Development and Application of Small Spaceborne Synthetic Aperture Radars (1998)

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

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.

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

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.

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.

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

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.

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 Repeat-pass 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

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.