The bouncing of radio waves off objects in the solar system, with reception and analysis of the "echoes".

Radar contact with the moon was first made in 1945 and with Venus in 1961. The travel time to radio reflections allows the distances of objects to be determined accurately.

Analysis of the reflected beam reveals the rotation period and allows the object's surface to be mapped. The rotation periods of Venus and Mercury were first determined by radar. Radar maps of Venus were obtained first by earth-based radar and subsequently by orbiting space probes.