Distance Measuring Equipment (DME) — Page 248, Lesson 242

Let’s pick this up right where the DME picture gets interesting — the saturation behaviour of the beacon. In Figure 15.9, we have aircraft A through G, and all of them are receiving ranges from the transponder. Aircraft B is just entering the coverage. Now, here’s the key mechanism: when the transponder becomes saturated, the receiver gain is reduced. That reduction in gain means the weaker interrogations — the ones from further away or at the edge of coverage — will no longer be answered. In the figure, aircraft A, B, E, F and G will be excluded and will unlock. So the system deliberately drops those aircraft. The aim is to give preference to the nearest aircraft, because the beacon responds to the strongest interrogations. That’s the whole logic: when it’s overloaded, it serves the closest users first. Now, station identification. Every DME transmits a 3-letter call sign every 30 seconds, usually in conjunction with an associated VOR. During that ident period, the random pulses — the ones that carry the ranging information — are replaced by regularly spaced pulses keyed with the station identification letters. That means range information is not available during the ident period. But the aircraft equipment has a 10-second memory circuit, so it continues to display the range it obtained before the ident started. And you can tell the DME identification apart from the VOR identification because it has a different tone — usually higher than the VOR. Finally, VOR/DME frequency pairing. To speed up frequency selection and reduce the pilot’s cockpit workload, VORs may be frequency paired with a DME or a military TACAN installation. What that means is the aircraft’s DME circuits are automatically activated when you select the appropriate VHF VOR frequency. Ideally, the VOR and the DME or TACAN beacons should be co-sited, so that a range and a bearing can be plotted from the same source. But that’s not always possible, and the table in the book explains the siting, frequency pairing, and call sign arrangements of VOR/DME or VOR/TACAN facilities. So, to tie it together: saturation is the beacon’s way of prioritising the nearest aircraft when it’s overloaded; the ident period is a brief window where you lose range data but the memory circuit covers it; and frequency pairing is what lets one VOR selection bring up the DME automatically.