missions. Two Soviet spacecraft (Venera 15 and Venera 16) imaged the planet in 1983 and 1984 using SAR and Radar altimeters. 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, which orbited Saturn between 2004 and 2017, provided images of 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 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 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). References External links Air Force fact sheet Globalsecurity.org page Northrop Grumman info sheet "The Space Based Radar Plan", John A. Tirpak, Air Force Magazine, August, 2002 Earth observation satellites Synthetic aperture radar