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

Let’s start with the identification rules, because that’s where the pairing logic really lives. When a VOR and a DME are associated, it means they are meant to be used together as one navigation aid. The book gives you three conditions for that association. First, both transmitters are co-located — that is, sitting at the same spot. Second, the maximum distance between the two transmitters is 30 metres, or 100 feet, but only when they are inside a TMA, a Terminal Manoeuvring Area. Third, if they are used elsewhere — outside a TMA — the maximum distance can be up to 600 metres, or 2000 feet. So the tolerance is much tighter in the terminal area. Now, the identification. Both the VOR and the DME transmit the same call sign. The book says there are four idents every 30-second period. Here is the split: the VOR transmits three of the four, and the DME transmits the fourth. So over any 30-second window, you hear the VOR ident three times and the DME ident once, but they are all the same call sign, so you cannot tell them apart by sound alone. Then there is a case called not associated but serving the same location. Here the first two letters of the ident are the same, but the last letter for the DME is ‘Z’. So you can tell them apart by that final letter. Finally, there is the case where the VOR and DME or TACAN are widely separated — more than 6 nautical miles apart. In that situation they may or may not be paired, and they will have totally different identifications. So the separation distance is the deciding factor: within 30 metres in a TMA, within 600 metres elsewhere, they are associated; beyond 6 nautical miles, they are not. Now let’s move to the DME range measurement for an ILS. When DME is paired with an ILS, the transponder is adjusted to give range to the threshold — the runway threshold — in UK systems. The ground installation cannot be placed at the threshold, so they compensate. They do this by reducing the time delay at the transponder. The goal is that the total time — the interrogation signal travelling from the runway threshold to the transponder, plus the delay at the transponder, plus the reply travelling back from the transponder to the threshold — equals 50 microseconds. Here is the worked example from the book. If the transponder is 1500 metres from the runway threshold, the time for the interrogation and reply pulses to travel between the threshold and the transponder is 5 microseconds each way. So the delay at the transponder must be reduced to 40 microseconds to give a range to the threshold. Let me check that arithmetic: 5 plus 40 plus 5 equals 50 microseconds. Exactly right. Now, range and coverage. DME transmissions obey the line of sight rule. So the higher the aircraft, and the higher the ground beacon, the greater the theoretical reception distance. Intervening high ground will block the line of sight range. There is also the effect of bank angle. When the aircraft banks, the aircraft antenna can be hidden from the transponder on the ground, causing an interruption in the flow of signals. However, the memory circuit ensures there is no major disruption to range measurement. So the system holds the last valid reading during that brief interruption. To overcome range errors caused by mutual interference between two or more facilities sharing the same frequencies, a Designated Operational Coverage — the DOC — is published for each DME. This protects a DME from co-channel interference under normal propagation conditions. The DOC is specified as a range and a height. If you use a DME beyond its DOC limitations, you may get range errors. Finally, to eliminate errors arising from reflections from the earth’s surface, buildings, or mountainous terrain, the aircraft receiver incorporates an Echo Protection Circuit. That circuit filters out the reflected signals so the range reading stays accurate. So, to tie it together: identification tells you which aid you are listening to, the ILS pairing adjusts the timing to give threshold range, and the coverage and echo protection circuits keep the measurement reliable.