Radar Altimeter

A radar altimeter, radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level.
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Aircraft instrument that makes use of the reflection of radio waves from the ground to determine the height of the aircraft above the surface. (FAA6)

A type of absolute altimeter which measures vertical distance to the surface below using radar technology. See also absolute altimeter. 1/12/93

Type d'altimètre absolu qui indique la distance verticale le séparant de la surface sous-jacente en utilisant le principe du radar. Voir aussi altimètre absolu. 1/12/93

(See RADIO ALTIMETER.)