Radio Altimeter

A radio altimeter, also called a radar altimeter or an electronic altimeter, is an aircraft instrument that measures true height above the ground directly beneath it, rather than height above mean sea level the way a barometric altimeter does. It transmits radio waves downward that reflect off the terrain and return to the aircraft, and it calculates altitude from the travel time of those waves and the known speed of light, most commonly using frequency modulated continuous wave radar in the E, Ka or S radio bands, typically reading accurately up to about 2,500 feet above the ground. The concept grew out of the wider development of radar at Bell Labs during the 1910s and 1920s; Lloyd Espenschied filed a patent in 1930 that was granted in 1936, a working model was demonstrated in early 1938 and publicly announced that October, and RCA began mass production of radio altimeters during the Second World War. Modern commercial airliners are required to carry at least two radio altimeters, since the instrument underlies ground proximity warning systems and gives autopilots and automatic landing systems the precise low altitude readings they need to flare and touch down safely.