Instrument that measures the altitude of the land surface or of any object, such as an airplane. The mechanical pressure altimeter measures atmospheric pressure relative to sea level through a series of bellows, gears, and springs, which move pointers on a dial. Radio altimeters measure the distance of an aircraft above the ground rather than above sea level by indicating the time a pulse of radio energy takes to travel from the aircraft to the ground and back; they are used in automatic navigation and blind-landing systems.

This entry comes from Encyclopædia Britannica Concise.
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