FUTURE NASA SPACEBORNE SAR MISSIONS

Two Earth-orbiting radar missions are planned for the near future by N ASA - Shuttle Radar Topography Mission (SRTM) and LightSAR. The SRTM w ill fly aboard the Shuttle using interferometric synthetic aperture ra dar (IFSAR) to provide a global digital elevation map. SRTM is jointly sponsored by NASA and the National Imagery and Mapping Agency (NIMA). The LightSAR will utilize emerging technology to reduce mass and life -cycle costs for a mission to acquire SAR data for Earth science and c ivilian applications and to establish commercial utility. LightSAR is sponsored by NASA and industry partners. The use of IFSAR to measure e levation is one of the most powerful and practical applications of rad ar. A properly equipped spaceborne IFSAR system can produce a highly a ccurate global digital elevation map, including cloud-covered areas, i n significantly less time and at significantly lower cost than other s ystems. For accurate topography over a large area, the interferometric measurements can be performed simultaneously in physically separate r eceive systems. The Spaceborne Imaging Radar C (SIR-C), successfully f lown twice in 1994 aboard the Space Shuttle Endeavour, offers a unique opportunity for global multifrequency elevation mapping by the year 2 000. The addition of a C-band receive antenna of approximately 60 m le ngth, extended from the Shuttle bay on a mast, and operating in concer t with the existing SIR-C antenna, produces an interferometric pair. I t is estimated that the 90% linear absolute elevation error achievable is less than 16 meters for elevation postings of 30 meters. The SRTM will be the first single-pass spaceborne IFSAR instrument and will pro duce a near-global high-resolution digital topography data set. Since LightSAR offers important benefits to both the science community and U S industry, an innovative government-industry teaming approach is bein g explored, with industry sharing the cost of developing LightSAR in r eturn for commercial rights to its data and operational responsibility . LightSAR will enable mapping of surface change. The instrument's hig h-resolution mapping, along with its quad polarization, dual polarizat ion, interferometric and ScanSAR modes will enable continuous monitori ng of natural hazards, Earth's surface deformation, surface vegetation change, and ocean mesoscale features to provide commercially viable a nd scientifically valuable data products. Advanced microelectronics an d lightweight materials will increase LightSAR's functionality without increasing the mass. Dual frequency L/X-band designs have been examin ed.