Measuring Altitude

Aircraft use an instrument called an altimeter to measure altitude. Airplanes have two different types of altimeters. The pressure altimeter measures a plane’s altitude above mean sea level. As an airplane climbs higher in the atmos­phere, the air pressure falls. Measuring the air pressure shows how high the plane is. A pressure altimeter, or baro­metric altimeter, is accurate to within about 20 feet (6 meters).

MEASURING ALTITUDE WITH SATELLITES

Airliners fitted with satellite naviga­tion can use it to measure altitude. Radio signals received from naviga­tion satellites are used to measure the distance between the plane and the satellites. (The positions of the satellites used are precisely known.) Signals received from three satellites enable a plane to determine its posi­tion on a map. Adding a signal from a fourth satellite also enables the plane to figure out its altitude.

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As an airplane descends for landing, however, it is more important to know its height above the ground than its pressure altitude. The radio altimeter is switched on when a plane descends below 2,500 feet (762 meters). The altimeter aims radio waves at the ground and measures the time it takes for them to reach the ground and travel back up to the airplane. It uses the measurement to calculate the distance between the plane and the ground. The radio alti­meter can measure a plane’s height above the ground to within about 2 feet (0.61 meters).

SEE ALSO:

• Air and Atmosphere • Pressure