Editing Space-based radar (section)

== 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 [[Project Apollo|Apollo]] and [[Apollo Lunar Module|LEM]]) belong to two categories: imaging radars and sounders.

'''Imaging radars''': [[Synthetic aperture radar]]s 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 (moon)|Titan]], the largest moon of [[Saturn]], in order to penetrate its opaque atmosphere. The radar of the [[Cassini probe|''Cassini'' probe]], which orbited [[Saturn]] between 2004 and 2017, provided images of [[Titan (moon)|Titan's]] surface during each fly-by of the moon. The ''Cassini'' radar was a multimode system and could operate as [[Synthetic Aperture Radar]], [[radar altimeter]], [[scatterometer]] and [[radiometer]].

'''Sounding radars''': these are low-frequency (normally, HF - 3 to 30&nbsp;MHz - or lower) ground-penetrating [[Radars]], used to acquire data about the planet sub-surface structure. Their low operating frequency allow them to penetrate hundreds 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 (radio)|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 [[Jet Propulsion Laboratory|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 (physics)|plasma]] probing) was embarked on the Japanese Martian mission [[Nozomi (probe)|Nozomi]] (launched in 1998 but lost).