The FAA's IFR handbook glosses over a keypoint:
The aircraft DME transmits interrogating radio frequency (RF) pulses, which are received by the DME antenna at the ground facility. The signal triggers ground receiver equipment to respond to the interrogating aircraft. The airborne DME equipment measures the elapsed time between the interrogation signal sent by the aircraft and reception of the reply pulses from the ground station. This time measurement is converted into distance in nautical miles (NM) from the station.
Which is: how does one airborne DME ignore the replies meant to other planes?
The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and, after a precise time delay (typically 50 microseconds), the ground station replies with an identical sequence of pulse-pairs. [emphasis added]
So is the pulse-pair unique to each unit? Or does the DME unit listen for replies and then picks a different pulse-pair for itself?
ICAO Annex 10 mentions 4 modes:
Mode W, X, Y, Z. A method of coding the DME transmissions by time spacing pulses of a pulse pair, so that each frequency can be used more than once.
But it is unclear to me how each airborne DME achieves a unique interrogating signal.
Just the basic concept would be great if the radio theory is too complicated.