Altimeter << al TIHM uh tuhr or AL tuh `mee` tuhr >> is an instrument that measures altitude. Aircraft and some satellites are equipped with altimeters. Mountain climbers, surveyors, and scientists also use altimeters.
There are three main kinds of altimeters: (1) pressure altimeters, (2) radar altimeters, and (3) laser altimeters. Pressure altimeters are standard equipment on aircraft. Some planes also carry radar altimeters. Earth satellites carry radar and laser altimeters to measure the height of the oceans, land, and icecaps. Scientists have used radar and laser altimeters mounted on space probes to map the surfaces of other planets and the moon.
A pressure altimeter resembles a type of gauge known as an aneroid barometer. Both devices measure the effect of air pressure on a metal chamber from which most of the air has been removed. A pressure altimeter determines an aircraft’s distance above sea level by measuring the pressure of the earth’s atmosphere. This pressure decreases as altitude increases.
A radar altimeter measures the time a radio signal takes to travel from a plane or satellite to the earth’s surface and back. A laser altimeter works in the same way, but uses pulses of laser light rather than radio waves.
To use a satellite-based altimeter to measure the height of features on the earth’s surface, scientists must first measure the satellite’s distance above sea level. They do this by tracking the satellite by lasers and radio from ground-based stations and from satellites of the Global Positioning System (GPS). The GPS is a worldwide navigation system that uses radio signals broadcast by satellites. The altitudes of the ground stations and the GPS satellites are precisely known. Thus, scientists can use those altitudes and the tracking data to determine a satellite’s distance above sea level.
Researchers have used measurements made by satellite-based altimeters to determine how the ocean shrinks and expands as its temperature changes. Scientists have also used satellite data to measure the speed and direction of ocean currents. In addition, satellite-based altimeters have measured the height of the icecaps that cover large areas of land in the polar regions. Researchers can use the information about the icecaps to determine whether the caps are growing or shrinking.
See also Barometer
