The use of pulsed radio signals in astronomical applications, such as the detection of meteor showers, the measurement of distances within the solar system and the mapping of surfaces of objects in the solar system. Radar signals transmitted by the 305-metre 1,000-foot radio telescope at Arecibo Observatory have successfully been used to map Venus and to characterize the size and shape of asteroids. The Magellan spacecraft, placed in orbit around Venus, used synthetic aperture radar to map the planet’s surface, which is concealed by opaque cloud. Radar is of fundamental importance in calibrating the distance scale within the solar system and thereby determining the value of the astronomical unit.