Just to clarify what Yancey posted. The spacing between the 2 pulses sent be the DME is fixed (12 uS in X and 30 or 36uS in Y Y I can remember the Y spacing from the aircraft...I'm old and forgetful) but the timing between the pairs being sent out is somewhat random. In general a single channel DME interrogates the ground station a little over 100 times a second till it locks on, then 23 times a second thereafter.