Diagram showing the face of a three-pointer sensitive aircraft altimeter displaying altitude in feet.

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. Image File history File links Size of this preview: 588 × 599 pixel Image in higher resolution (638 × 650 pixel, file size: 30 KB, MIME type: image/png) Source: Instrument Flying Handbook (IFH) FAA-H-8083-15. ... Image File history File links Size of this preview: 588 × 599 pixel Image in higher resolution (638 × 650 pixel, file size: 30 KB, MIME type: image/png) Source: Instrument Flying Handbook (IFH) FAA-H-8083-15. ... Altitude is the elevation of an object from a known level or datum. ... Bathymetry is the underwater equivalent to topography. ...

Aircraft Instrumentation

Pressure altimeter

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. An Airbus A380, currently the worlds largest passenger airliner An aircraft is any vehicle or craft capable of atmospheric flight. ... A barometer is an instrument used to measure atmospheric pressure. ... Air pressure can refer to: Atmospheric pressure, the pressure of air environmentally Pressure of air in a system Air pressure is the pressure of the atmosphere pushing down on the earth. ... A static port in an airplane is usually nothing more than a small circular opening in the airplanes skin. ... The bar (symbol bar) and the millibar (symbol mbar, also mb) are units of pressure. ... For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ...

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. Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ... A clock face is the part of an analog clock that tells time through the use of a fixed numbered dial or dials and moving hand or hands. ...

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. Image File history File links Size of this preview: 800 × 564 pixel Image in higher resolution (1002 × 707 pixel, file size: 56 KB, MIME type: image/png) Source: Instrument Flying Handbook (IFH) FAA-H-8083-15. ... Image File history File links Size of this preview: 800 × 564 pixel Image in higher resolution (1002 × 707 pixel, file size: 56 KB, MIME type: image/png) Source: Instrument Flying Handbook (IFH) FAA-H-8083-15. ... Paul Kollsman (February 22, 1900 in Germany – March 17, 1982 in Beverly Hills, California) was an American inventor. ...

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. For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ... QNH is a Q code. ... QFE is a three letter acronym which can have meanings in aviation, in software development, and in internet usage. ...

Kollsman-type barometric aircraft altimeter as used in North America

The calibration formula for an altimeter, up to 36,090 feet (11,000 m), can be written as: Image File history File linksMetadata Download high resolution version (1632x1232, 578 KB) Aircraft altimeter set at 75 ft (23 m) showing a pressure of 29. ... Image File history File linksMetadata Download high resolution version (1632x1232, 578 KB) Aircraft altimeter set at 75 ft (23 m) showing a pressure of 29. ...

where h is the indicated altitude in feet, P is the static pressure and P_ref is the reference pressure (use same units for both). This is derived from the barometric formula using the scale height for the troposphere. The barometric formula, sometimes called the exponential atmosphere or isothermal atmosphere, is a formula used to model how the pressure (or density) of the air changes with altitude. ... Atmosphere diagram showing the mesosphere and other layers. ...

Radar altimeter

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. A Radar Altimeter measures altitude above the terrain presently beneath the aircraft. ... A Radar Altimeter measures altitude above the terrain presently beneath the aircraft. ... Terrain-following radar is an aerospace technology that allows a very-low-flying aircraft to automatically maintain a constant altitude. ... This article or section does not cite its references or sources. ...

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. A 1929 Duesenberg j350 Willoughby on display at the 2005 United States Grand Prix 1930 Duesenberg J Walker Legrande Torpedo Phaeton 1932 Duesenberg J Murphy-bodied coupe convertible Wikimedia Commons has more media related to: Duesenberg vehicles Duesenberg was a US-based luxury automobile company active from the 1910s until...

Mountaineers use wrist-mounted barometric altimeters when on high-altitude expeditions, as do skydivers. Mountaineering is the sport, hobby or profession of walking, hiking and climbing up mountains. ... Skydiver about to land Parachuting, or skydiving, is an activity involving the breaking of a free-fall from a height using a parachute. ...

Scientific Uses

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.

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. This long range Radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll[1]. Radar is a system that uses radio waves to determine and map the location, direction, and/or speed... A spacecraft is a vessel, craft or device designed to operate beyond the surface of the Earth in outer space. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ... Relationship of the atmosphere and ionosphere The ionosphere is the part of the atmosphere that is ionized by solar radiation. ...

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. “Gravity” redirects here. ...

See also

• Flight level
• seasat, TOPEX/Poseidon are satellites that deployed extremely accurate altimeters

External links

Wikimedia Commons has media related to:

Altimeter

• History of the Kollsman altimeter
• A Flash 8 Based Simulator for Altimeter Errors Caused by Variations in Temperature and Pressure