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. 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 nino.

SARs applications are countless: they range from geology to crop monitoring, from sea-ice mapping to disaster monitoring to vessels 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:

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: Imaging (i.e., Synthetic aperture radars) are the only instruments capable to penetrate cloud covers such as that of Venus. And Venus has, in fact, been the first target for such missions. It has been imaged by two soviet spacecraft, Venera 15 and Venera 16 in 1983/84 (carrying also a Radar altimeter), and by the American spacecraft Magellan in 1990/94.

The other Solar system's body targeted by an imaging radar mission has been Titan, the largest moon of Saturn, 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.

Its antenna also serves as main antenna for communication with Earth.

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. Thanks to their low operating frequency they can 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 has been 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 similar instrument (primarily devoted to ionospheric plasma probing) was embarked on the Japanese martian mission Nozomi (launched in 1998 but lost). A radar sounder is also used on the Japanese moon probe SELENE, launched September 14, 2007.

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 2000. SBR is a less-ambitious version of Discoverer II.

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