Terminology
For clarification:
Radar = RAdio Detection And Ranging: The transceiver transmits a signal burst, then listens on the same antenna, much like ATC radar. Radar altitude is computed from delay between transmit and reception of signal. . Smaller civilian and military aircraft typically use radar altimeters.
The more common Radio Altimeter transceiver on larger aircraft transmits a continuous signal through a transmit antenna, and receives on a separate but identical antenna. Radio Altitude is computed from phase difference of modulation on transmitted and received signals.
Radio altimeters provide more reliable altitude data, and are designed primarily as a landing aid, to the point of being Flight Critical sensors during Autolands. Airbus and McDouglas have used dual radio altimeters for dual/dual Autoland architecture, while Boeing uses three of them in its Triplex architecture. . Radio altitude has many other uses, of course, including GPWS.
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