Altimeter

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Diagram showing the face of the dangerous "three-pointer" sensitive aircraft altimeter displaying an altitude of 10,180 feet. (See: United Airlines Flight 389)

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

A pressure altimeter (also called barometric altimeter) is the traditional altimeter found in most aircraft. In it, an aneroid barometer measures the air pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude — about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) near sea level.

The altimeter is calibrated to show the pressure directly as an altitude, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale. Modern aircraft use a "sensitive altimeter" which has a primary needle that makes multiple revolutions, and one or more secondary needles that show the number of revolutions, similar to a clock face. In other words, each needle points to a different digit of the current altitude measurement.

Diagram showing the internal components of the sensitive aircraft altimeter.

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 temperature and pressure.

In aviation terminology, 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.

Kollsman-type barometric aircraft altimeter as used in North America displaying an altitude of 80 feet.

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.

Main article: Radar altimeter

A radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure height above ground level during landing in commercial and military aircraft. Radar altimeters are also a component of terrain avoidance warning systems, warning the pilot if the aircraft is flying too low, or if rising terrain ahead is a hazard to be avoided. Radar altimeter technology is also used in terrain-following radar allowing fighter aircraft to fly at very low altitude.

Handheld GPS devices can also determine altitude by trilateration with multiple satellites. However, altitude determined using GPS is not accurate enough to be certified by local aviation authorities without using some method of augmentation.

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.

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

To measure the actual local air pressure (also called ambient 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.

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.