A device that, on receiving radar signals, transmits coded signals in response to help navigators determine their position WordNet 3.0 & Open English WordNet

Radar beacon is – according to article 1.103 of the International Telecommunication Union's ITU Radio Regulations – defined as "A transmitter-receiver associated with a fixed navigational mark which, when triggered by a radar, automatically returns a distinctive signal which can appear on the display of the triggering radar, providing range, bearing and identification information." Wikipedia

A beacon that, on detecting a radar signal, responds by transmitting a coded navigation signal. Wiktionary