A radar altimeter is a specialized radar used by aircraft to determine their height over ground (i.e., altitude). Modern implementations are more accurate than the barometric altimeters previously used, but a radar altimeter is not a perfect instrument. For example, the receiver, on a moving aircraft, may pick up a secondary reflection (i.e., multipath effect) of the signal, and assume that delayed signal is coming from the ground immediately underneath.
If the ground is highly absorptive of the radar frequency in use, it may not be possible to get an adequate measurement. Radar altimeters also can become confused by wave motion, when flying over rough water at low level.