It is strange. One notices this when flying a DME arc (velocity=0 is a clue you are in parallel with the arc) but you have no idea which way it might go.
The data encoding shows why. No sign bit.
Pretty obviously the DME (this is a KN63) does not use doppler; it uses two consecutive readings and works it out. Surprisingly the velocity is returned to 0.01nm.
Is this the signal out of the DME receiver to some display device? I recall DME receiver just calculates distance by measuring a delay between pseudo random sequence sent and received it back from ground station.
I guess speed is calculated by receiver and technically can be negative if distance is increasing over time?
Fun fact that I learned. DME ground station adds 50ms delay before replying the original sequence. But can be changed to trick DME in airplane to calculate distance to threshold even if DME ground station is installed beyond it.
It is just odd that the protocol does not transmit the sign of the velocity, even though it must obviously have it internally.
The delay is 50us (microseconds), BTW. AIUI it is there to make DMEs function even when you are overhead.
The nonzero offset (on many terminal DMEs) is a different thing. That is a fake delay delta, enabling the DME to be physically placed somewhere away from the touchdown zone. That is incidentally why terminal DMEs should not be used in DME-DME RNAV.
The DME ground station is a reverse transponder, it receives pulse pairs from aircraft DME broadcasts on one frequency, delays, and then broadcasts the same pulse pair sequence on another frequency. Pulses from all DME are mixed together in the received and transponded pulse trains. Each DME randomizes the time between pulse pairs and a DME receiver matches up its own delayed sequence timing. The round trip timing delay is used by the receiver to calculate a distance. There isn’t any distance information in the protocol, much less velocity or direction. The ground station is a simple transponder and can handle about 100 DME simultaneously. When receiving more than 100 DME, the strongest signals will override weaker ones. So the receiver in the aircraft is where distance is calculated and if velocity is determined from the change in distance, is where a sign could be determined.
