruction of these vital and expensive parts of our global infrastructure. As the recent RTI study presented above shows, a day without PNT would be a bad day, a year without PNT would be a major disaster. We must ensure that this does not happen.

Chapter 9

PNT on Other Worlds, and Our World Beyond PNT

Introduction We now turn to the future, both here and on other worlds. First, we take a look at the first out-of-this-world PNT system that is in development, which will also be the first internationally designed and operated system, and then we will consider the world beyond our current understanding of PNT and what may come next. As a part of its Artemis project to return to the Moon and to establish a permanent presence there, NASA and its partners are developing a PNT system called LunaNet for the Moon and for use in Cislunar space, the region of space between the Earth and the Moon and around the Moon, as shown below in Fig. 9.1. This will provide PNT, data, internet, and communications in an integrated system for spacecraft, bases, rovers, and individuals. This is a significant technical challenge, being 384,000 km from the Earth. The goal is to provide ~10-m position capability anywhere on the Moon, plus a 50-m landing capability. The architecture is still being designed but may include use of terrestrial PNT side-lobe signals, doppler radio signal processing, and, eventually, a constellation of smallsats in Lunar orbit, as shown below in Fig. 9.2. One of the unique aspects is that this will be the first international PNT system, developed and operated by NASA and the other 35 signatories (and growing) of the Artemis Accords. The system will have an integrated design providing four quite different capabilities: PNT and navigation of spacecraft, vehicles, and individuals in space and on the Moon, on demand networking and internet, threat detection (for dangerous Solar flares) and information sharing, and radio/optical science data services between the Moon and the Earth. The PNT system includes a dedicated Search and Rescue capability for vehicles and individuals on the Moon. The first iteration will use Doppler radio systems for locating vehicles and individuals on the surface, but further iterations may include dedicated smallsats providing PNT services. There will be interesting constraints, including maintaining the far side of the Moon © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_9

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Fig. 9.1  A notional image of a LunaNet interface on a tablet on the Moon. Image courtesy NASA/ Reese Patillo. (Reprinted from https://www.nasa.gov/humans-­in-­space/lunanet-­empowering-­ artemis-­with-­communications-­and-­navigation-­interoperability/ © 2024, NASA/Reese Patilio)

Fig. 9.2  A possible LunaNet PNT architecture. (Reprinted from https://www.nasa.gov/humans-­ in-­space/lunanet-­empowering-­artemis-­with-­communications-­and-­navigation-­interoperability/ © 2024, NASA/Reese Patilio)

as a totally shielded zone protected for radio astronomy. It will be fascinating to see how this all develops. The system is also seen as the initial prototype of a future MarsNet system, for when humans first venture there. We live in interesting times. The Lunar GNSS Receiver Experiment (LuGRE), is a joint NASA and Italian Space Agency payload, and is schedules to be flown as early as 2024 to characterize

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weak-signal GPS and Galileo use in the Lunar environment and on the surface of the Moon. This is to determine if existing signals, already very weak, will be able to support human missions to the Moon. The mission will be flown on the FireFly BlueGhost1 mission, scheduled to be one of the first commercial landers on the Moon (https://fireflyspace.com/blue-­ghost/). The LunaNet development for Cislunar space and the Moon is a new step forward, and such a truly international system will be the first attempt at international cooperation in PNT as well as the first system beyond the Earth. Perhaps we can collaborate on other worlds in PNT, if not here at home.

Beyond PNT And we must also understand that all existing paradigms must pass. This current one, begun by the U.S. GPS in the early 1970’s, is now over half a century old, and that is a very long time in terms of modern technology development. GPS-like systems too, will someday be superseded, it is only a question of when and with what. Just as GPS was a giant leap beyond the Transit Doppler system, the several PNT systems of today will surely also be replaced by some new and more capable, less expensive, and equally revolutionary technology. This new paradigm will open its own new commercial opportunities, provide better public safety and security, and will bring with it a new set of legal, moral, ethical, and social challenges for humanity. We are beginning to see the first glimpses of what may be coming next. Just as GPS replaced Transit, many researchers in governments and academia are looking at what might be the next great leap in providing positioning, timing, and navigation. We know that GPS-style MEO satellite ranging systems are very expensive, have significant limitations, and cannot address several important needs. They cannot work in buildings, underground, or underwater, and this is a significant public safety issue. Firefighters need to be able to navigate inside burning buildings and search in the rubble after an earthquake. There is also the huge cost (over US$22 billion for the U.S.  GPS so far) of managing such MEO constellations and their associated ground systems. And we have not yet addressed the disposal of all those MEO satellites. Solar storms can affect the satellite signals (and destroy satellites), but the largest concern by PNT system operators is the potential for spoofing, jamming and denial of service through either destroying the satellites or denying receivers access to the signals in areas of conflict. We are already seeing this today in the Middle East and Ukraine. There are several new approaches to the determination of positioning and timing data that extend beyond the current generation of PNT technology. In 2021, researchers at the Center for Automated Vehicle Research with Multimodal Assured Navigation (CARMEN) conducted an experiment where they were able to use the carrier signals from SpaceX Starlink communications satellite constellation in LEO to derive a position on the Earth within 7.7 m of its true location. The carrier signal alone was used, and this could provide an early and practical

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alternative to GPS (https://spaceref.com/press-­release/spacex-­satellite-­signals-­ used-­like-­gps-­to-­pinpoint-­location-­on-­earth/). Conceptually, Starlink, with its thousands of very low orbit satellites could become the basis of a very robust and even, potentially, more precise future PNT system. This is due to the very low orbit, higher power transmission, and large number of satellites in view at any time. (https://www.digitaltrends.com/cool-­tech/spacex-­starlink-­gps/). Such a system may be of interest to SpaceX, as it could provide the very high precision and guaranteed availability PPP (Precise Point Positioning) navigation required for Tesla automated driving, precision agriculture, and other related commercial and military uses relevant to their businesses. Hybrid and Autonomous Positioning Systems, or HAPS, is an advanced concept where once a navigation and timing system is initialized, it can operate independently (including inside buildings) for extended periods without requiring frequent external updating. The ultimate goal is to have a very small (single integrated chip) PNT system that includes its own, low-cost atomic clock and a high-quality ring laser gyro Inertial Navigation System (INS) that does not drift in the way that current generation systems do. There are currently PNT systems that are linked with INS systems, for commercial aviation for example, but these drift significantly over time. Such a HAPS system, at least in a cost-effective and portable version, is not available at this time, but new developments in miniaturized, chip-scale atomic clocks and miniaturized inertial measurement systems could be integrated with an advanced PNT system in the future. Remember what the first military GPS unit looked like, and how far we have come since then. Advanced concepts such as real-­ time data comparison with the real world could make available indoor positioning that would be very useful for a variety of applications such as military, firefighting, disaster response, and public safety. Advanced hybrid PNT systems are also under development, integrating external data for updates. DARPA, the U.S. Defense Advanced Research Projects Administration, funded the development of Transit, and was instrumental in the development and evolution of GPS. It is now in the forefront of conducting research in technologies that might replace the current PNT model. DARPA has funded several studies of interesting concepts, and some of these are described in the open literature, some are classifed. The Director of DARPA has stated “Position, navigation, and timing are as essential as oxygen for our military operators. Now we are putting new physics, new devices, and new algorithms on the job so our people and our systems can break free of their reliance on GPS.” One article describes five DARPA concepts that might replace the current paradigm, as described briefly below, as an indication of where things might be going. (https://www.darpa.mil/news-­events/2014-­07-­24). Adaptable Navigation System (ANS). This project includes the development of significantly improved inertial measurement unit (IMU) systems that would apply quantum physics to allow IMUs to be able to operate for extended periods without updating from external sources, as are required today. This would allow small, portable systems that, once aligned, would be able to operate in buildings, underwater, etc. without external updating. This involves very advanced technology, including cold-atom interferometry measuring the rotation of a cloud of atoms as the reference. Another area of interest is the use of what are referred to as ‘non-navigational

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Fig. 9.3  The DARPA-­ developed “PNT on a chip’ on a U.S. penny. (Reprinted from https:// www.darpa.mil/news-­ events/2013-­04-­10. © 2024, DARPA. All rights reserved)

electromagnetic signals’ for additional spatial referencing. These could include non-PNT satellite transmissions, radio and TV, and even lightning strikes tor navigation in areas where PNT signals are degraded or denied. MicroPNT- Microtechnology for Positioning, Navigation, and Timing. This project is focused on Size, Weight, and Power (SWAP) improvements for PNT capabilities. It involves the use of DARPA-developed microelectromechanical systems, or MEMS, as shown in Fig. 9.3 above to significantly miniaturize future PNT systems. This project has developed a prototype chip-scale device with multiple gyroscopes, accelerometers and a master clock, all on a single chip smaller than a penny, as shown above in Fig. 9.3. The QuASAR project, for Quantum-Assisted Sensing and Readout, is devoted to making the very large, expensive, and delicate atomic clocks used in every PNT system smaller, less expensive, more robust, and with a timing error of less than 1 s in 5 billion years. The development of low cost, small, and reliable atomic clocks has many different applications beyond PNT systems. The PULSE project, Program in Ultrafast Laser Science and Engineering, is related to the QuASAR project, and deals with improvements in Pulsed laser technologies to create atomic clock frequency synchronization, allowing multiple devices to access precise corrections across long distances. Finally, there is STOIC, the Spatial, Temporal, and Orientation Information in Contested Environments program, which is seeking totally non-PNT technologies that can provide the same capabilities in contested environments where existing PNT systems may be degraded or totally unavailable. It involves robust and long-­ range reference signals, stable clocks, and sharing of information among several users.

Back to the Past In addition to these very advanced concepts, the U.S. military services, and presumably others globally, have several projects underway to revisit previous technologies. For example, the U.S. Navy is developing an Automated Celestial Navigation

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System (ACNS) and an improved Inertial Navigation System concept. This would automatically compute a ship’s position day and night using traditional celestial navigation techniques, independent of GPS. The updated position would automatically be fed to the ship’s Inertial Navigation System and moving chart displays. The Naval Sea Systems Command in 2021 awarded the Charles Stark Draper Laboratory a contract to produce ACNS systems through FY 2027 (https://govtribe.com/opportunity/federal-­contract-­opportunity/automated-­celestial-­navigation-­system-­acns-­ n0002422r5232). This system will work day and night, but not, as currently envisioned, under cloud cover. They also have put traditional celestial navigation skills back into the curriculum of the U.S. Naval Academy. Interestingly, the U.S. Air Force used an analog Astroinertial navigation system for navigating its SR-71 s for decades; allowing it to make its Mach 3 dashes around the world at 85,000  feet altitude. (https://theaviationgeekclub.com/the-­sr-­71-­ astroinertial-­navigation-­system-­aka-­r2-­d2-­was-­crucial-­in-­blackbird-­mission-­heres-­ why/). The Nortronics NAS-14 V2 Astroinertial Navigation System provided 90-m positioning using an optical and analog computing system that was quite advanced. The Air Force has a renewed interest in automated celestial and inertial navigation technologies and is investigating this as a redundant capability to GPS. It has at least one other related project that is focused on Resilient Embedded GPS and INS, similar to some of the DARPA work mentioned above (https://www.gao.gov/products/gao-­22-­106010). There are, most certainly, other studies underway in the U.S. and around the world that are more closely guarded. We have become so dependent on PNT that the thought of it being denied has become unacceptable and so alternatives are being actively pursued. Improved, automated celestial navigation systems that are integrated with INS and navigation systems have significant potential and will surely be developed into operational capabilities, either as a backup to GPS or, more likely, integrated with existing systems. It is important to note that Russia, China and Saudi Arabia have all maintained and still use their versions of the LORAN-C radio-based navigation and timing systems, which was decommissioned by the U.S. in 2010. Both Russia and China operate multiple coastal transmission towers which provide a redundant, if slightly less precise timing and navigation capability with no space component (https:// rntfnd.org/wp-­content/uploads/2018/04/China-­eLoran.pdf). China’s system covers the western Pacific Ocean out to Japan and the Philippines and is being extended to cover the mainland and western portions of the country, far from the ocean (https:// rntfnd.org/wp-­content/uploads/2018/04/China-­eLoran.pdf). Russia’s LORAN-C system called Chayka (Seagull) operates 5 segments around its massive coastlines. There is an important transmission location on the Crimean Peninsula, and Russia is reportedly using the Chayka system in their war in Ukraine. The idea of other nations fielding upgraded LORAN-type systems as a relatively low-cost redundant backup to PNT systems is an interesting possibility. The U.S. spent millions developing the eLORAN system in the 1990–2010 timeframe, but after this work was completed and a full design was completed, all LORAN systems were decommissioned when GPS became operational. Perhaps it is time to bring an advanced eLORAN system back? (https://rntfnd.org/wp-­content/

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uploads/eLoran-­Definition-­Document-­0-­1-­Released.pdf). Such a very high power, low frequency ground-based system would be a powerful compliment to the low power, high frequency space-based PNT systems, and could be fielded for relatively low cost compared to a new space system. It would provide a powerful redundant capability for both timing and position. China is reported to have also been developing and fielding thousands of km of buried fiber optic lines dedicated to providing a robust national precise timing system that, again, has no space component. All with the goal of providing a redundant and secure timing capability that cannot be degraded by attacking their space-based BeiDou system (https://www.nytimes.com/2024/03/28/world/asia/as-­threats-­in-­ space-­mount-­us-­lags-­in-­protecting-­key-­services.html). Such an approach, buried fiber optic lines, provides a hardened and redundant capability that may also be copied by others in the future. As with all other space systems, PNT satellites will certainly follow the trend of becoming smaller, using less power, and being less expensive. We may even see micro-scale systems, and we cannot know how far these trends can go. As satellites become smaller and more efficient, and launch systems become ever more reliable and less expensive, we may indeed see many more national systems, once the costs become manageable for a larger group of nations. There will also continue to be a move from MEO to LEO orbits, joint use LEO mega constellation systems, and more. We will even see purely private and commercial systems, but there will be significant legal and policy issues to be overcome, given the dual use nature of these technologies. The first of these new, commercial PNT systems is already in development in the U.S.  The California-based company Xona, partly funded by the venture arm of Lockheed Martin, is developing a LEO commercial PNT constellation of between 250 and 300 smallsats for the aerial drone, self-driving vehicles, and military markets. They announced in 2024 a contract with Belgian Aerospacelabs to construct the first batch of satellites (https://www.xonaspace.com/news). And in Europe, ESA in 2024 has funded two similar projects with 78.4 million Euros approved for each in a program called LEO-PNT. The contracts went to GMV Aerospace and Defence, along with OHB systems, and the second went to Thales Alenia Space. Both contracts are focused on providing PNT services in LEO to augment the existing Galileo system, providing stronger signals in urban zones, IOT, and to provide additional anti-jamming capabilities. The program was oversubscribed at the ESA ministerial, an indication of the interest in the topic, and there was also funding for Cislunar ideas. (https://www.esa.int/ESA_Multimedia/ Images/2022/07/LEO_PNT). We will certainly see a continued concern with the threats all satellite-based systems face regarding signal jamming and spoofing that can only be addressed with long-term system stability offered by some of these new research developments. And, finally, there may indeed be a totally unforeseen new technology, completely unrelated to our current concepts of PNT, that may emerge to begin the technology development cycle again.

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One interesting possibility is the use of X-Ray pulsars from deep space for timing and navigation. This has been referred to as X-NAV, or PulseNav, and such a system could not be jammed or spoofed. A sensor would compare pulsar X-ray signals and compare these with known pulsar locations in the universe. Several experiments have been conducted in the U.S., Europe, and China for both spacecraft and also for aircraft (https://www.nature.com/articles/d41586-­018-­00478-­8) https:// www.nasa.gov/missions/tech-­demonstration/nasa-­navigation-­tech-­shows-­timing-­ really-­is-­everything/. There is also research into using localized microvariations in the Earth’s magnetic field to provide updates to Inertial Reference Systems without satellites, but this would be in the future. In conclusion, PNT in all its various aspects is a fascinating story. Born in the cold war for military purposes, very smart people created a completely new technology that has impacted nearly every aspect of our lives today. And this will only increase in the future with the growing interconnected digital world that is rapidly emerging. It is proper to allow the visionary Sir Arthur C. Clarke, who foresaw this all in 1945, the last words: Before you become too entranced with gorgeous gadgets and mesmerizing video displays, let me remind you that information is not knowledge, knowledge is not wisdom, and wisdom is not foresight. Each grows out of the other, and we need them all.——Arthur C. Clarke.

Chapter 10

Top Ten Things to Know About Positioning, Navigation, and Timing

1. Space-based PNT systems play an important and growing role in our lives and have become an invisible utility serving a growing number of purposes around the globe for all of us. They impact everyone daily, regardless of who you are and where you live. 2. Nation states will continue to support their own PNT systems for their own benefit. 3. Space PNT technologies are relevant to all nations, regardless of their level of economic or technological development. They provide public safety, commercial, and dual use capabilities to all, and have significant potential to better the lives of billions in the less developed parts of our planet. 4. A day without PNT would be a very bad day on the ground, and many military, transportation, financial, IT, and other services would be disrupted. A long-term stoppage would be very, very damaging. There needs to be serious planning regarding how we would respond to such an outage and plans need to be in place to react quickly to minimize the financial impact and social disruption that would be caused. 5. The integration of many different types and sources of data using Geomatics is the key to the future. In the coming years, PNT data will be increasingly integrated with other types of information and services. The Internet of Everything (IOE) is coming fast, and PNT systems will be a major, if hidden, part of all of this. It is happening now. 6. PNT systems have quickly become an invisible utility that supports many practical benefits that people do not consider to be involving space, and these systems depend upon complex and expensive government-operated systems that are provided to the public free of charge. People often do not realize how serious the consequences would be if vital ‘free’ government services such as weather forecasting, space weather forecasting, disaster response, and PNT systems were to disappear. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4_10

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7. There will be many more competing (or complimentary) systems in the very near future, with much redundant cost and limited overall benefits to the end users. This is driven by the dual use nature of the technologies, tremendous commercial potential, and national pride, among other things. Such redundancy will be beneficial if we do loose a system due to accident, intentional acts, or space weather event such as a coronal mass ejection. Some redundancy is good, but an event that takes one system out is likely to impact many or even all. 8. It is uncertain if all PNT providers will continue to offer free and ‘in the clear’ services to all, or if we will devolve into competing, protected regional markets mandating the use of the local technologies within protected markets. Some PNT providers already require local products to use their own services, and this may continue and expand. 9. In the future we will find even more uses of PNT services, new commercial markets, and it will all become integrated with the emerging Internet of Things, the Internet of Everything, and more. But there are very important legal, moral, and ethical issues to be addressed. Technology always outpaces the regulatory and legal framework, and this will only accelerate. We must consider the ethical and moral implications of what we do in space and here on the Earth. 10. Perhaps one day we will have a single, integrated, efficient and international PNT capability, but this will not happen in the immediate future. The LunaNet project is a step in this direction. Perhaps we can do this together.

Selected Bibliography

Andrade, A. A. L. (2001). The global navigation satellite system: Navigating into the new millennium (Ashgate studies in aviation economics and management). Ashgate Pub Ltd. ISBN: 0754618250. Capderou, M. (2005). Satellites, orbits and missions. Springer Verlag France. Davis, K. (1990). Ionospheric Radio. Peter Peregrinus Ltd.. European Space Agency. (2004). Galileo: Galileo reference troposphere model for the user receiver. ESA-APPNG-REF/00621-AM. European Space Agency, Centre National d’ Etudes Spatiales. (2009). User guide for EGNOS application developers. European Communities. Hofmann-Wellenhof, B., Legat, K., & Wieser, M. (2003). Principles of positioning and guidance. Springer. Hofmann-Wellenhof, B., Lichtenegger, H., & Wasle, E. (2008). GNSS-global navigation satellite systems (GPS, Glonaess, Galileo and more). Springer. Hudnut K., Bock Y., Galetzka J., Webb F., & Young W. The Southern California integrated GNSS network (2001, March 19–22). 10th FIG international symposium on deformation measurements, Orange. Lau, L., Pattinson, M., & Sheridan, K. (2003, April 22–25). Adapted precise point positioning within Galileo development and validation activities. In Proceedings of European navigation conference. Madry, S. (2014). Space systems for disaster warning, response and recovery (p. 146). Springer Press. http://www.springer.com/us/book/9781493915125 Madry, S. (2019). Disruptive space technologies and innovations: The next chapter (p.  252). Springer Nature. Madry, S., Martinez, P., & Laufer, R. (2018). Innovative design, manufacturing and testing of small satellites (p. 161). Springer Nature. O’Connor, A. C., Gallaher, M. P., Clark-Sutton, K., Lapidus, D., Oliver, Z. T., Scott, T. J., Wood, D. W., Gonzalez, M. A., Brown, E. G., & Fletcher, J. (2019, June). Economic benefits of the Global Positioning System (GPS). RTI Report Number 0215471. Sponsored by the national institute of standards and technology. RTI International. Parkinson, B. W., & Enge, P. K. (1996). Differential GPS. In B. W. Parkinson & J. J. Spilker (Eds.), Global positioning system: Theory and applications (Vol. II, pp. 3–115). American Institute of Aeronautics and Astronautics. Parkinson, B. W., Spilker, J. J., Jr., & (eds). (1996). Global positioning system: Theory and applications (Vol. I). AIAA. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 S. Madry, Global Navigation Satellite Systems and Their Applications, SpringerBriefs in Space Development, https://doi.org/10.1007/978-3-031-74488-4

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Selected Bibliography

Pelton, J., Madry, S., & Camacho-Lara, S. (Eds.). (2017). Handbook of satellite applications (Vol. 2, 2nd ed., p. 1556). Springer. Ruiz, Jennifer L., & Frey, Charles H., (2005, September). Geosynchronous satellite use of GPS. In Proceedings of the 18th international technical meeting of the satellite division of the institute of navigation (ION GNSS 2005). (pp. 1227–1232). Schaer, S., Beutler, G., Rothacher, M., Brockmann, E., Wiget, A., & Wild, U. (1999, July 21). The impact of the atmosphere and other systematic errors on permanent GPS networks. Paper Presented at the IAG symposium on positioning. Shaw, M., Turner, D. A., & Sandhoo, K. (2002). Modernization of the global positioning system. In Proceedings of GPS JIN 2002. Japan Institute of Navigation. Skone, S. H., & Shrestha, S. M. (2003). Strategies for 4-D regional modeling of water vapour using GPS. Wuhan University Journal of Natural Sciences, 8, 627–635. van Diggelen, F. (2009). A-GPS, Assisted GPS, GNSS, and SBAS. Artech House. Wang, J., Rizos, C., & Han, S. (2003). Predicting atmospheric biases for real time ambiguity resolution in GPS/GLONASS reference station networks. Journal of Geodesy, 76, 617–628. Zogg, J.-M. (2010). GPS: Essentials of navigation (Compedium). u-blox AG. Thalwill. Available at: http://bit.ly/fhT71T

Selected Global PNT Web Sites and Resources ESA Navipedia. https://gssc.esa.int/navipedia/index.php?title=Main_Page GISCorps. http://www.giscorps.org Glonass. https://glonass-­iac.ru/en/about_glonass/ GPSWorld. http://gpsworld.com Inside GNSS. http://www.insidegnss.com QGIS Open Source GIS software. http://qgis.org The current US GPS System and GPS modernization performance standards. http://www.gps.gov/ technical/ps/2008-­SPS-­performance-­standard.pdf The Future of GNSS. https://courseware.e-­education.psu.edu/downloads/geog862/Lesson4_ FutureGNSS.pdf Trimble Online GNSS Planning Tool. https://www.gnssplanning.com/#/settings U.S. Coast Guard navigation center. http://www.navcen.uscg.gov U.S.  Dept. of Transportation What is PNT?. https://www.transportation.gov/pnt/ what-­positioning-­navigation-­and-­timing-­pnt U.S.  Naval Information Warfare Center Satellite-Based Positioning, Timing, and Navigation (PNT). https://apps.dtic.mil/sti/trecms/pdf/AD1169371.pdf UN Office of Outer Space Affairs (OOSA). http://www.oosa.unvienna.org UNAVCO. http://www.unavco.org UNAVCO GPS/GNSS Data Page. http://www.unavco.org/data/gps-­gnss/gps-­gnss.html UNOOSA GNSS Educational curriculum. http://www.oosa.unvienna.org/pdf/icg/2013/Ed_ GNSS_eBook.pdf US Government GPS site. gps.gov. http://www.gps.gov