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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