Space-based Radar - Earth-observing Radars

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:

• 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 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. 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 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 orbits 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:

• 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 Luftwaffe, )
• TerraSAR-X (SAR Germany, 2007)
• TanDEM-X (SAR Germany, 2010)
• COSMO-SkyMed (SAR, Italy, 2007)
• 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)