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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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS 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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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS 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. 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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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS 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. 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. 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. 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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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS 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. 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). 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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DEVELOPMENT AND APPLICATION OF SMALL SPACEBORNE SYNTHETIC APERTURE RADARS 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.