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. the dme receiver in the aircraft searches for reply pulse-pairs (x-mode= 12 microsecond spacing) with the correct interval and reply pattern to its original interrogation pattern. (pulse-pairs that are not coincident with the individual aircraft's interrogation pattern e.g. not synchronous, are referred to as filler pulse-pairs, or squitter. also, replies to other aircraft that are therefore non-synchronous also appear as squitter).