Altimeter

Altimeter, instrument used for estimating the height of an aircraft above sea level. Most are modified aneroid barometers and work on the principle that air pressure decreases with increased altitude, but these must be constantly recalibrated during flight to take account of changing meteorological conditions (local ground temperature and air pressure). Radar altimeters, which compute absolute altitudes (the height of the aircraft above the ground surface immediately below) by measuring the time taken for radar waves to be reflected to the aircraft from the ground, are essential for blind landings.

See also: Barometer.