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

Let’s start with the core problem the DME has to solve. When an aircraft interrogates a ground beacon, the beacon replies, and the time between your transmission and the reply tells you your slant range. But here’s the catch: if a hundred aircraft are all interrogating the same beacon, how does your receiver know which reply belongs to you? The answer is a technique called lock-on, and it all begins with the way your interrogator transmits. Your aircraft’s interrogator is programmed to transmit its paired pulses at random intervals. That means the sequence of pulses is irregular — we call this jittered. This irregularity is what differentiates your pulses from everyone else’s. At the instant you transmit, your receiver sets up what we call gates — electronic windows that match the random pulse repetition frequency, the PRF, of your own transmitted twin pulses. Now, the responses coming back on the transponder’s carrier frequency include your paired pulses plus those from all the other aircraft. Your receiving equipment is designed so that only the responses matching your randomized PRF are allowed through the gates. When that happens, we say the pulses have achieved lock-on, and the DME enters what we call the tracking mode. Here’s the elegant part. As your range from the station increases or decreases — unless you’re circling — 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 the returning twin pulses are continuously tracked. And that offset in time between transmission and reception is precisely the measure of your slant range from the DME transponder. Why twin pulses? Why not single pulses? Because the DME system uses twin pulses to ensure the receivers never accept matching randomized single pulses that could possibly come from other sources — for example, other radars, ignition systems, or lightning. The twin-pulse structure is a form of discrimination against false returns. Now let’s talk about how the system actually finds you in the first place. This is the range search. To achieve a rapid lock-on, 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 within that period, it reduces the rate to 60 ppps and maintains that rate until there is a range lock-on. Once lock-on is achieved, the system settles into a random PRF of 27 ppps for normal tracking. During the range search, the range counters or the pointer of your indicator 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 again. Finally, let’s look at beacon saturation. The output of a modern ground beacon is a constant 2700 pulse pairs per second. In the absence of any aircraft interrogations, those pulses 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 is to exclude the transmissions from aircraft whose interrogation pulses are weaker. This equates to about 100 aircraft using the DME at the same time. So the whole picture is this: your interrogator jitters its pulses, your receiver gates on that jitter, lock-on is achieved, and the time offset between transmission and reception gives you slant range. The twin pulses protect against false returns, the range search gets you locked on quickly, and beacon saturation manages the load when many aircraft are using the same station.