- Altimeter
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 termbathymetry , the measurement of depth underwater.Aircraft Instrumentation
Pressure altimeter
A pressure altimeter (also called barometric altimeter) is the altimeter found in most
aircraft . In it, ananeroid barometer measures theatmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude—approximately 100 millibars per 800meters or oneinch of mercury per 1000 feet nearsea 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 theInternational Standard Atmosphere (ISA). Older aircraft used a simple 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 aclock face . In other words, each needle points to a different digit of the current altitude measurement.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 millibars (or hectopascals) 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 ofpressure system s in the atmosphere.In
aviation terminology, the regional or local air pressure at mean sea level (MSL) is called theQNH 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 convert|36090|ft|m, can be written as:
:h = frac{(1-(P/P_{ref})^{0.19026}) imes 288.15}{0.00198122}
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 thetroposphere .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 interrain-following radar allowingfighter aircraft to fly at very low altitude.Global Positioning System
Global Positioning System (GPS) receivers can also determine altitude bytrilateration with four or moresatellite s. 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 an altimeter.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. [http://www.stolaf.edu/people/hansonr/soaring/altimetr.htm] ."
Satellites
A number of satellites (see links) use advanced dual-band
radar altimeters to measure height from aspacecraft . That measurement, coupled withorbit al elements (possibly augmented by GPS), enables determination of theterrain . The two differentwavelengths of radio waves used permit the altimeter to automatically correct for varying delays in theionosphere .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 bywater vapor in theatmosphere . Other corrections are also required to account for the influence of electrons in theionosphere 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 ofocean current s and the amount and location of heat stored in the ocean, which, in turn, reveals globalclimate variation s.See also
*
Flight instruments
*Flight level
*seasat ,TOPEX/Poseidon are satellites that deployed extremely accurate altimeters
*United Airlines Flight 389
*Jason-1 ,Ocean Surface Topography Mission/Jason-2 are current satellite missions that use altimeters to measure sea surface heightExternal links
* [http://www.kollsman.com/company/index.asp History of the Kollsman altimeter]
* [http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx A Flash 8 based simulator for altimeter errors caused by variations in temperature and pressure]
* [http://www.biber.fsnet.co.uk/altim.html The use of altimeters in height measurement] - for hillwalkers
* [http://www.longcamp.com/baro.html Early use of barometers on surveys]
* [http://www.dutchops.com/Portfolio_Marcel/Articles/Instruments/Air_Data_Instruments/Altimeter.htm The altimeter and the types of altitude]
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