DME Freq Diff RNAV
After the aircraft transmits its interrogation pulses, it then listens out for the response to its own "jiggered" pulses.
If the aircraft listened out on the same frequency, there is every chance that it could mistake reflections from the ground as the response from the DMEDME —Distance Measuring Equipment beacon — and thus display the distance to the ground and not to the DMEDME —Distance Measuring Equipment. This problem is solved entirely by having the DMEDME —Distance Measuring Equipment respond on a frequency that is 63 MHz removed from the aircraft transmission. (This 63 MHz difference is also an exam fact.)
If the aircraft hears the response on the same frequency that it transmitted on, it must be a reflection and can be ignored.