/ æltɪmətər /
ETYM Late Lat. altimeter; altus high + metrum, Greek metron measure: cf. French altimčtre.
An instrument that measures the height above ground; used in navigation.
Instrument for taking or showing altitude.
Instrument used in aircraft that measures altitude, or height above sea level. The common type is a form of aneroid barometer, which works by sensing the differences in air pressure at different altitudes. This must continually be recalibrated because of the change in air pressure with changing weather conditions. The radar altimeter measures the height of the aircraft above the ground, measuring the time it takes for radio pulses emitted by the aircraft to be reflected. Radar altimeters are essential features of automatic and blind-landing systems.