The study of bodies in the Solar System by bouncing radio pulses off them. Radar work requires very large (and hence sensitive) telescopes, such as the 305-m radio dish at Arecibo Observatory. Radar astronomy can be used to determine the accurate distances of the planets (by measuring the time delay of the reflected signal), rotation rates (by Doppler broadening of the signal), and to map surface features (by detailed analysis of the echoes). Notable achievements in radar astronomy include the accurate measurement of the astronomical unit; determination of the rotation periods of Mercury and Venus; and the mapping of the cloud-covered surface of Venus. Earth-based radar has also been used to study the surface of Mars, the larger moons of Jupiter and Saturn, the rings of Saturn, asteroids, comets, and meteor trails. Several spacecraft sent to Venus have carried radar mapping equipment, notably Pioneer Venus Orbiter, Veneras 15 and 16, and Magellan. In addition, the Cassini spacecraft has mapped Saturn’s moon Titan by radar.

https://www.jpl.nasa.gov/news/news.php?release=2011-011