The Shuttle Radar Topography Mission produced the most complete, highest‐resolution digital elevation model of the Earth. The project was a joint endeavor of NASA, the National Geospatial‐Intelligence Agency, and the German and Italian Space Agencies and flew in February 2000. It used dual radar antennas to acquire interferometric radar data, processed to digital topographic data at 1 arc sec resolution. Details of the development, flight operations, data processing, and products are provided for users of this revolutionary data set.

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Two Earth-orbiting radar missions are planned for the near future by NASA-Shuttle Radar Topography Mission (SRTM) and Lig,htSAR. The SRTM will fly aboard the Shuttle using interferometric synthetic aperture radar (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 civilian applications and to establish commercial utility. LightSAR is sponsored by NASA and industry partners. The use of IFSAR to measure elevation is one of the most powerful and practical applications of radar. A properly equipped spaceborne IFSAR system can produce a highly accurate global digital elevation map, including cloud-covered areas, in significantly less time and at significantly lower cost than other systems. For accurate topography over a large area, the interferomehic measurements can be performed sitnultaneously in physically separate receive systems. The Spaceborne Imaging Radar C (SIR-C), successfully flown twice in 1994 aboard the Space Shuttle Endeavour, offers a unique opportunity for global multifrequency elevation mapping by the year 2000. The addition of a C-band receive antenna of approximately 60 m length, extended from the Shuttle bay on a mast, and operating in concert with the existing SIR-C antenn~produces an interferometric pair. It is estimated that the 90 percent linear absolute elevation error achievable is less that 16 meters for elevation postings of 30 meters. The SRTM will be the first single-pass spaceborne IFSAR instrument and will produce 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 being explored, with industry sharing the cost of developing LightSAR in return for commercial rights to its data and operational responsibility. LightSAR will enable mapping of surface change. The instrument's high-resolution mapping, along with its quad pohuization, dual polarization, interfkrometric and ScanSAR modes will enable continuous monitoring of natural hazards, Earth's surface deformation, surface vegetation change, and ocean mesoscale features to provide commercially viable and scientifically valuable data products. Advanced microelectronics and lightweight materials will increase LightSAR's functionality without increasing the mass. Dual frequency L/X-band designs have been examined.

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Twenty-five years ago, the two flights of the SIR-C/X-SAR Synthetic Aperture Radar (SAR) system on the Space Shuttle Endeavour blazed a trail toward the future with a series of radar system innovations-that nearly every spaceborne SAR flown since then has benefited from, and in some cases improved upon. Many of the SAR techniques adopted by SAR system designers worldwide as part of their toolkit, such as: ScanSAR, Spotlight mode, along-track interferometry, polarization diversity and polarimetry, polarimetric calibration, variable length and bandwidth pulses, and onboard processing, can trace their heritage back to this first-of-a-kind, civil-use SAR system. The electronic steering capability of SIR-C's phased array antenna, combined with the exquisite orbit track control provided by the Space Shuttle, paved the way for systematic mapping of the Earth's topography by the Shuttle Radar Topography Mission, and later by TanDEM-X. Some techniques, such as multi-frequency SAR, multi-frequency repeat-pass interferometry and onboard processing have yet to be fully exploited. The richness of the SIR-C/X-SAR data set has proved to be a treasure trove for opening up entirely new remote sensing techniques, such as Polarimetric SAR Interferometry (or PolInSAR), and GPS or now GNSS reflections (also known as Signals of Opportunity), which were both demonstrated from archive data, years after the 1994 flights. The groundbreaking legacy of SIR-C/X-SAR lives on in the many SAR systems collecting data in Earth orbit today, and in those planned for the future.