Distance Measuring Equipment (DME) — Page 248, Lesson 240

I want to walk you through how a DME — Distance Measuring Equipment — actually measures your range, and the clever way it sorts your own pulses out from everyone else's. The core problem is this. A ground DME transponder answers every aircraft that interrogates it, and it answers them all on the same frequency. So when your receiver hears a reply, how does it know that reply is meant for you, and not for the aircraft next to you? The answer is a technique called lock-on, and it starts with your interrogator being programmed to transmit its paired pulses at random intervals. That irregular, jittered transmission sequence is what differentiates your pulses from everyone else's. Here's the sequence of events. At the instant you transmit, your receiver sets up what we call gates — electronic windows in time — that are matched to the random PRF, the pulse repetition frequency, of your own transmitted twin pulses. Now, the responses coming back on the transponder's carrier frequency include your paired pulses, but they also include replies to other aircraft. Your receiving equipment is designed so that only the responses which match your randomized PRF are allowed through the gates. The moment your pulses pass through, you've achieved lock-on, and the DME enters what we call the tracking mode. Once you're tracking, here's the beautiful part. As your range from the station increases or decreases — unless you're circling, which keeps range constant — the gates move to accommodate the corresponding change in the time between transmission and reception of the twin pulses. This is the lock-and-follow technique. It ensures your returning twin pulses are continuously tracked, no matter how your distance changes. And that offset in time between transmission and reception is the direct measure of your slant range from the DME transponder. That's the fundamental principle — time delay equals distance. Now, why twin pulses? Why does the system use a pair of pulses rather than a single one? Because the use of twin pulses ensures the receivers never accept matching randomized single pulses that could possibly come from other sources — for example, other radars, ignition systems, or even lightning. A single random pulse from some noise source might slip through a gate, but a matched pair at your specific random spacing is far more distinctive. Let me talk about range search — how the system finds you quickly in the first place. To achieve a rapid lock-on during the range search, the DME interrogator transmits at 150 pulse pairs per second — that's 150 ppps — for 15,000 pulse pairs, which works out to 100 seconds. If lock-on is not achieved in that time, it reduces the rate to 60 ppps and maintains that rate until there is a range lock-on. And at the moment of lock-on, the system settles down to operate at a random PRF of just 27 ppps. During that range search, watch your indicator. The range counters, or the pointer, rotate rapidly from zero nautical miles through to the maximum range. In modern equipment this sweep takes 4 to 5 seconds; in older systems it takes 25 to 30 seconds. If no response is achieved within that period, the pointer or counters return rapidly to zero and the search starts all over again. Finally, let's look at beacon saturation. The output of a modern ground beacon is a constant 2700 pulse pairs per second, which, in the absence of any aircraft interrogations, are generated at random intervals. When a ground beacon is receiving 2700 ppps, it becomes saturated, and it then reduces its receiver gain. The effect of this gain reduction is to exclude transmissions from aircraft whose interrogation pulses are weaker — the distant ones. This equates to about 100 aircraft using the DME at the same time. So the beacon effectively prioritises the stronger, closer signals when it's busy. That's the complete picture of DME range measurement — the jittered PRF for identification, the gates and lock-and-follow tracking, the search sequence with its rates and timings, and how the beacon manages saturation.