radio altimeter

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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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The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Radio Altimeter


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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

radio altimeter

[′rād·ē·ō al′tim·əd·ər]
An absolute altimeter that depends on the reflection of radio waves from the earth for the determination of altitude, as in a frequency-modulated radio altimeter and a radar altimeter. Also known as electronic altimeter; reflection altimeter.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

radio altimeter

Altimeters employing radio waves to measure the height of the aircraft above the surface. The frequency of the transmitted and received signal is the same but the transmitted frequency is continuously changed. The time difference between the transmitted and the received signals is converted into height and indicated in feet or meters on the indicator. Most radio altimeters do not indicate height when an aircraft is above a certain distance from the ground. Some errors are likely when an aircraft is banked.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
References in periodicals archive ?
The block diagram of a single-antenna FM radio altimeter is shown in Figure 1.
The single-antenna FM radio altimeter comprises a transmitter (1), a receiver (2) and a processing block (3).
The FM radio altimeter operates in the CW mode until the aircraft lands.
In the CW mode, the FM radio altimeter operates with a constant modulation period [T.sub.m0].
The Dutch Safety Board has issued a warning to Boeing to prevent faulty radio altimeters influencing the autopilot.

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