Space-based radar or spaceborne radar is a radar operating in outer space; orbiting radar is a radar in orbit and Earth orbiting radar is a radar in geocentric orbit. A number of Earth-observing satellites, such as RADARSAT, have employed synthetic aperture radar (SAR) to obtain terrain and land-cover information about the Earth. Military In the United States, Discoverer II was a proposed military space-based radar program initiated in February 1998 as a joint Air Force, DARPA, and NRO program. The concept was to provide high-range-resolution ground moving target indication (GMTI), as well as SAR imaging and high-resolution digital mapping. This program was cancelled by Congress in 2007. SBR is a less-ambitious version of Discoverer II. Space-based radar (SBR) is a proposed constellation of active radar satellites for the United States Department of Defense. The SBR system would allow detection and tracking of aircraft, ocean-going vessels (similar to the Soviet US-A program), and potentially land vehicles from space. This information would then be relayed to regional and national command centers, as well as E-10 MC2A airborne command posts. Active military radar satellites Indian RISAT American Lacrosse Russian Kondor Japanese Information Gathering Satellite German SAR-Lupe Chinese Huanjing Earth-observing radars Use of radar sensor for Earth observation purposes was started by NASA/JPL's Seasat satellite, which carried three different radar sensors: a synthetic aperture radar (SAR) for high-resolution imaging a radar altimeter, to measure the ocean topography a wind scatterometer to measure wind speed and direction After Seasat, SARs, altimeters and scatterometers have been flown on several other space missions. While the SAR, in principle, is similar to its airborne counterparts (with the advantage of the increased coverage and worldwide access offered by the satellite platform), the other two are specific to satellite operations. A satellite radar-altimeter is a nadir-looking radar with very high range resolution, which measures the ocean surface topography with an accuracy in the order of few centimeters. Additionally, analysis of the echo amplitude and shape can extract information about the wind speed and wave height, respectively. Some radar-altimeters (like CryoSat/SIRAL) employ synthetic aperture and/or interferometric techniques: their reduced footprint allows mapping of rougher surfaces like polar ices. A wind scatterometer observes the same portion of the ocean surface from different (at least 3) angles of view as the satellite passes by, measuring the echo amplitude and the corresponding surface reflectivity. Reflectivity being affected by the ocean surface "roughness", which in