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radio altimeter:see altimeteraltimeter
, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises.
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an instrument for determining the flight altitude of an aircraft or spacecraft by measuring the time required for radio waves radiated by the instrument to be reflected from the surface below; the time is used to determine the flight altitude based on the known propagation velocity of the radio waves. A distinction is made between radio altimeters using frequency modulation (FM altimeters) and pulse modulation (pulse altimeters) of the emitted radio waves.
FM altimeters are used in aviation mainly at low altitudes, for example, during a landing approach. In this case, continuous radio signals are emitted whose frequency is varied periodically according to a specified law. The altitude of the aircraft is determined by the frequency difference between the radiated and reflected radio signals, which is shown on the instrument’s indicator.
Pulse altimeters are used in aviation (for example, in aerial photography at high altitudes) and in space flights (for example, to give a command for firing a spacecraft’s retrorocket engine at a certain altitude above a planet’s surface). In this case, the instrument emits short radio pulses. The altitude of the aircraft is determined by measuring the time delay of the reflected radio pulses relative to the radio pulses received directly from the transmitter.