DME

Interrogator and transponder

DME operates in the UHF (ultra-high frequency) band, and for the first time the measurement itself is a two-way radio exchange between the system's two basic components: the interrogator in the aircraft and the transponder on the ground. The aircraft's interrogator transmits a timed interrogation; the ground station's transponder replies on a different frequency, and the airborne equipment measures the time between question and answer. Radio waves travel at a known speed, so the round-trip time, less the transponder's fixed reply delay, halves into the distance. The reply deliberately sits on a different frequency from the interrogation, so the receiver can never mistake its own transmitted pulse, or its echo, for the station's answer.

Slant range, not ground range

What the timer measures is the straight line from antenna to antenna, and that line leaves the ground the moment the aircraft does: the distance measured by DME is slant range, the hypotenuse of the triangle whose other sides are the aircraft's height and the distance over the ground. Directly overhead the station, the ground distance is zero and the DME reads the aircraft's height and nothing else. The exam's favourite arithmetic converts one side of that triangle into another:

The circle and the arc

One DME reading does not give a point: a DME position line is a circle with the station at its centre, every point on it at the same range. Approach design turns that circle into a road: a DME arc, flown in accordance with ICAO (International Civil Aviation Organization) Doc 8168 Volume 1, keeps the aircraft at a constant DME distance while it curves around the station, typically to join a final approach course from the side. The technique is a string of short straight legs: with a VOR/DME pair, keep the RMI (radio magnetic indicator) needle near the wingtip, fly a few degrees, let the range drift a fraction, correct toward or away, and the aircraft rides the circle without ever turning continuously.

Paired with the VOR

DME rarely stands alone. The pairing of VHF (very high frequency) and UHF frequencies (VOR/DME) enables the selection of two items of navigation information from one frequency setting: tune the VOR's published VHF frequency, and the paired UHF DME channel is selected with it, giving bearing and distance together, the rho-theta fix a single station can provide. When a DME is co-located with a VOR or an ILS (instrument landing system), the frequencies are paired and the identification is shared: selecting the published VHF frequency tunes both, and both transmit the same Morse letters in turn. The ear can still tell them apart: the identification signal with the higher-tone frequency is the DME, identifying itself approximately every 40 seconds, once for every few of the VOR's idents, in a noticeably higher tone. The military runs the same idea in its own band: military UHF TACAN (tactical air navigation aid) stations may be used for DME information by civil aircraft, the distance half of TACAN being an ordinary DME answer.

One hundred aircraft

A DME transponder is one transmitter answering everybody, and it transmits its reply pulses at a fixed maximum rate: a ground station can generally respond to a maximum of about 100 aircraft. Roughly 100 interrogating aircraft exhaust that reply rate, and when more interrogations arrive than the station can answer, it protects itself by reducing its receiver's sensitivity, so the weakest interrogations stop being heard: the aircraft farthest away are denied a DME range first, while the strong, close-in signals keep their service.

The groundspeed read-out

A DME system may combine a GS (groundspeed) and a time-to-station read-out with the distance read-out, all three derived from the same measurement. The groundspeed is derived from the rate at which the range changes, and the derivation carries the slant-range subtlety with it: the equipment can only see how fast the slant range is changing, the component of the aircraft's motion along the line to the station. The two speeds agree only when flying straight at or away from the station at long range, where the slant line and the ground run coincide; any other geometry sends part of the motion across the line rather than along it, so the GS (groundspeed) read-out from a DME can be less than the actual groundspeed. The extreme case is the arc: on a DME arc the range is constant by definition, nothing moves along the line at all, and the read-out is zero while the aircraft is flying at full speed around the circle.