From Glossary of Meteorology
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An instrument that determines the altitude of an object with respect to a fixed level.

There are several types of altimeters: 1) the pressure altimeter, which measures barometric pressure and converts it to altitude above sea level when referenced to the existing measured sea level barometric pressure, or to pressure altitude when referenced to a standard pressure; 2) the radio or radar altimeter, which deduces altitude by measuring the time it takes a radio signal to travel from a transmitter to the underlying surface and back to the receiver; and 3) the Global Positioning System (GPS), which determines altitude by measuring time of travel of radio signals between GPS-equipped satellites and the receiver.