Altimeter

Altimeter, mechanical or electronic device commonly used in aircraft to measure vertical height above the surface of the earth. Two main types of altimeter exist: pressure and radio. The more common pressure altimeter operates on the principle that atmospheric pressure decreases with an increase in altitude. Pointers on the graduated face of the altimeter dial connect through a system of gears and levers to an aneroid capsule, a hollow, metallic disk partially evacuated of air that expands and contracts slightly with changes in atmospheric pressure, that is, with altitude (see Barometer). Radioaltimeters, radar devices modified to measure vertical distance only, beam a pulse of electromagnetic radiation downward from the aircraft. A receiving antenna on the craft then detects the radio waves reflected by the surface of the earth. By measuring the time difference (t) between sending and receiving the pulse, the altitude (h) can be computed in the equation