Abstract: A DME apparatus and method for determining the distance of an aircraft from a ground station and the groundspeed thereof. Replies to the transmitting aircraft are distinguishd from replies to other aircraft and squitter by digitally storing information indicative of the times of receipt of all replies received during a first cycle and determining whether any of the stored times are identical to the times of receipt of replies received during the cycles following the first cycle for a predetermined number of successive cycles. If this criterion is met distance information is derived and displayed and a range gate is generated and combined with incoming replies. If replies fall within the range gate for at least a predetermined percentage of cycles then the apparatus is switched into the track state in which all replies are ANDed with the range gate before storage and the coincidence determination is effected.