DME Jiggered PRF Reason RNAV

When the aircraft interrogates a DMEDME —Distance Measuring Equipment, that DMEDME —Distance Measuring Equipment may be responding to many other aircraft at the same time. Consequently, there is the problem that each aircraft has to distinguish which of the DMEDME —Distance Measuring Equipment responses is for it and which are for other aircraft. The way it does this is to "jigger" — that is, randomly change — the "rhythm" of the pulse pairs it sends to the DMEDME —Distance Measuring Equipment. When the DMEDME —Distance Measuring Equipment responds, it will respond with the same "jiggered" pulses that it was interrogated with.

A useful way to think of this is imagine the aircraft is sending a "song" to the DMEDME —Distance Measuring Equipment, and waiting for that song to be played back to it. When the aircraft hears its own song, it knows that that response pulse is for it. Just be careful not to think each aircraft has always transmits the same, unique song however — this is not true, as the PRFPRF —Pulse Repetition Frequency is randomly changed.