Distance Measuring Equipment (DME)
Distance measuring equipment (DME) functions by means of two-way transmissions of signals between the aircraft and the DME site. Paired pulses at a specific spacing are sent out from the aircraft and are received by the ground station. The ground station then transmits paired pulses back to the aircraft on a different frequency. The time required for this signal exchange is measured in the airborne DME unit and is translated into distance (nautical mile [NM]) from the aircraft to the ground station. Distance information received from DME is slant range distance and not actual horizontal distance. Accuracy of the DME system is within ±0.5 NM or three percent of the distance, whichever is greater.
DME is collocated with most Canadian VHF omnidirectional range (VOR) installations (VOR/DME) and with many instrument landing system (ILS) and localizers (LOCs). In some cases, DME are also collocated with non-directional beacons (NDBs) to provide improved navigation capability. For collocated sites, a single keyer is used to key both the VOR/ILS/LOC and the DME with the three-letter location indicator. The VOR/ILS/LOC transmits three consecutive indicator codes in a medium pitch of 1 020 Hz followed by a single DME indicator code transmitted on the DME frequency (ultrahigh frequency [UHF]) and modulated at a slightly higher pitch of 1 350 Hz. In the event that synchronization from the VOR/ILS/LOC should fail, the DME identification will be transmitted independently.
The DME system is in the UHF band and therefore is limited to line-of-sight reception with a range similar to that of a VOR. Most DME “X” and “Y” channels are paired with VOR and LOC frequencies. As a result, the receiving equipment in most aircraft provide automatic DME selection through a coupled VOR/ILS receiver. Otherwise, the DME interrogator must be selected to the paired VOR or LOC frequency. Distance information from an independent tactical air navigation aid (TACAN) facility can be obtained by selecting the appropriate paired VOR frequency.
From Nav Canada