An instrument, typically an aneroid barometer,
that is used for determining the elevation or height above sea
level. Aneroid barometer style altimeters operate by precisely
measuring the change in atmospheric pressure, that decreases
with increasing height above sea level, since there is less air
exerting pressure at a point at higher elevations than at lower
elevations. Altimeters need to be calibrated each day at a
known elevation, to account for weather-related changes in
atmospheric pressure.
Before 1928 there was no possible way for pilots to
know how far above the ground they were. The German
inventor Paul Kollsman invented the first reliable and accurate
barometric altimeter. The altimeter measured altitude by barometric
pressure. Pilots still use the barometric altimeter today.
In 1924 Lloyd Espenschied invented the first radio
altimeter. In 1938 Bell Labs demonstrated the first radio
altimeter. A radio altimeter uses radio signals that bounce off
of the ground and back to the receiver in the plane showing
pilots the altitude of the aircraft. A radar altimeter works
much in the same way except it bounces the signal off of an
object in the air thus telling the height of the object above the
ground. A laser altimeter can measure the distance from a
spacecraft or satellite to a fixed position on Earth. The measurement
when compiled with radial orbit knowledge can
provide the topography of the Earth.
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