An altimeter is an active instrument used to measure the altitude of an object above a fixed level. For example, a laser altimeter can measure height from a spacecraft above an ice-sheet. That measurement, coupled with radial orbit knowledge, will enable determination of the topography.
The traditional altimeter found in most aircraft works in measuring the air pressure from a static port in the airplane. Air pressure decreases with altitude - about one millibar per 27 feet close to sea level. The altimeter is calibrated to show the pressure directly as altitude in accordance with a mathematical model defined by the International Standard Atmosphere (ISA).
The reference pressure can be adjusted by a setting knob. This is nescessary since sea level air pressure varies with the weather. In pilot's jargon, the regional or local air pressure at mean sea level is called the QNH, 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. Difference in temperature from the ISA model will therefore cause error in indicated altitude.
In a spreadsheet, the calibration formula for an altimeter (up to 36090 feet) can be written as:
h =(1-(P0/Pref)^0.19026)*288.15/0.00198122
where h is the indicated altitude in feet, P0 is the static pressure and Pref is the reference pressure (use same units for both).
Other types of altimeter are the radar altimeter that measures the altitude more exactly using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure the exact height during the landing procedure of commercial aircraft.
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