What Is an Altimeter Used for?

An altimeter is an instrument used within planes, satellites and by individuals at elevations to measure the altitude of an object above a fixed level, generally sea level. The types of altimeter include a pressure altimeter (measures atmospheric pressure from a static port), radar altimeter (measures time taken for a sent radio signal to reflect) and a global positioning system (measures altitude by triangulation with 3 or more satellites).

Several Inventors for Several Methods

Louis Paul Cailletet, a French physicist who lived from 1832 to 1913, is credited with inventing the first altimeter which measured changes in air pressure. Paul Kollsman, (1900 to 1982), a German inventor, invented a more accurate altimeter that took into account variations in temperature and barometric pressure to determine an airplane's height above sea level. Today, the adjustable setting gage in an altimeter is referred to as the "Kollsman window." Lloyd Espenschied, (1889 to 1986), an American electrical engineer, created the first radio altimeter in 1924 which bounced radio signals off the ground to show pilots the altitude of their aircraft.

Most Popular Personal Altimeters

Most aircraft today use a pressure or barometric altimeter. Air pressure decreases altitude increases. For every 800 meters you ascend, about 100 millibars of pressure increases. Older aircraft used a simple device with a single needle that would make a revolution around the face from zero to full as the airplane climbed. Today's aircraft have a primary needle that makes several revolutions and a secondary needle (or more than 2) that show the number of revolutions. It is like a clock with a seconds and minute hand.

Commercial Altimeters

During landings and for commercial and military aircraft, a radar altimeter is used due to its increased accuracy. Such altimeters are used in terrain-warning systems to alert aircraft if there is rising terrain or if the aircraft is too low. Radar altimeter allows greater terrain-following capabilities and is used by fighter aircraft to fly at low altitudes.

Calibration

Altimeters are calibrated to show height above ground at a given airfield taking into account local air pressure at mean sea level. One factor that will cause inaccurate readings is variations in air temperature. Hence, radar is used when greater accuracy is needed since temperature does not affect radio waves.

Global Positioning Systems and Space

Global positioning system (GPS) receivers are available to some aircraft and institutions that have at least 3 satellites around the globe that can triangulate altitude. This method is still not as accurate as radar and is generally used in conjunction with other tools to determine accurate height. Many spacecraft use dual-band radar altimeters and GPS to determine altitude. The most modern altimeter is spaceborne radar that is used by governments and organizations like NASA to map ocean topography. This tool broadcasts a microwave pulse to the ocean's surface and then measures how long it takes to bounce back. A computer onboard corrects delays possibly caused by water vapor, electrons in the ionosphere and the atmosphere's mass. Such readings are so accurate that sea surface heights can be determined within inches. The shape and strength of the returning signal can also detail information on wind speed and the ocean wave height.