Navigational aid going up or down (5)
I believe the answer is:
radar
This could be a double definition.
'navigational aid' is the first definition.
(I've seen this before)
'up or down' is the second definition.
'going' is the link.
This may not be correct. It may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for radar that I've seen before include "System for locating distant objects" , "Range-finding and detection system" , "Electronic rangefinder" , "Kind of gun" , "Locator that uses a microwave echo" .)