Altimeter

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Image:Aircraft altimeter.JPG An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The traditional altimeter found in most aircraft works by measuring the air pressure from a static port in the airplane. Air pressure decreases with an increase of altitude — about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) close to sea level. The altimeter is calibrated to show the pressure directly as altitudes, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA).

The reference pressure can be adjusted by a setting knob. The reference pressure, in inches of mercury, is displayed in the Kollsman Window, visible at the right side of the aircraft altimeter shown here. This is necessary, since sea level air pressure varies with the weather. In pilot's jargon, the regional or local air pressure at mean sea level (MSL) is called the QNH, and the pressure which will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field. An altimeter cannot, however, be adjusted for variations in air temperature. Differences in temperature from the ISA model will, therefore, cause errors in indicated altitude.

The calibration formula for an altimeter, up to 36,090 feet (11,000 m), can be written as:

<math>h = \frac{(1-(P_0/P_{ref})^{0.19026}) \times 288.15}{0.00198122}</math>

where h is the indicated altitude in feet, <math>P_0</math> is the static pressure and <math>P_{ref}</math> is the reference pressure (use same units for both). This is derived from the barometric formula using the scale height for the troposphere.

A radar altimeter measures altitude more exactly, using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure the exact height during the landing procedure of commercial aircraft. Radar altimeters are also a component of terrain avoidance warning systems, telling the pilot that the aircraft is flying too low or that terrain is rising to meet the aircraft.

Mountaineers use wrist-mounted barometric altimeters when on high-altitude expeditions, as do skydivers.

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Scientific Uses

A number of satellites (See links) use exotic dual-band radar altimeters to measure height from a spacecraft. That measurement, coupled with orbital elements (possibly from GPS), enables determination of the topography. The two lengths of radio waves permit the altimeter to automatically correct for varying delays in the ionosphere.

Over water, detailed satellite altitude information has proven amazingly useful. Humps in the water indicate gravitational concentrations, permitting a computer program to construct a map of undersea features such as mountains. The altimeters can also measure wave heights, wave directions, and wave spectra. This information permits computer programs to measure the speed of ocean currents and produce detailed maps of wind speeds and directions at the surface, even in extremely stormy conditions.

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