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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