radar beacon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fixed device that sends or receives, amplifies, alters, and returns a radar signal, permitting a distant receiver to determine its bearing and sometimes its range.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a device that, on receiving radar signals, transmits coded signals in response to help navigators determine their position