The radio altimeter is often erroneously described as a "radar altimeter". In fact it doesn't use radar principles beyond the fact that it responds to reflected signals.
Pedant on. Read his post and while he comes across as somewhat knowledgeable his brief of how they operate was not complete. He only talks of modulated frequency and not pulse. As to what they are called the manufacturers these days refer to them mostly as radar altimeters, although the radio is used infrequently. Radio/radar it matters not.
http://search.honeywell.com/search?p...meter&x=7&y=10
FreeFlight Systems Radar Altimeters and Indicators
The manual of the aircraft I flew describes it as a radio altimeter. Depending on the particular piece of kit fitted it operates "to send a signal to the ground, receive an echo, and measure the phase or time difference between the transmitted and received signals. In the pulse system the time may be measured, and in the continuous wave systems the phase is measured".
Pedant off. (Sorry BOAC and forget)