Distance Measuring Equipment (DME) — Page 257, Lesson 243

Let's start with the identification of VOR/DME and TACAN stations, because that's where this section opens. The key idea is that a DME is almost always paired with a VOR or a TACAN, and the way they identify themselves tells you exactly how they're related. First, the term "Associated" means the VOR and DME transmitters are co-located, or the maximum distance between them is 30 metres, which is 100 feet, in TMAs — that's Terminal Manoeuvring Areas. Or, the maximum distance can be up to 600 metres, which is 2000 feet, for use elsewhere. When they're associated, both transmitters transmit the same call sign. There are four idents every 30-second period. The VOR transmits three of those four, and the DME transmits the fourth. So you hear the same letters, but the DME's ident is the one that's slightly different in timing. Then there's "Not associated but serve the same location." In that case, the first two letters of the ident are the same, but the last letter for the DME is 'Z'. So you can tell from the call sign that they're not physically paired, but they're serving the same spot. Finally, "VOR/DME-TACAN widely separated," meaning more than 6 nautical miles apart. These may or may not be paired, and they have totally different identifications. So the call sign tells you the relationship. Now, the DME range measurement for ILS. When DME is paired with an ILS, the transponder is adjusted to give range to the threshold in UK systems. The ground installation can't be placed at the threshold, so they reduce the time delay at the transponder. The goal is that the time for the interrogation signal to travel from the runway threshold to the transponder, plus the delay at the transponder, plus the time for the reply to travel back from the transponder to the threshold, equals 50 microseconds. For example, if the transponder is 1500 metres from the threshold, the travel time each way is 5 microseconds. So the delay at the transponder must be reduced to 40 microseconds to give you range to the threshold. Then range and coverage. DME transmissions obey the line-of-sight rule. The higher the aircraft and the ground beacon, the greater the theoretical reception distance. Intervening high ground blocks the line of sight. Bank angle hides the aircraft antenna from the transponder on the ground, causing an interruption in the signal flow. But the memory circuit ensures there's no major disruption to range measurement. To overcome range errors from mutual interference between facilities sharing the same frequencies, a Designated Operational Coverage, or DOC, is published for each DME. This protects the DME from co-channel interference under normal propagation conditions. The DOC is specified as a range and a height. Using a DME beyond its DOC limitations may result in range errors. Finally, to eliminate errors from reflections off the earth's surface, buildings, or mountainous terrain, the aircraft receiver incorporates an Echo Protection Circuit. That figure shows beacon saturation, which is a separate concept, but it's relevant to how DME handles multiple aircraft. Let me know if you want to explore that.