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The first operationally effective barometric altimeter was designed by Paul Kollsman and first tested in 1929. It was designed and is still primarily used as an aircraft guidance instrument. Kollsman, who immigrated to the United States from Germany, was determined to find a way for aeroplanes to fly at night or in low visibility. The barometric altimeter makes that possible.
The barometric altimeter works by measuring air pressure and correlating it to a specific altitude. At higher altitudes, air pressure decreases; at low altitudes, air pressure increases. A barometric altimeter measures the air pressure just like a meteorological barometer and then translates that reading into a measurement of altitude. A barometric altimeter must be calibrated for adjustments to local air pressure to obtain the most accurate reading.
In Modern Aircraft
Barometric altimeters in aeroplanes take pressure readings from a "static port," which is a vent in the fuselage protected from the interference of speed and wind. The static port, which is also used in modern aircraft to calculate temperature and other data, gives the most accurate pressure readings, which can then be interpreted to altitude measurements accurate within 3 meters.
Air pressure is affected by temperature and velocity, among other variables, and the measurement can be complicated. A pitot-static system combines measurements from a static port with measurements from an open port on the wing of an aeroplane, and specially calibrated computers calculate the necessary adjustments to obtain an accurate reading.
Global positioning system (GPS) altimeters are also in use today, but they require readings from four separate satellites and are not as accurate as barometric altimeters, which are still the most reliable altimeters on earth.
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