RADIO ALTIMETER- Aircraft equipment which makes use of the reflection of radio waves from the ground to determine the height of the aircraft above the surface.
RADIO BEACON-
(See NONDIRECTIONAL BEACON.) ...
Autoland requires the use of a radio altimeter to determine the aircraft's height above the ground very precisely so as to initiate the landing flare at the correct height (usually about 50 feet).
An Alert Height, as read from the radio altimeter, is a height defined for operational use above which a Category III approach would be discontinued and a missed approach initiated if a failure occurred in one of the required aircraft or ground ...
The original Ground Proximity Warning System (GPWS) concept relied on information from the air data system and radio altimeter.
 See also: Range, Expect, Emergency, Direct, Speed
  
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