altimeter

[Latin altus, high + -METER.]

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Any device which measures the height of an aircraft. The two chief types are the pressure altimeter, which measures the aircraft's distance above sea level, and the radar altimeter, which measures distance above the ground.

Pressure altimeter

A pressure altimeter precisely measures the pressure of the air at the level an aircraft is flying and converts the pressure measurement to an indication of height above sea level according to a standard pressure-altitude relationship. In essence, a pressure altimeter is a highly refined aneroid barometer since it utilizes an evacuated capsule whose movement or force is directly related to the pressure on the outside of the capsule. Various methods are used to sense the capsule function and cause a display to respond such that the pilot sees the altitude level much as one looks at a watch.

Because altitude measured in this manner is also subject to changes in local barometric pressure, altimeters are provided with a barosetting that allows the pilot to compensate for these weather changes, the sea-level air pressure to which the altimeter is adjusted appearing in a window of the dial. Flights below 18,000 ft (5486 m) must constantly contact the nearest traffic center to keep the altimeters so updated. Flights above 18,000 ft and over international waters utilize a constant altimeter setting of 29.92 in. Hg, or 1013.2 millibars (101.32 kilopascals), so that all high-flying aircraft have the same reference and will be interrelated, providing an extra margin of safety. See also Air navigation.

Radar altimeter

A radar altimeter is a low-power radar that measures the distance of an aircraft (or other aerospace vehicle) above the ground. Radar altimeters are often used in aircraft during bad-weather landings. They are an essential part of many blind-landing and automatic navigation systems and are used over mountains to indicate terrain clearance. Special types are used in surveying for quick determination of profiles. Radar altimeters are used in bombs, missiles, and shells as proximity fuses to cause detonation or to initiate other functions at set altitudes. Radar altimeters have been used on various spacecraft, starting with Skylab in 1973, to measure the shape of the geoid and heights of waves and tides over the oceans. Other spacecraft altimeters provide topographic information on other planets, particularly Venus. See also Automatic landing system; Ground proximity warning system.

Like other radar devices, the altimeter measures distance by determining the time required for a radio wave to travel to and from a target, in this case the Earth's surface. If the Earth were a perfectly flat horizontal plane or smooth sphere, the signal would come only from the closest point, and would be a true measure of altitude. Actually, the Earth is not smooth, and energy is scattered back to the radar from all parts of the surface illuminated by the transmitter. For the radar to measure distance to the ground accurately, it must distinguish between the energy from points near the vertical and that from more distant points.

Most radio altimeters use either pulse or frequency modulation, the former being more popular for high altitudes, and the latter for low altitudes. In a typical pulse altimeter the radio-frequency carrier is modulated with short pulses (under 0.25 microsecond). The short pulse permits measurements, even at low altitudes, of the time delay between the leading edge of the transmitted pulse and that of the pulse returned from the ground. Early pulse altimeters displayed the received signal on a cathode-ray tube with circular sweep, allowing the pilot to determine the leading-edge position of the echo signal. Modern pulse altimeters use a tracking gate system. One gate is kept close to the leading edge by a servo system that adjusts the position of the gate to the optimum delay point. A simple single-gate system can be used, but most pulse altimeters use two or three gates to achieve better distance measurement in the presence of noise and fading. See also Pulse modulator.

In a frequency-modulated (FM) altimeter, the frequency of a continuous carrier is swept in some manner, usually to give a triangular frequency-time curve. The difference in frequency between that received from the ground (but transmitted earlier) and that being transmitted is a measure of the time delay. See also Frequency modulation.

An instrument used to plot altitude. Altimetry is the measurement of heights. An altimetric frequency curve shows how often various heights of land occur within a given area. Such curves may be used to identify the remnants of erosion surfaces.

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Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an altitude of 10,180 feet.

An altimeter is an 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.

Contents 1 Aircraft instrumentation 1.1 Pressure altimeter 1.2 Radar altimeter 1.3 Global Positioning System 2 Other modes of transport 3 Measuring air pressure 4 Satellites 5 See also 6 External links 7 References

Aircraft instrumentation

Pressure altimeter

Digital barometric pressure sensor for altitude measurement in consumer electronic applications

A pressure altimeter (also called barometric altimeter) is the altimeter found in most aircraft. In it, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude—approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet near sea level.

The altimeter is calibrated to show the pressure directly as an altitude above mean sea level, 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.

On a sensitive altimeter, the sea level reference pressure can be adjusted by a setting knob. The reference pressure, in inches of mercury in Canada and the US and hectopascals (previously millibars) elsewhere, is displayed in the Kollsman window, visible at the right side of the aircraft altimeter shown here. This is necessary, since sea level reference atmospheric pressure varies with temperature and the movement of pressure systems in the atmosphere.

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:

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.

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 there is rising terrain ahead. Radar altimeter technology is also used in terrain-following radar allowing fighter aircraft to fly at very low altitude.

Global Positioning System

Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. However, altitude determined using autonomous GPS is not precise or accurate enough to supersede the pressure altimeter for aviation use without using some method of augmentation.

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 altimeters.

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

Measuring air pressure

The local atmospheric pressure or ambient pressure is displayed in the Kollsman window of a sensitive altimeter, when it is adjusted to read zero altitude.^[1]

Satellites

Main article: Satellite altimetry

This graph shows the rise in global sea level (in millimeters) measured by the NASA/CNES ocean altimeter mission TOPEX/Poseidon (on the left) and its follow-on mission Jason-1. Image credit: University of Colorado

A number of satellites (see links) use advanced dual-band radar altimeters to measure height from a spacecraft. That measurement, coupled with orbital elements (possibly augmented by GPS), enables determination of the terrain. The two different wavelengths of radio waves used permit the altimeter to automatically correct for varying delays in the ionosphere.

Spaceborne radar altimeters have proven to be superb tools for mapping ocean-surface topography, the hills and valleys of the sea surface. These instruments send a microwave pulse to the ocean’s surface and time how long it takes to return. A microwave radiometer corrects any delay that may be caused by water vapor in the atmosphere. Other corrections are also required to account for the influence of electrons in the ionosphere and the dry air mass of the atmosphere. Combining these data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimetres (about one inch). The strength and shape of the returning signal also provides information on wind speed and the height of ocean waves. These data are used in ocean models to calculate the speed and direction of ocean currents and the amount and location of heat stored in the ocean, which, in turn, reveals global climate variations.

See also

• Flight instruments
• Flight level
• seasat, TOPEX/Poseidon are satellites that deployed extremely accurate altimeters
• United Airlines Flight 389, an accident attributed to misreading of an altimeter
• Turkish Airlines Flight 1951, an accident attributed to a malfuctioning radio altimeter
• Jason-1, Ocean Surface Topography Mission/Jason-2 are current satellite missions that use altimeters to measure sea surface height

External links

Wikimedia Commons has media related to: Altimeter

• MS5561C Micro Altimeter for GPS, 1m resolution
• History of the Kollsman altimeter
• A Flash 8 based simulator for altimeter errors caused by variations in temperature and pressure
• The use of altimeters in height measurement - for hillwalkers
• Compact digital pressure sensor for altimeters
• Early use of barometers on surveys
• The altimeter and the types of altitude
• Evolution of the Modern Altimeter - Flight archive

References

1. ^ http://www.stolaf.edu/people/hansonr/soaring/altimetr.htm

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Dansk (Danish)
n. - højdemåler

Nederlands (Dutch)
hoogtemeter

Français (French)
n. - altimètre

Deutsch (German)
n. - Höhenmesser

Ελληνική (Greek)
n. - υψομετρικό όργανο, (τεχνολ.) όργανο ένδειξης ύψους πτήσης

Italiano (Italian)
altimetro

Português (Portuguese)
n. - altímetro (m) (Aer.)

Русский (Russian)
альтиметр

Español (Spanish)
n. - altímetro

Svenska (Swedish)
n. - höjdmätare

中文（简体）(Chinese (Simplified))
高度计

中文（繁體）(Chinese (Traditional))
n. - 高度計

한국어 (Korean)
n. - 고도계

日本語 (Japanese)
n. - 高度計

العربيه (Arabic)
‏(الاسم) مقياس, الارتفاعات‏

עברית (Hebrew)
n. - ‮מכשיר במטוסים למדידת הגובה מעל פני הים או הקרקע, מד-גובה, מד-רום‬

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