a vital role in complementing and enhancing the understanding of the space-based measurements, as have systems developed in Germany (E-SAR) and the Netherlands (PHARUS). The accompanying research and analysis & program has also played an indispensable role in advancing the utility of this technology.

In spite of these commendable advances, there is no doubt that SAR systems remain less familiar and are less frequently employed than are more conventional electro-optical sensing systems. While both kinds of systems can be used to produce images of the Earth, the interpretation of the images is necessarily quite different between the two. As a result, the research and operational user communities have had a lengthier period to go through in learning how to use SAR data, and a major part of the learning has involved significant research in determining what the data show. That research continues. The moisture and frequency-dependent variable surface and vegetation penetration of microwaves, for example, certainly requires a reorientation of the thinking of image analysts. The problems of layover and shadowing also pose challenges in the interpretation of radar data. Lastly, until some of these issues are better understood, the research community cannot effectively include SAR data in processing algorithms that link near, short-wave, and long-wave infrared information.

At the same time, however, the additional learning the community has undergone can pay dividends. Electro-optic sensors, as powerful as they have become, are inherently limited by cloud-cover, fog, and dust--all of which may be persistent phenomena in some regions of the world, or which may be expected to accompany natural disasters. Indeed, in most regions of the world, one cannot rely on being able to obtain a surface image from an electro-optical sensor at the time the image is most needed. Because of their day-night, all-weather capability, microwave systems may represent the only reliable approach to collecting data on a given region at a particular time. In addition, unlike electro-optical systems, the signals returned by radar systems are sensitive to the physical structure and moisture content of the surface being sensed and may offer avenues to obtaining results that are important for research and applications but are not otherwise obtainable.

For all of the above reasons, there are some who believe with possible justification that, while radar imaging systems today play a secondary role to the electro-optical sensing systems, the role will be reversed in the future. Whether this “bullish” view proves to be correct or not is less important than is the acknowledgment that active microwave systems are demonstrating their worth, and that room exists for still further technological enhancement of their capabilities. Thus, although it is understandable why active microwave sensors have not occupied a more prominent role in the early development of the planning for the Mission to Planet Earth, it should be expected that they will become increasingly important in the future--and likely be indispensable in some applications.

Putting aside for the moment the committee's generally favorable view of the long-term potential of SAR measurements for the Mission to Planet Earth, the committee recognizes that a major immediate issue that you are facing is deciding whether or not to seek a third flight of the Shuttle Radar Laboratory (SRL-3). Based only on scientific considerations, it is the committee's judgment that such a flight would produce good scientific results, if the current instrumentation were simply reflown, but that it would produce especially worthwhile results if it were modified

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