turn is affected by the wind and also dependent on its direction, this instrument can determine the wind speed and direction. These three types of radar are currently used on several satellites. Scatterometers are of high value for operational meteorology, allowing reconstruction of wind fields on a global scale. Data from radar altimeters 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 many: they range from geology to crop monitoring, from measurement of sea ice 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 SRTM 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 other Earth observation satellites, radar satellites often use Sun-synchronous orbits so that diurnal variations of vegetation are ignored, allowing long-term variations to be more accurately measured. List of Earth-observing radar satellites Earth-observing radar satellites include: RISAT-1 (SAR, ISRO India, 2012) RORSAT (SAR, Soviet Union, 1967-1988) Seasat (SAR, altimeter, scatterometer, US, 1978) RADARSAT-1 (SAR, Canadian, 1995) RADARSAT-2 (SAR, Canadian, 2007) SAR Lupe 1-5 (SAR satellites of the German Air Force) TerraSAR-X (SAR Germany, 2007) TanDEM-X (SAR Germany, 2010) COSMO-SkyMed (SAR, Italy, 2007) SAOCOM (L band SAR constellation, Argentina) TecSAR (SAR, Israeli, 2008) TOPEX/Poseidon (altimeter) Jason 1 / Jason 2 (altimeter) Shuttle Imaging Radar (see Shuttle Radar Topography Mission) (SAR) JERS-1 (SAR) Geosat (altimeter) ERS-1 & ERS-2 (European Remote-Sensing Satellite) (altimeter, combined SAR/scatterometer) Envisat (SAR, altimeter) Tropical Rainfall Measuring Mission (Precipitation Radar) Cloudsat (cloud radar) Metop (scatterometer) QuickScat (scatterometer) NISAR (satellite) Almaz Sentinel-1 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