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

I want to walk you through the final part of the DME chapter now — the operational details that govern how the system behaves in the real world, starting with a situation called beacon saturation. Picture Figure 15.9, where we have seven aircraft, labelled A through G, all receiving ranges from the same DME transponder. Aircraft B is just entering the coverage area. Now, a DME transponder can only handle a limited number of interrogations at once — when that limit is reached, we say the transponder is saturated. Here's the critical part: when saturation occurs, the transponder automatically reduces its receiver gain. That reduction in gain means the weaker interrogations — the ones from further away — are no longer heard. In our figure, aircraft A, B, E, F and G will be excluded and will unlock. They lose their range lock. The design intent here is deliberate: the beacon responds to the strongest interrogations, so the nearest aircraft get preference. That's the whole philosophy — when capacity is tight, the closest users are served first. Now let's move to station identification. Every DME transmits a three-letter call sign every 30 seconds, and this is usually done in conjunction with an associated VOR. Here's the mechanism: during the identification period, the normal random pulses — the ones that carry range information — are replaced by regularly spaced pulses that are keyed with the station identification letters. So the pulses are now Morse code for the call sign instead of random ranging pulses. The consequence is direct: range information is not available during the ident period. But the aircraft equipment has a 10-second memory circuit, and that circuit continues to display the range that was obtained before the ident began. So the pilot doesn't see the range drop out during identification. One more distinction you need: the DME identification is distinguished from the VOR identification by having a different tone — usually higher than the VOR. That's how you tell which beacon you're actually listening to. Finally, we come to VOR/DME frequency pairing. The purpose is twofold — to facilitate and speed up frequency selection, and to reduce the pilot's cockpit workload. Here's how it works: a VOR may be frequency paired with a DME, or with a military TACAN installation. When you select the appropriate VHF VOR frequency, the aircraft's DME circuits are automatically activated — you don't have to tune the DME separately. Now, ideally, the VOR and the DME or TACAN beacons should be co-sited, meaning physically at the same location, so that a range and a bearing can be plotted from the same source. That gives you a single point of origin for both measurements. But — and this is the practical caveat — this is not always possible. The chapter includes a table that explains the siting and frequency pairing and call sign arrangements of VOR/DME or VOR/TACAN facilities, so you can see the different configurations in practice. That's the operational picture: saturation behaviour with nearest-aircraft preference, the identification cycle with its 10-second memory, and frequency pairing to cut cockpit workload.