Altimeter

Information about Altimeter

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.

Aircraft Instrumentation

Pressure altimeter

A pressure altimeter (also called barometric altimeter) is the traditional altimeter found in most aircraft. In it, an 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.

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

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

$\text{[math]}$

where h is the indicated altitude in feet, $\text{[math]}$ is the static pressure and $\text{[math]}$ is the reference pressure (use same units for both). This is derived from the barometric formula using the scale height for the troposphere.

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.

GPS

Handheld GPS devices can also determine altitude by triangulation with multiple satellites.

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.

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.

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.

External links

For other uses see Altitude (disambiguation)

Altitude is the elevation of an object from a known level or datum (plural: data). Common data are mean sea level and the surface of the WGS-84 geoid, used by GPS.
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Bathymetry is the underwater equivalent to altimetry. The name comes from Greek βαθυς, deep, and μετρον, measure.
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aircraft is a vehicle which is able to fly through the air (or through any other atmosphere). All the human activity which surrounds aircraft is called aviation. (Most rocket vehicles are not aircraft because they are not supported by the surrounding air).
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A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather.
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Air pressure can refer to:

Atmospheric pressure, the pressure of air environmentally
Pressure of air in a system

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A Pitot-static system consists of a system of pressure-sensitive instruments and the means by which the appropriate pressures are obtained. A Pitot-static system is generally composed of the pitot-static instruments, a Pitot tube and a static port.
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The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. They are not SI units, but they are accepted for use with the SI.
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This article or section relies largely or entirely upon a .
Please help [ improve this article] by introducing appropriate of additional sources. ()
This article has been tagged since December 2006.
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International Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes.
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Clock Face is an area in the borough of St Helens, Merseyside, England. It was formerly the site of a coal mine which closed in the 1960s.

A clock face
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Paul Kollsman (February 22 1900 in Germany – March 17 1982 in Beverly Hills, California) was an American inventor. He invented barometers and instruments for instrument flight in airplanes.

Kollsman studied civil engineering in Stuttgart and Munich.
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QNH is a Q code. It is a pressure setting used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain defined region.
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QFE is a three letter acronym which can have meanings in aviation, in software development, and in internet usage. It can refer to

QFE, a Q code used by pilots and air traffic control (ATC)
A software development acronym for Quick Fix Engineering.

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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.
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not to scale.]]

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and aerosols.

The average depth of the troposphere is about 11 km in the middle latitudes.
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A radar altimeter, radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which
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Terrain-following radar is an aerospace technology that allows a very-low-flying aircraft to automatically maintain a constant altitude. It is sometimes referred-to as ground hugging, terrain hugging or nap-of-the-earth flight.
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fighter aircraft is a military aircraft designed primarily for attacking other aircraft, as opposed to a bomber, which is designed to attack ground targets, primarily by dropping bombs. Fighters are comparatively small, fast, and maneuverable.
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Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its
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triangulation is the process of finding coordinates and distance to a point by calculating the length of one side of a triangle, given measurements of angles and sides of the triangle formed by that point and two other known reference points, using the law of sines.
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satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
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Duesenberg was a United States-based luxury automobile company active in various forms from 1913 to 1937, most famous for their extremely high-quality, record-breakingly fast roadsters.
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Mountaineering is the sport, hobby or profession of walking, hiking, trekking and climbing up mountains. It is also sometimes known as alpinism, particularly in Europe.
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Parachuting is an activity involving a preplanned drop from a height using a deployable parachute.

One type of parachuting is skydiving, which is recreational parachuting, also called sport parachuting.

The history of parachuting isn't clear.
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Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain.
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spacecraft is a vehicle or device designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters outer space but then returns to the planetary surface (such as Earth) without making a complete orbit.
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ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
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