The measurement of the surface relief of a planetary body by recording the travel time of reflected radar waves. The most spectacular success of the technique has been the establishment of the detailed surface topography of Venus by the US probes Pioneer Venus 1 and 2 launched in 1978, and the Soviet Venera 15 and 16 launched in 1983. Radar scanning of the Earth’s surface from aircraft or satellites provides information on topography, vegetation cover, and lithology, and is particularly valuable where cloud cover prevents normal surveying. Over the oceans, Seasat altimetry data provide information on wave height and motion, and on ocean bathymetry, and assist in definition of the geoid.