Space - based radar Space - based radar refers to space - borne radar systems that may have any of a variety of purposes . A number of earth - observing radar satellites , such as RadarSat , have employed synthetic aperture radar ( SAR ) to obtain terrain and land - cover information about the Earth . 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 RORSAT 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 . Earth - observing radars . Use of radar sensor for Earth observation purposes was started by NASA / JPL ' s Seasat satellite , which carried 3 different radar sensors : After Seasat , SARs , altimeters and scatterometers have been flown on several other space missions . While the SAR is , in principle , 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 allows to measure ( with an accuracy in the order of few centimeters ) the sea surface profile . Additionally , analysis of the echo amplitude and shape allows to 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 . Being it affected by the ocean surface " roughness " , in turn affected by the wind and also dependent from its direction , it is possible to determine the wind speed and direction . These three types of radar are currently used on several satellites . Scatterometer s are of high value for operational meteorology , allowing reconstruction of wind fields on a global scale . Data from Radar altimeter s are used for the accurate determination of the geoid , monitoring of tides , ocean currents and other large - scale ocean phenomena such as El Niño . SARs applications are countless : they range from geology to crop monitoring , from sea - ice mapping to disaster monitoring to vessel traffic surveillance , not to forget the military applications ( many civilian SAR satellites are , in fact , dual - use systems ) . SAR imaging offer the great advantage , over its optical counterparts , of not being affected by meteorological conditions such as clouds , fog , etc . , making it the sensor of choice when continuity of data must be ensured . Additionally , SAR interferometry ( both dual - pass or single - pass , as used in the STRM mission ) allows accurate 3 - D Reconstruction . Other types of radars have been flown for earth observation missions : precipitation radars such as the Tropical Rainfall Measuring Mission , or cloud radars like the one used on Cloudsat . Like the majority of earth - observing satellites , radar satellites often use sun - synchronous orbit s so that diurnal variations of vegetation are ignored , allowing long - term variations to be more accurately measured . Some of the former and current earth - observing radar satellites are : Planetary radars . Most of the radars flown as payload in planetary missions ( i . e . , not considering avionics radar , such as docking and landing radars used in Apollo and LEM ) belong to two categories : imaging radars and sounders . Imaging radars : Synthetic aperture radars are the only instruments capable of penetrating heavy cloud cover around planets such as Venus , which was the first target for such missions . Two Soviet spacecraft ( Venera 15 and Venera 16 ) imaged the planet in 1983 and 1984 using SAR and Radar altimeter s . The Magellan probe also imaged Venus in 1990 and 1994 . The only other target of an imaging radar mission has been Titan , the largest moon of Saturn , in order to penetrate its opaque atmosphere . The radar of the Cassini probe , in orbit around Saturn , is currently providing images of Titan surface at each fly - by of the moon . The Cassini radar is a multimode system and can operate as Synthetic Aperture Radar , radar altimeter , scatterometer and radiometer . Sounding radars : these are low - frequency ( normally , HF - 3 to 30 MHz - or lower ) ground - penetrating Radars , used to acquire data about the planet sub - surface structure . Their low operating frequency allow them to penetrate for hundred of meters , or even kilometers , below the surface . Synthetic aperture techniques are normally exploited to reduce the ground footprint ( due to the low operating frequency and the small allowable antenna dimensions , the beam is very wide ) and , thus , the unwanted echo from other surface objects . The first radar sounder flown was ALSE ( Apollo Lunar Sounder Experiment ) on board Apollo 17 in 1972 . Other sounder instruments flown ( in this case around Mars ) , are MARSIS ( Mars Advanced Radar for SubSurface and Ionosphere Sounding ) on board the European Space Agency ' s Mars Express probe , and SHARAD ( mars SHAllow RADar sounder ) on JPL ' s Mars Reconnaissance Orbiter ( MRO ) . Both are currently operational . A radar sounder is also used on the Japanese moon probe SELENE , launched September 14 , 2007 . A similar instrument ( primarily devoted to ionospheric plasma probing ) was embarked on the Japanese martian mission Nozomi ( launched in 1998 but lost ) . Defense radars . 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 .