DME Frequencies
 

Can anyone give me info on DME frequencies, how channel spacing is split between X and Y, what are the pulse pair timings etc. etc. Each different ref book I look at seems to give part of the picture but not the whole, with some even giving contradictory info.

Aircraft transmit pulsed interrogation. Ground station replies after 50microseconds. Received by aircraft. Elapsed time is appropriately converted into a distance for display.
[Time in microseconds-50]/ 12.359 = slant range in nm
SIGNAL CHARACTERISTICS
All pulse pairs
Interrogation (aircraft to ground)
Pulse width= 3.5 microsec
Pulse spacing ModeX= 12 microsec
ModeY= 36microsec
Radio frequency 1041-1150 MHz
Channel spacing 1 MHz

Reply (ground to aircraft)
Pulse width= 3.5 microsec
Pulse spacing ModeX=12microsec
ModeY=30microsec
Radio frequency 978-1213MHz
Channel spacing 1 MHz

X AND Y
Reply freqs are 63MHz removed from interrogation freqs. For example on channel 17X interr freq=1041Mhz (the lowest) and replies freq=978MHz
channel 17Y interr freq=1041MHz replies freq=1104MHz
There are only 100 interrogation frequencies in ICAO (more in tacan).
So there are 100 mode X channels for reply and 100 modeY channels for reply. Total 200.
Handy that, as there are 160 VOR frequencies and 40 ILS frequencies. They are paired off.
For example the lowest VOR frequency is 108.00 Mhz. Its paired DME channel is 17X. A colocated DME, if fitted to the ground beacon, receives only on 1041 MHz. If it receives a signal from an aircraft, it replies on 978 MHz.
To keep things easy in the cockpit, when the NAV receiver is set to 108.00 in the VHF band the DME interrogator is automatically set to broadcast on 1041Mhz in the UHF band, and to listen out on 978.
There is much more!