altimeter

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. more...

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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.

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.

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

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.

Read more at Wikipedia.org