(1) An instrument determining radial velocity of an object by comparing the frequency of radiation returned from an object with the frequency of radiation transmitted toward the object. e.g., Doppler radar on an aircraft determines the velocity of the aircraft, referred to a coordinate system on the aircraft, by measuring the Doppler shift experienced by microwaves transmitted towards the Earth and scattered back to a receiver at the aircraft. The radio waves are transmitted as suitably oriented beams. At least three beams are required; a fourth beam has recently been added (in radars operating coherently) for a check. The term Doppler radio system is preferable, since the so called Doppler radar is not, properly speaking, radar. (2) An instrument which determines the radial velocity of an object by comparing the frequency of radiation received from the object with the known frequency of the radiation emitted by it. This type of Doppler radar has been used for determining the trajectories of spacecraft sent to other planets, etc. The term is also applied to equipment and apparatus incorporating an instrument of either of the types described in the definitions. The term Doppler radio system is preferable, since the so called Doppler radar is not, properly speaking, radar.