Distance Measuring Equipment
Distance measuring equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of UHF radio signals. The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder) on the ground. The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and the ground station replies with an identical sequence of reply pulse-pairs with a precise time delay (typically 50 microseconds). The time difference between interrogation and reply minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement in nautical miles which is then displayed in the cockpit.