Navigational aid the same both ways (5)
I believe the answer is:
radar
I believe this is a double definition.
'navigational aid' is the first definition.
(I've seen this before)
'the same both ways' is the second definition.
(Other definitions for radar that I've seen before include "Instrument to detect and locate distant objects" , "waves to expose foe" , "Radio detection system" , "location finder" , "Remote detection system" .)