Development and Application of Small Spaceborne Synthetic Aperture Radars [1 ed.] 9780309582650

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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS

Space Studies Board National Research Council

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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS

Committee on Earth Studies Space Studies Board Commission on Physical Sciences, Mathematics, and Applications National Research Council

NATIONAL ACADEMY PRESS Washington, D.C. 1998

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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by Contract NASW 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor. Copyright 1998 by the National Academy of Sciences. All rights reserved. Copies of this report are available from Space Studies Board National Research Council 2101 Constitution Avenue, N.W. Washington, D.C. 20418 Printed in the United States of America

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COMMITTEE ON EARTH STUDIES MARK R. ABBOTT, Oregon State University, Chair OTIS B. BROWN, Rosenstiel School of Marine and Atmospheric Science DANIEL J. JACOB, Harvard University CHRISTIAN J. JOHANNSEN, Purdue University VICTOR V. KLEMAS, University of Delaware M. PATRICK McCORMICK, Hampton University BRUCE D. MARCUS, TRW ARAM M. MIKA, Lockheed Martin Missiles and Space RICHARD K. MOORE, University of Kansas DALLAS L. PECK, U.S. Geological Survey (retired) WALTER S. SCOTT, EarthWatch GRAEME L. STEPHENS, Colorado State University KATHRYN D. SULLIVAN, Columbus Ohio's Center of Science and Industry FAWWAZ T. ULABY, University of Michigan THOMAS T. WILHEIT, JR., Texas A&M University EDWARD F. ZALEWSKI, University of Arizona ARTHUR A. CHARO, Senior Program Officer INA B. ALTERMAN, Senior Program Officer (from May 1997) CARMELA J. CHAMBERLAIN, Senior Project Assistant

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SPACE STUDIES BOARD CLAUDE R. CANIZARES, Massachusetts Institute of Technology, Chair MARK R. ABBOTT, Oregon State University JAMES P. BAGIAN,* Environmental Protection Agency DANIEL N. BAKER, University of Colorado LAWRENCE BOGORAD, Harvard University DONALD E. BROWNLEE, University of Washington JOHN J. DONEGAN,* John Donegan Associates, Inc. GERARD W. ELVERUM, JR., TRW Space and Technology Group ANTHONY W. ENGLAND, University of Michigan MARILYN L. FOGEL, Carnegie Institution of Washington MARTIN E. GLICKSMAN,* Rensselaer Polytechnic Institute RONALD GREELEY, Arizona State University BILL GREEN, former member, U.S. House of Representatives ANDREW H. KNOLL, Harvard University JANET G. LUHMANN,* University of California, Berkeley ROBERTA BALSTAD MILLER, CIESIN BERRIEN MOORE III, University of New Hampshire KENNETH H. NEALSON,* University of Wisconsin MARY JANE OSBORN, University of Connecticut Health Center SIMON OSTRACH, Case Western Reserve University MORTON B. PANISH, AT&T Bell Laboratories (retired) CARLÉ M. PIETERS, Brown University THOMAS A. PRINCE, California Institute of Technology MARCIA J. RIEKE,* University of Arizona PEDRO L. RUSTAN, JR., U.S. Air Force (retired) JOHN A. SIMPSON, Enrico Fermi Institute GEORGE L. SISCOE, Boston University EDWARD M. STOLPER, California Institute of Technology RAYMOND VISKANTA, Purdue University ROBERT E. WILLIAMS, Space Telescope Science Institute JOSEPH K. ALEXANDER, Director (as of February 17, 1998) MARC S. ALLEN, Director (through December 12, 1997)

* Former member.

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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS ROBERT J. HERMANN, United Technologies Corporation, Co-chair W. CARL LINEBERGER, University of Colorado, Co-chair PETER M. BANKS, Environmental Research Institute of Michigan WILLIAM BROWDER, Princeton University LAWRENCE D. BROWN, University of Pennsylvania RONALD G. DOUGLAS, Texas A&M University JOHN E. ESTES, University of California at Santa Barbara MARTHA P. HAYNES, Cornell University L. LOUIS HEGEDUS, Elf Atochem North America, Inc. JOHN E. HOPCROFT, Cornell University CAROL M. JANTZEN, Westinghouse Savannah River Company PAUL G. KAMINSKI, Technovation, Inc. KENNETH H. KELLER, University of Minnesota KENNETH I. KELLERMANN, National Radio Astronomy Observatory MARGARET G. KIVELSON, University of California at Los Angeles DANIEL KLEPPNER, Massachusetts Institute of Technology JOHN KREICK, Sanders, a Lockheed Martin Company MARSHA I. LESTER, University of Pennsylvania NICHOLAS P. SAMIOS, Brookhaven National Laboratory CHANG-LIN TIEN, University of California at Berkeley NORMAN METZGER, Executive Director

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FOREWORD

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Foreword

Observations of Earth from space have value for scientific research, for commerce, and for the public welfare. Synthetic aperture radar (SAR) is one sophisticated observational technique receiving rapidly increased attention from all three sectors in the United States, Europe, and Japan. A major impediment to rapid exploitation of SAR has been the cost associated with orbiting the massive and complex instrumentation that has heretofore been necessary. This report addresses issues associated with achieving effective SAR capabilities in the context of a “smaller, faster, cheaper” implementation, a so-called “small SAR.” The report assesses the current state of the technology and the science, and it makes recommendations designed to enhance the success of a small-SAR program. These include the need to focus the mission objectives and concentrate on key enabling technologies, which are important characteristics of achieving more efficient and cost-effective missions in other areas of space research as well. Other considerations about necessary additional research, the interaction between research and commercial interests, and international coordination are more specific to SAR. Success in implementing a more affordable SAR could have profound implications for understanding our planet ecology, the assessment of natural disasters, and commercial agriculture, to name a few. This report is intended to aid in achieving that potential. Claude R. Canizares, Chair Space Studies Board

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FOREWORD viii

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ACKNOWLEDGMENTS

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Acknowledgments

This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report: Herbert Friedman, E.O. Hulbert Center for Space Research, Naval Research Laboratory, Gordon Pettengill, Massachusetts Institute of Technology, Jack L. Walker, Environmental Research Institute of Michigan, and R. Keith Raney, Johns Hopkins University Applied Physics Laboratory.

While the individuals listed above have provided many constructive comments and suggestions, responsibility for the final content of this report rests solely with the authoring committee and the NRC.

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ACKNOWLEDGMENTS x

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CONTENTS xi

Contents

EXECUTIVE SUMMARY 1

1 INTRODUCTION Overview Request from NASA The Small SAR Option for NASA 6 6 7 9

2 SAR ISSUES Background and History Trade-offs in Synthetic Aperture Radar Small SAR Mission Enhancements 12 12 13 22

3 VALIDATION RESEARCH Vegetated Surfaces (Forests, Rangeland, and Agriculture) Ice Sheets and Glaciers Oceanography Hydrology Solid Earth Land Use 28 28 30 30 31 32 33

4 DATA COLLECTION AND DISSEMINATION 34

5 RECOMMENDED STRATEGY Six-Point Strategy Conclusion 36 36 39

REFERENCES 40

APPENDIXES A ACRONYMS AND ABBREVIATIONS B LETTER REPORT OF APRIL 1995 47 49

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CONTENTS xii

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EXECUTIVE SUMMARY

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Executive Summary

SPACEBORNE SYNTHETIC APERTURE RADAR Background and Task Following a decline in imaging radar research in the 1970s and 1980s, the 1990s have witnessed a resurgence of activity as researchers apply active and passive microwave capabilities to Earth observations. In the past few years, in particular, there has been a remarkable increase in studies based on European, Canadian, and Japanese free-flying synthetic aperture radars (SARs), as well as on the series of Shuttle-based SAR flights (SIR [Shuttle Imaging Radar]-A, SIR-B, and the U.S.-Germany-Italy SIR-C/X-SAR). SAR interferometry is among the capabilities driving exciting applications in solid-earth studies. In addition, biomass estimation, ecosystem delineation, ice dynamics characterization, and biological water monitoring also have progressed. Multifrequency and multipolarization SAR systems are rekindling interest in the variety of unique Earth parameters that can be measured. The present study originated in 1994 with a request from the National Aeronautics and Space Administration's (NASA's) Office of Earth Science (OES—formerly the Office of Mission to Planet Earth) to assess the utility of a third SIR-C/X-SAR mission. In a letter report dated April 4, 1995 (see Appendix B), the Committee on Earth Studies of the Space Studies Board concluded that a third flight would produce useful scientific results if the existing instrumentation were simply reflown, but that it would produce especially worthwhile results if it were modified for dual-antenna interferometric measurements of terrain topography. In the 1995 letter report, the committee also summarized the current capabilities of SAR applications in ecology, ice sheets and glaciers, oceanography, hydrology, and solid-earth studies. During the period following release of the letter report, events have unfolded regarding a proposed NASA small spaceborne SAR program, often referred to as “LightSAR” but called “small SAR” in this report to avoid confusion with a specific proposal from the Jet Propulsion Laboratory (JPL).1 The stated objective of the LightSAR program is “to validate key advances in synthetic aperture radar technology, and related systems, that will reduce the cost and enhance the performance of this and future U.S. [Earth-imaging] SAR missions.”2 NASA's interests in a small SAR are twofold: (1) to exploit the scientific utility of SAR data and (2) to investigate the opportunity for an innovative industry-government partnership for a small SAR that would take advantage of the potentially high commercial interest in SAR applications. On December 5, 1996, NASA requested an update on the committee's perspective since the SAR study began. Specifically, NASA requested comments on the “value added” of a multifrequency small SAR as an alternative to a single-frequency operation, which was the baseline proposal, and an analysis of other SAR-related issues, such as reducing system costs,

1

The term “LightSAR” has been associated with proposals from NASA's Jet Propulsion Laboratory. However, unless otherwise noted, the term “small SAR” is used in this report to denote a generic class of comparatively small and inexpensive spaceborne synthetic aperture radars. 2 Business Development and System Design Definition Study Contracts for the LightSAR Program, Commerce Business Daily Procurement Alert, November 20, 1996.

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EXECUTIVE SUMMARY

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optimizing weight and power requirements, and increasing mission focus. In addition, NASA requested guidance in developing a strategy for a space-based, science-oriented, interferometric small SAR. This report responds to those requests, expanding on ideas presented in the committee's April 1995 letter report. In addition, this report emphasizes that a strategy for a space-based, science-oriented, interferometric small SAR must also consider mission focus, design trade-offs, and options for data availability. STRATEGY AND RECOMMENDATIONS Existing SAR systems have been severely constrained by their very large volume, mass, and power requirements. Such demands have inhibited the approval of even experimental systems but are especially problematic for operational systems whose requirements for coverage (geographic and repeat cycle) lead to system design concepts that require maintaining several spacecraft continuously in orbit. NASA and National Oceanic and Atmospheric Administration (NOAA) studies of future sensing needs describe research and operational requirements leading to a need for multiple spacecraft with markedly differing characteristics (e.g., Winokur, 1996). However, the LightSAR baseline design proposed by JPL appears to incorporate new technologies in instrument design and antennas that could result in significant size, mass, power, and cost savings compared to existing international SAR systems, but it does not adequately address coverage requirements for multiple users. In the committee's opinion, if NASA proceeds with a small SAR, it should give preference to a mission that optimizes for a specific scientific goal or related application. Additionally, consideration should be given to meeting the needs of public use and commerce within design constraints imposed by the science requirements. In addition, the goal or application should be selected to address ongoing public needs (e.g., natural disaster assessment and global topographic mapping), future high-profile commercial potential (e.g., forestry or agricultural assessment), or specific science demonstrations (e.g., ice-flow dynamics and volcanic lava flow rates). The duty cycle should be used to build orbit-by-orbit data sets related to these applications so that over the life of the mission, experience would increase and the global dimensions of the objectives could be further quantified and validated. In the committee's judgment, spaceborne SAR will become increasingly important in achieving the objectives of NASA's Earth Science Enterprise (ESE—formerly Mission to Planet Earth) science strategy, which is a deeper understanding of the five major components of the Earth system: hydrological, biogeochemical, atmospheric, ecological, and geophysical processes. Different uses for small SAR will likely require different data acquisition modes, which may lead to conflicts unless a clear policy is defined early in the mission design process. The committee recommends that NASA consider the following strategy for a small SAR program. 1. Develop a well-defined focus for any small SAR mission. It is important for NASA to consider what objectives are to be served by a potential spaceborne SAR system, in a broad sense, and their relative priorities. Three general areas are recognized: (1) providing scientific data (e.g., of the type required by ESE), (2) providing

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EXECUTIVE SUMMARY

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information in support of the general public good (e.g., environmental monitoring and hazard assessment), and (3) providing data for commercial interests (e.g., digital elevation models for cartographic applications, mineral exploration, or forest management). The committee recommends that the relative priorities and interests of all three use categories be weighed at the outset of the mission design process. End-to-end system engineering can then be optimized to serve the prioritized suite of information needs. The committee notes that of the several proposed operating frequencies associated with SARs, the L-band is especially useful in forest and desert ecology applications, but other applications such as agriculture may lead to a small SAR design based on C-, X-, or Ku-band frequencies. These frequencies require smaller antennas than does the L-band, which may simplify deployment from a small spacecraft. In the committee's view, design parameters such as frequency, polarization, resolution, and swath width should be chosen to match the mission focus, while the results of all available research, including that from the 1960s and 1970s, are considered. .

2. Adopt new technologies to reduce SAR costs. In the committee's view, many new technologies may come from outside NASA. In addition, new technologies currently available for data capture and processing can be used to lower overall SAR system costs. Many of these technologies can be evaluated without resorting to costly spacecraft missions. According to JPL's LightSAR point design report (JPL, 1997), the estimated end-to-end mission cost of the baseline design is $125 million, which is only a fraction of the estimated mission costs of the single-frequency, single-polarization SAPs of ERS-1 ($750 million) and Radarsat ($640 million). The significant reduction in cost is attributed to the incorporation of new technologies. As examples of cost-reducing technologies, L-band antennas are seven times lighter and require only half as much power as SIR-C's antenna. Small SAR synthesizers are seven times lighter than SIR-C's and require one-tenth the power. 3. Continue support of a vigorous research and analysis program in radar remote sensing. Although considerable progress has been made in recent years in understanding signal-terrain interactions, there are many areas in which the physical link between the SAR signal and the geophysical phenomenon is less well known. For example, there is a soil-moisture signal present in SAR imagery that relates to the material's dielectric properties, but this component is difficult to extract from signal influences related to surface roughness and topography (Evans et al., 1995). More research is necessary to learn how to quantify the effects of roughness, topography, and surface cover on the soil-moisture signals. NASA should continue appropriate air- and spaceborne studies to strengthen these links. Such calibration and validation studies might be a suitable focus for a small SAR mission. 4. Establish a clearly defined small SAR data policy that will protect commercial interests while ensuring free and open access by the public and research communities. SAR imagery has potentially important applications for research, the public sector, and commercial users. If NASA continues to seek commercial partners for a small SAR mission (to

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EXECUTIVE SUMMARY

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reduce costs), it must define data policies clearly to protect the proprietary interests of commercial entities while ensuring open access to other user communities. In addition, widely distributed data processing and dissemination may both lower costs and increase access to small SAR data. Such a “federated” approach is consistent with the strategy being pursued for the Earth Observing System Data and Information System (EOSDIS). It is expected that much data will be dual-use in nature and can serve multiple interests (science, public use, and commerce). Innovative data access policies could protect both research and commercial communities. For example, most commercial applications may have a relatively short shelf life. When they no longer have commercial value, these older data sets may still be valuable to the research community. However, given the longer time scales for many terrestrial applications (e.g., glacial recessions), the commercial shelf life may be many years for some data sets. Consideration of public use and commercial interests complicates issues of data access and distribution because of conflicting needs. At the same time, public use of data should also be expected and encouraged, but many such users may not be able to afford commercial rates for access to data. The committee recognizes that flexibility must be maintained in the data acquisition and dissemination system. The concept of life-cycle mission design should be applied to minimize conflicts in scheduling SAR operating modes. The various scientific objectives of the LightSAR science plan imply that conflicts will arise and require dissimilar data types over common areas. Such conflicts will be exacerbated by the differing needs of public and commercial users. Mission life-cycle planning can be used to balance conflicting needs weighted by relative priority. 5. Consider an enhanced multifrequency small SAR configuration. There is sufficient evidence to warrant consideration of a multifrequency small SAR (Evans et al., 1995; Dobson et al., 1997). Single-frequency, single-polarization spaceborne SAR systems cannot meet all of the scientific objectives outlined in the LightSAR science plan. These needs might be met at some level of accuracy by a single-frequency polarimetric small SAR as defined in the baseline plan. For some applications, the accuracy level is known to increase markedly with the addition of higher-frequency SAR data. Such increased accuracy may be very compelling to industrial partners seeking to satisfy commercial demands. Industry teams can be expected to pay close attention to the end-to-end costs of any system enhancements relative to the expected commercial value of such a system. The commercial value of LightSAR enhancements is currently being evaluated by the marketplace via the LightSAR business development and system design definition studies (JPL, 1997). It may be prudent for NASA to conduct a parallel evaluation of the relative scientific value of potential enhancements (e.g., C-band dual polarization and X-band single polarization). 6.

Continue coordinating small SAR with other international SAR missions in an Integrated Global Observing Strategy framework.

Although there are difficult issues associated with international coordination of radar missions, the committee believes that NASA should continue to coordinate small SAR with other international SAR missions within the Integrated Global Observing Strategy (IGOS) framework.

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EXECUTIVE SUMMARY

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No single nation has the resources to deploy the constellation of satellites necessary to exploit this technology fully or to test the advantages and disadvantages of different combinations of spectral bands or types of data from different sensors.

CONCLUSION Recent technological advances that can significantly reduce the size and cost of spaceborne SAR, advances in data capture and processing, the advantages of SAR over electro-optical imaging, and potential trade-offs to reduce the weight of a SAR all led the committee to conclude that focused applications of a multifrequency small SAR mission, as opposed to one with a single-frequency system, could provide more and better information and understanding of earth, ocean, and atmospheric processes at lower costs than were heretofore possible.

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INTRODUCTION

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1 Introduction

OVERVIEW The Committee on Earth Studies (CES) of the Space Studies Board is a standing committee charged with examining all areas of remote sensing of the Earth from space for civilian and related purposes. The charter includes the satellite-based Earth observation programs of the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), as well as the merged polar-orbiting environmental satellites of NOAA and the former Defense Meteorological Satellite Program. In 1995, at the request of NASA's Office of Earth Science (formerly Office of Mission to Planet Earth), the committee began a two-part study on issues related to the development and utility of spaceborne synthetic aperture radar (SAR). The potential of SAR for Earth science commercial and civil applications is being advanced by several satellite systems, including Shuttle-based SAR flights (SIR [Shuttle Imaging Radar]-A, SIR-B, SIR-C/X-SAR1) and the European Space Agency's (ESA's) ERS-1 and ERS-2. Significant contributions are also being realized from Japan's JERS-1 and Canada's Radarsat. Future systems such as ESA's Advanced Synthetic Aperture Radar (ASAR) and Japan's Ministry of International Trade and Industry (MITI) SAR-2 and Phased Array type L-band Synthetic Aperture Radar (PALSAR), an instrument proposed for the Advanced Land Observing Satellite (ALOS), promise to add even more data and processing knowledge to the global pool of SAR experience (Table 1.1). Aircraft support data from NASA's AIRSAR, Germany's E-SAR, and the Netherlands' PHARUS, among others, are used to complement these space measurements but also have helped to advance general understanding of radar reflection from the ground. These airborne systems continue to validate earlier NASA and commercial airborne radar applications and to expand understanding of signal-terrain interactions. Nevertheless, the development of satellite SAR systems has lagged behind electro-optical systems popularized by the Landsat program inaugurated in 1972. The Landsat paradigm reoriented the remote sensing community toward detection of temporal change and away from the 1960s paradigm of spectral analysis of landscapes. The inertia generated by more than a decade of such focus translates today into a nation whose familiarity with SAR data lags far behind its understanding of data sets derived from sensors operating in the visible near-infrared (VNIR) and thermal infrared (TIR) spectra. Data from all spectral regions, including the microwave region, produce unique and, in some cases, vital information for Earth system science. Determination of the potential information content, however, and means of extracting that information from SAR

1

The Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) is the most advanced imaging radar system to fly in Earth orbit. Carried in the cargo bay of the Space Shuttle Endeavor in April and October of 1994, SIR-C/X-SAR simultaneously recorded SAR data at three wavelengths (L-, C-, and X-bands; 23.5, 5.8, and 3.1 cm, respectively). In addition, the full polarimetric scattering matrix was obtained by the SIR-C instrument at L- and C-band over a variety of terrain and vegetation types. The integrated system is steerable in look angle (electronically in the case of SIR-C, mechanically in the case of X-SAR) to obtain data in the angular range of 15 to 60 degrees. Imaging resolution varies from about 10 to 50 meters, depending on the geometry and data-taking configuration.

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INTRODUCTION

7

images require much development. As a result of the global priority for electro-optical sensor data, there has been a dearth of accessible satellite SAR data for translating airborne applications to validated satellite applications. Consequently, the science community has had a delayed learning curve for using SAR data, a major part of which has involved research in data interpretation. Validating SAR's utility for measuring soil moisture is one of several deferred developments, even though studies have shown that soil-moisture patterns can be detected under known signal-terrain circumstances. Electro-optical sensors, as popular as they have become, are physically limited by changing atmospheric conditions (e.g., cloud cover, fog, and dust), which may be persistent phenomena locally or regionally or which may be expected to accompany natural disasters. In many regions of the world, one cannot reliably acquire a surface image from an electro-optical sensor when it is most needed. Given these considerations, there are several advantages to SAR: (1) because of their day-night, all-weather capability, microwave systems represent the best approach to collecting interpretable data for a given region at a specific time; (2) unlike those from electro-optical systems, signals returned by radar systems are sensitive to the physical structure and moisture content of the surface being sensed and may offer avenues for obtaining important results for research and applications that are not otherwise available; and (3) depending on how the data are processed (e.g., as images, or as interferograms), SAR data provide Earth scientists with unique means for extracting information at scales of reference not possible with electro-optical systems. For the reasons given above, the secondary role of radar imaging systems relative to electro-optical systems could be reversed in the future for certain applications. Whether or not this comes to pass, in the committee's view it is important to recognize that active microwave systems have already demonstrated their usefulness in Earth system science and that still further development of active microwave capabilities is possible. Active microwave sensors have not had a prominent role in the Earth Observing System (EOS), but an affordable spaceborne SAR could play an important role in the future and, for some applications, might be indispensable. Throughout the committee's deliberations, it was evident that developing a more advanced application for validated airborne radar and satellite SAR results would be a complicated but important effort. SAR is proving too valuable for the science and applications community to be content with its relegation to secondary status in the electromagnetic spectrum. The neglect of SAR capability in Earth studies has led to a need to develop interpretation algorithms. There are recognized needs for further validation studies for standard image analysis as well as for interferogram applications. To ensure these developments, it will be necessary to make SAR data vastly more accessible to the science community. REQUEST FROM NASA The SAR study originally requested by NASA posed eight questions regarding the utility of a third Shuttle Radar Laboratory (SRL) mission. These questions were addressed in a letter report to Dr. Charles Kennel dated April 4, 1995 (see Appendix B), which suggested that such a mission would continue the learning curve initiated by the SIR-C/X-SAR experiments and might, in particular, permit a limited scope for dual-antenna interferometric analyses. Also in that letter, the committee summarized the current state of SAR applications in ecology, ice sheets and

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INTRODUCTION 8

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INTRODUCTION

9

glaciers, oceanography, hydrology, and solid earth studies. Between the letter report and this report, events have continued to unfold regarding a proposed small SAR mission. Small in this context refers to comparatively inexpensive spaceborne SAR, not to the specific LightSar baseline proposal of the Jet Propulsion Laboratory (JPL).2 The stated objectives of the LightSAR program are “to validate key advances in synthetic aperture radar technology, and related systems, that will reduce the cost and enhance the performance of this and future US [Earth-imaging] SAR missions.”3 On December 5, 1996, NASA requested an update on the committee's perspective since the SAR study began. Specifically, NASA requested comments on the value added of a multifrequency small SAR as an alternative to a single-frequency operation, which was the baseline proposal, and an analysis of issues raised by the LightSAR baseline proposal. THE SMALL SAR OPTION FOR NASA NASA's interests in a small SAR are twofold: (1) to exploit the scientific utility of SAR data and (2) to investigate the opportunity for an innovative industry-government partnership for small SAR that would take advantage of the potentially high commercial interest in SAR applications. Objectives It is important to consider what objectives are to be served by a spaceborne SAR system. Three general areas are recognized: (1) providing scientific data (e.g., of the type required by ESE); (2) providing information in support of the general public good (e.g., environmental monitoring and hazard assessment); and (3) providing data to commercial interests (e.g., for cartographic application or mineral exploration). In the committee's view the interests of all three categories should be considered carefully in the mission design process. The committee recognizes the profound need for consistent regional to global SAR observations at moderate to high resolutions to support the science requirements of NASA's Office of Earth Science (OES).4 It also recognizes the difficulty of assessing the full extent of nascent needs. Notably lacking from NASA's request is consideration of needs outside the OES framework. The second area of objectives for using spaceborne SAR encompasses information needs of society that have limited scientific or commercial value. Regional to global environmental monitoring by international agencies and individual countries represents a potentially large market for SAR data. One example is the United Nations Food and Agricultural Organization mandate to compile forest stand records for the Tropical Forest Action Plan. Other examples include the need to assess natural hazards and disasters (e.g., seismic events, weather-related events, and pestilence) that affect public health. It is not realistic to expect that these data will be

2

Typically, with total costs of less than approximately $100 million for the space segment, launch vehicle, mission operations, and processing for the technology validation phase of the program. 3 Business Development and System Design Definition Study Contracts for the LightSAR Program, Commerce Business Daily Procurement Alert, November 20, 1996. p. 1. 4

Formerly the Office of Mission to Planet Earth.

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INTRODUCTION

10

purchased at commercial rates. The interests of this use category have not been represented adequately in earlier designs, but in the committee's opinion, they should be. The information needs of commercial interests are being defined by the Phase B design studies of the LightSAR program. Existing markets include surveillance and cartography. Most surveillance applications require high spatial resolution and near-real-time to real-time access to data. Emerging markets are difficult to assess because the applications are often poorly developed. Often the potential market does not currently use such information or has developed alternative methods for acquiring it. Some examples of these types of applications include exploring for nonrenewable resources, monitoring and assessing renewable resources, and predicting and monitoring natural hazards. As the CES report was being compiled, the expected sizes of these markets and the extent to which commercial needs might modify the small SAR baseline design were being evaluated in the LightSAR System Design Definition Study (JPL, 1997). The committee recommends that the relative priorities and interests of all three categories of objectives be carefully weighed at the outset of the mission design process. End-to-end system engineering can then be optimized to serve the prioritized suite of information needs. The associated costs of the end-to-end information system may be estimated by category and should serve to guide relative costing in government-industry teaming arrangements. Relative Priorities The JPL LightSAR baseline design provides a valuable illustration of the mission design process. Only NASA science requirements and commercial interests are under active consideration in this design. Satisfying NASA science requirements is mandatory in system design and therefore can be considered to have top priority. The science requirements are assigned approximate priorities, with repeat-pass interferometry as the highest.5 Repeatpass interferometry should cover seismically active areas in Japan and western North America at every available opportunity. Because of data rate limitations, interferometry and polarimetry are, in practice, mutually exclusive modes of operation. Hence, the baseline mission plan would preclude the collection of noninterferometric SAR data for these regions either in support of the public good (since these uses are not fully acknowledged or considered) or in support of other enumerated scientific, or yet to be specified, commercial objectives (because of lower relative priority). It must also be recognized that many data can have dual uses. Data collected to serve one interest can also serve another. For example, the proposed LightSAR baseline design science requirements request that Earth's boreal, temperate, and tropical forest belts be imaged at least annually over a 1-month period to address questions related to the carbon cycle. Such data might be valuable also for environmental monitoring by national governments and nongovernmental organizations in the service of the public good. Such data could also be valuable in commercial forestry for stand-level assessment, provided that resolution is adequate. Dual use is desirable in that it expands the application base with minimal impact on hardware design. However, dual use does present challenges with respect to data access and

5

A detailed explanation of interferometry techology is given in Chapter 2.

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INTRODUCTION

11

privileges. Significant policy questions are raised regarding data access and rights that are beyond the scope of this report but must be addressed by NASA. Questions related to validating SAR applications (i.e., reducing system costs, optimizing weight and power requirements, and defining mission focus) are primary considerations in formulating a NASA SAR strategy. These issues form the basis of the committee's response in this second and final report to NASA. This report expands on ideas presented in the letter report of April 1995 (see Appendix B). In keeping with NASA's narrow charge to the committee on December 5, 1996, issues of international cooperation and competition in the development of SAR technology and spacecraft are addressed only briefly in this report.

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SAR ISSUES

12

2 SAR Issues

BACKGROUND AND HISTORY Imaging radar was originally developed for military reconnaissance. Both real-aperture radars (RARs) and synthetic-aperture radars were developed in the 1950s. RARs became available for civilian use in 1965. Both produce the same kind of images, except for full-polarization SAR images. Thus, research with RAR applies to SAR and vice versa. However, for spaceborne purposes, SAR, which can have resolution smaller than 10 m, is normally used, because RAR from space cannot obtain ground images with resolutions better than several kilometers. The exception is the Russian-Ukrainian Okean series of RARs that produce images with ground resolution comparable to that of the Advanced Very High Resolution Radiometer, about 1 km. In 1965, images from the airborne Westinghouse AN/APQ-97 K-band multipolarized RAR became available to the civilian community. These images had a resolution of 7.5 m across-track (range resolution) and 1.1R m along-track (azimuth resolution), where R is the range (distance in kilometers) from radar to target. Thus, at a range of 10 km, airborne RAR produced better-resolution images (7.5 m × 11 m) than did the spaceborne SARs flown to date. Extensive research conducted with these images demonstrated that active microwave sensors could be used for a wide range of applications in studies of the solid earth, as well as for natural and cultural (e.g., agriculture) resource assessments. The AN/APQ-97 was diverted to commercial use in the late 1960s, resulting in more than a decade-long hiatus in data availability for the research community. Not until the late 1970s, with the launch of the short-lived Seasat mission, and the 1980s, when JPL, the Canada Centre for Remote Sensing, and others began SAR flights, did images become available again on a more routine basis. During the interim period, the APQ-97 and similar Canadian systems were flown commercially, but the data were considered to be confidential by the companies or governments that paid for data acquisition. Many countries in the tropics were mapped by these systems, but the largest project was RADAM in Brazil. Maps of nearly the entire country were produced for vegetation, geology, potential settlement, and other specialized uses. This information continues to be fused with multispectral data from resource satellites but is now being replaced by satellite SAR data from ERS-1 and 2, JERS-1, and Radarsat. Also during the interim period, extensive groundbased research into radar backscatter properties was conducted in the United States, Netherlands, France, Japan, and Canada. This provided the basis for our knowledge of radar response to crop, snow, sea-ice, ocean, soil, and soil-moisture conditions. In part because of technological limitations, these studies concentrated on the amplitude of the radar backscatter and did not investigate phase information. Many current studies use SAR imagery to confirm these earlier results and extend them in the spatial domain. However, recent technological advances have made possible the exploitation of relative phase information that JPL began collecting in 1984-1986 by the airborne L- and C-band polarimetric SAPs. This capability led to two significant analytic advances: polarimetry and interferometry. Polarimetry makes the complete

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SAR ISSUES

13

scattering matrix available for analysis. Interferometry relies on the relative phase stability of the scene for determination of digital elevation models. Interferometry Interferometric SAR is a technology that uses the phase difference between two coherent synthetic aperture radar images of a scene, obtained by two receivers separated by a cross-track distance called the baseline, to measure the height of the imaged surface. The interferometric technique was first applied to SAR by Graham (1974), who used two slightly displaced antennas mounted on an aircraft to form an interferometric beam (Figure 2.1a). When two antennas are used to receive simultaneously the complex signal scattered by the scene, subsequent to illumination by one of them, and then are processed together to produce height information, such a scheme is called cross-track SAR inteferometry (see Box 2.1). Seasat in 1978 and SIR-B in 1984 generated many SAR images of Earth's surface. Goldstein et al. (1988) extended the two-antenna interferometric SAR technique to a new capability by demonstrating how multiple-pass Seasat SAR images can be combined to realize the interferogram from which height information is derived (Figure 2.1b). Improvements in oscillator phase stability allowed Zebker and Goldstein (1986) to generate threedimensional maps of large areas at resolutions that are practical for many topographic applications. A similar demonstration was conducted using SIR-B images (Gabriel and Goldstein, 1988). In the 1990s the technique has been applied to numerous multiple-pass image combinations recorded by ERS-1 and 2, JERS-1, and Radarsat. Polarization Another recent development uses a full-polarization matrix, including phase, to obtain information not easily acquired in other ways. Much research since the 1980s shows that for some applications, phase differences between the polarizations convey information on the structure of the surface or of vegetation. Although radars of the 1960s had multipolarization capability, they could not measure the phases between polarizations and thus could not provide information conveyed by the phase. Rather, they provided only the amplitude differences, with those between like- and cross-polarizations being most useful. TRADE-OFFS IN SYNTHETIC APERTURE RADAR A small SAR system design must consider not only parametric and cost trade-offs, but also issues involving applications, data access, and data dissemination. A well-conceived end-to-end system should include applications undertaken for the public good as well as those driven primarily by science and commerce. Many of the potential applications cited by Evans et al. (1995) would be beneficial to multiple user sectors but are viewed differently by each of them. Intuitively, some applications are oriented more toward a single user sector. However, most

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SAR ISSUES

FIGURE 2.1a Interferometric radar data collection approaches. (a) Dual antenna. Interferometric radar data can be collected in a single pass where both antennas are located on the same platform. One antenna transmits and both antennas receive the returned echoes. (b) Repeat pass. In the repeat pass approach two spatially close radar observations of the same scene are made at two different times. The time interval may range from minutes to years.

14

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SAR ISSUES

15

BOX 2.1 DUAL-ANTENNA VERSUS MULTIPASS INTERFEROMETRY Cross-track interferometry can produce two types of information about Earth's surface: terrain height and surface deformation. Terrain height can be extracted from phase difference information between two images recorded by a two-antenna SAR in a single pass over the scene or by a single-antenna SAR in two separate passes with a horizontal separation (baseline) between them. In contrast, surface deformation, which is the change in terrain height between two instances in time, can be measured only by a multipass system. These two approaches are described below. Terrain Height The uncertainty (standard deviation) associated with the measured estimate of the surface height of a given image pixel, relative to a reference three-dimensional framework established on the basis of ground control points of known (x, y, z) coordinates, is given by the approximate relation:

where R is the range from the radar antenna to the pixel, λ is the wavelength, θ is the angle of incidence, and B is the baseline (horizontal separation between the two antennas or the two passes). Other factors that influence σh include signal-to-noise ratio, receiver noise, and the number of looks in processing the two SAR images (to reduce speckle) prior to combining them. According to the above relationship, to generate threedimensional topographic maps with small height uncertainties it is best to use a system with a relatively short-range R, as short a wavelength as practicable, and as large a baseline as possible. All of these considerations, as well as their relationships to transmitter power requirements, spatial resolution, swath width, antenna size and weight, and data rate were factored in the final design configuration of the Shuttle Radar Topography Mission (SRTM), planned for flight in September 1999. The SRTM instrument uses the SIR-C radar modified by adding a second set of C-band and X-band receivers at the end of a 60-m deployable mast. This instrument is designed to map Earth's topography between 60° north and south latitudes at 13- to 16-m vertical/30-m horizontal resolution. The 12-day mission will not include repeat passes, and so no change detection measurements will be made. The two major advantages of a dual-antenna, single-pass approach over a single-antenna repeat-pass (sometimes called multipass) approach are as follows: 1.

2.

There is no temporal decorrelation between the two backscattered complex signals because they are measured at the same time. Decorrelation can be a serious problem with multipass, particularly for vegetation-covered terrain, because of the random movements of the scattering elements. The decorrelation problem is wavelength dependent; X-band is more vulnerable to decorrelation than is Cband, which in turn is more vulnerable than is L-band. By maintaining the baseline physically constant, all parts of the world imaged by a single-pass will be mapped at about the same vertical resolution.

The major advantage of a repeat-pass approach over a single-pass configuration is that a much larger baseline (distance between orbits) can be used, thereby improving the vertical resolution of the measured terrain height. Surface Deformation Whereas a larger baseline leads to better height resolution, the inverse is true with surface deformation measurement. The ideal orbital configuration is one in which the two passes follow exactly the same orbit. To measure the surface deformation associated with earthquakes and other superficial changes at accuracies on the order of millimeters, the repeat orbits should remain within a circular tube with a diameter of 250 m. This would be sufficient to produce deformation maps with a horizontal resolution of 3 m and a vertical resolution of 2 mm. The relatively tight orbital constraint can be relaxed by using three orbits, two to produce a terrain height database, and the third, in interferometric combination, to produce a map of height changes between the times of the height baseline and the “differential” orbit. Using this method, accuracies on the order of a few millimeters have been reported (Meade and Sandwell, 1996). JPL and others have demonstrated direct measurement of surface movement on the order of 1 m/s or less. The technique used is along-track SAR interferometry, which requires antennas to be separated in the flight direction, rather than cross-track. The measurement is applicable to oceanic and coastal currents.

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SAR ISSUES

16

applications are not yet well-enough documented scientifically to enable assessment of their primary value for the public good, for commercial gain, or for Earth system science. In the following section, trade-offs among design parameters form the basis of the discussion. In the committee's view, a separate study should address NASA's strategy for optimizing applications for these various constituencies. Historically, SARs have been physically large systems requiring large amounts of power. This legacy is perpetuated by SAR designs that have been, and are being, developed to address multiple applications. Designers for new spaceborne SAPs often optimize for versatility to attract the widest range of users. The results have been expensive, large systems with substantial power and data requirements. However, if the design is tuned to specific user needs, then trade-offs may be possible among critical SAR parameters that could reduce cost, size, weight, and power requirements. If, for example, one wishes only moderate resolution (500 m or larger) in the range direction, the power and weight can be quite small. If one wishes the finest possible resolution, power and weight are large, although advances in technology now permit much smaller (but not lower-power) units than were previously possible. The only element in a spaceborne SAR that must be large is the antenna, and its size depends on the choice of frequency. It is the committee's opinion that approaches to small SAR should address design trade-offs. However, incomplete understanding of these trade-offs by users and managers has sometimes resulted in overly complex and expensive systems. Since different applications require different parameters, the primary application should be selected prior to making other trade-offs. As these are made, the design can be optimized for cost, size, and weight. Table 2.1 lists some of the instrument requirements as derived from the LightSAR report (JPL, 1997) and publications in the literature. With all of these parameters available for trade-off, selection of a major focus for any SAR mission, particularly a small SAR mission, is extremely important. Most applications can be addressed to some extent with any choice of parameters, but the selected focus and cost of other constraints should drive the choice. Table 2.2 itemizes some of the more important design options by primary application and by the type of processing required (interferometric, polarimetric) and maturity (demonstrated, validated, or operational) (JPL, 1997). The entries are brief because of the tabular format, but they can help guide choices and also show the importance of application focus to parameter choice. Impacts of Various Parameters Choosing a primary application for a small SAR mission will govern its design. For example, few traditional (i.e., none in noninterferomatic) applications require ground resolution finer than a few tens of meters. The increasing cost of higher power and greater weight to obtain a very fine resolution, in the judgment of the committee, does not appear to be justified for most mission foci. Certain missions might require very fine resolution, although these are probably not good candidates for a small SAR. If necessary, one can achieve very fine resolution using a spotlight mode, as discussed briefly below.

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SAR ISSUES

TABLE 2.1 Spaceborne SAR Instrument Requirements Minimum Configuration Scientific Configuration for Added Discipline Capability Ecology Biomass, L-quad + C-quad regrowth, cover+ P-quad type classification Vegetation L-quad + C-quad moisture + P-quad Vegetation height L-copol interf +L-dual interf and roughness C-copol interf +C-dual interf Complete map of L-dual + C-dual boreal and tropical forests for long-term studies Hydrology Soil moisture L-dual + C-dual + P-dual Snow hydrology X-quad + C-quad Floods L-HH + L-dual, L-quad Tropical basins L-HH + P-HH + L-dual, L-quad + P-quad + P-quad Geology Tectonics and L-copol interf + C-copol interf surface change (repeat pass) Soil erosion, L-dual + C-dual desertification Subsurface L-dual + P-quad mapping Glaciology Ice stream L-copol interf + C-copol interf velocity (repeat pass) Surface C-copol interf + L-copol interf topography (repeat pass) Oceanography Thin-ice L-quad + C-quad thickness + P-quad Major ice types L-quad + C-quad Sea-ice motion C-HH + L-HH + C-quad Summer openL-quad water ice concentration Ice melt ponds L-HH + C-HH Eddies, internal L-copel +C-copol waves, films, + P-copol underwater topography L-HH + C-HH Wave climatology and generation

17

Resolution (m)

Data-Take Frequency (once per)

Special Requirements

25-50

3 months

None

25-50

10-15 days

None

25-50

3 months

None

100

1 year

None

25-50

10 days

Low angle

25-50 25-50

10 days 10 days

High altitude None

100

6 months

None

25-50

10 days

None

25-50

6 months

None

25-50

None

25-50

10 days

Polar orbit None

25-50

10 days

None

25

10 days

25-50 100

10 days 10 days 10 days

Polar orbit NFσ < −30 dB Polar orbit >300-km : Swath >50-km : Swath