An altimeter is a device that measures the height or altitude of an aircraft above sea level or, alternatively, the surface immediately below. Many aircraft altimeters are modified aneroid barometers and work on the principle that air pressure decreases with increased altitude, but these must be constantly recalibrated throughout the flight to take account of changing meteorological conditions (local ground temperature and air pressure reduced to sea level). Altimeters based on aneroid barometers are also used by scientists, surveyors, and mountain climbers.
Radar altimeters which compute absolute altitudes (the height of the aircraft above the ground surface immediately below) from the time taken for radio waves to be reflected to the aircraft from the ground, are frequently used by commercial aircraft for approach and landing, especially in conditions of low visibility and also automatic landings. Spacecraft altimeters are also of this type.