Space-based radar

A number of Earth-observing satellites, such as RADARSAT, have employed synthetic aperture radar (SAR) to obtain terrain and land-cover information about the Earth.

The concept was to provide high-range-resolution ground moving target indication (GMTI), as well as SAR imaging and high-resolution digital mapping.

The SBR system would allow detection and tracking of aircraft, ocean-going vessels (similar to the Soviet US-A program), and potentially land vehicles from space.

A satellite radar-altimeter is a nadir-looking radar with very high range resolution, which measures the ocean surface topography with an accuracy in the order of few centimeters.

Some radar-altimeters (like CryoSat/SIRAL) employ synthetic aperture and/or interferometric techniques: their reduced footprint allows mapping of rougher surfaces like polar ices.

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.

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.

A similar instrument (primarily devoted to ionospheric plasma probing) was embarked on the Japanese Martian mission Nozomi (launched in 1998 but lost).