Radar altimeter

A radar altimeter, electronic altimeter, reflection altimeter, radio altimeter (RADALT), low range radio altimeter (LRRA) or simply RA, used on aircraft, measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to reflect from the ground and return to the plane. This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined datum, usually mean sea level.

ITU definition

From the legal point few radio altimeter is – according to article 1.108 of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) – defined as «Radionavigation equipment, on board an aircraft or spacecraft, used to determine the height of the aircraft or the spacecraft above the Earth's surface or another surface.» Radionavigation equipment shall be classified by the radiocommunication service in which it operates permanently or temporarily. The utilization of radio altimeter equipment is categorised as so-called safety-of-life service, must be protected for Interferences, and is essential part of Navigation.