Altimeter

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Kollsman-type barometric aircraft altimeter as used in North America

An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.

1 Aircraft Instrumentation
2 Other Modes of Transport
3 Scientific Uses
- 3.1 Measuring air pressure (barometer vs. absolute)
- 3.2 Satellites
4 See also
5 External links

[edit] Aircraft Instrumentation

The traditional altimeter found in most aircraft works by measuring the air pressure from a static port. 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 or "altimeter setting", 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:

$h = \frac{(1-(P/P_{ref})^{0.19026}) \times 288.15}{0.00198122}$

where h is the indicated altitude in feet, $P$ is the static pressure and $P r e f$ 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.

[edit] Other Modes of Transport

The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some high-performance luxury cars which were never intended to leave paved roads, such as the Duesenberg in the 1930s, have also been equipped with an altimeter; their ability to ascend hills can thereby be noted by the driver.

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

[edit] Scientific Uses

[edit] Measuring air pressure (barometer vs. absolute)

There are two ways to use an aircraft altimeter for measuring air pressure:

To measure the actual local air pressure, adjust the altimeter to read 0 ("feet") elevation. Then the pressure reading will be in absolute pressure (eg. in USA, "inches of Hg, absolute"). See the last paragraph of this reference.

"Barometric" pressure, on the other hand, means "corrected to sea-level" instead of "absolute". Adjust the altimeter to read the actual local elevation, and then the pressure reading will be in corrected "barometric" pressure just like airports and weather reports use. See this reference.

[edit] Satellites

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 terrain. 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.