Distance Measuring Equipment (DME) — Page 257, Lesson 246

Let’s start with the accuracy of the system, because that sets the professional standard you’ll be judged against. For DME used for navigation — that’s the designation DME/N — the system accuracy is based on a 95% probability. That means 95 times out of 100, the total system error must not exceed plus or minus 0.25 nautical miles, plus 1.25% of the range. So the error budget grows slightly with distance. For precision systems, designated DME/P, the accuracy is much tighter: plus or minus 100 feet on final approach. Now, that total system error isn’t just the box in your aircraft. It includes errors from the airborne equipment, the ground equipment, propagation effects through the atmosphere, and random pulse interference. All of those combine into the total system limit. Next, the difference between slant range and ground range. This is a big one. The DME measures slant range — the straight-line distance from your aircraft to the beacon. But on a chart, you usually think in ground distance. The difference between the computed slant range and the actual ground distance grows the higher and closer you are to the DME. As a general rule, that difference becomes significant when your range is less than three times your height. So if you’re at 10,000 feet, watch out below 30,000 feet of range. Here’s the extreme case: when you’re directly over the DME — zero ground distance — the equipment will indicate your height in nautical miles. So it’s not showing zero; it’s showing your altitude converted to nautical miles. There’s also a small cone of confusion directly over the beacon, but the range indications keep being computed because the equipment has a 10-second memory circuit. Let me walk you through the worked example in the figure. An aircraft at 36,840 feet. We convert that height to nautical miles: 36,840 divided by 6080 feet per nautical mile gives 6 nautical miles. Now, if the slant range is 10 nautical miles, we use Pythagoras: slant range squared equals height squared plus ground range squared. So 10 squared minus 6 squared equals 100 minus 36, which is 64. The square root of 64 is 8. So the ground range is 8 nautical miles. That’s the geometry in Figure 15.10. Now, ground speed computation. The indicated ground speed is computed from the rate of change of slant range. And here’s the catch: it becomes more inaccurate, and it under-reads the actual ground speed, the closer and higher you are to the DME beacon. Think about circling the beacon at a constant range — your slant range isn’t changing at all, so the equipment computes a ground speed of zero knots, even though you’re flying a circle. A ground speed reading is only valid when you’re homing directly to, or flying directly away from, a VOR/DME or TACAN. Now let me give you the DME summary, because this pulls everything together. Frequency: DME operates in the UHF band, from 962 to 1213 MHz, with 1 MHz spacing, giving 252 channels. There’s a plus or minus 63 MHz difference between the transmitted and received frequencies. The channel is selected by a paired VHF frequency — either a VOR or an ILS. When DME is paired with ILS, the range is referenced to zero at the ILS runway threshold. Uses: a circular position line around the beacon, ground speed and time to and from the station, DME arcs, range and height checks during let-downs, accurate ranges to the threshold, and RNAV — area navigation. Principle of operation: your aircraft has an interrogator and receiver. It transmits pairs of pulses at random intervals, omni-directionally — that is, in all directions. The ground station transponder re-transmits all pulses at plus or minus 63 MHz, after a delay of 50 microseconds. Your receiver identifies its own pulses and determines range from the time interval between transmitted and received pulses, minus that 50-microsecond delay. Pulse characteristics: a twin pulse is used to avoid interference. Jittered pulses — pulses with random timing — are used to identify your own pulses. And a frequency change prevents your aircraft from locking on to reflections. Finally, range search. Initially, the pulse rate is 150 pulse pairs per second. After 15,000 pulse pairs — that’s the total count — the rate is reduced to 60 pulse pairs per second. That reduction happens once the equipment has found and locked onto the reply. So the whole picture: DME measures slant range by timing pulses, with a 50-microsecond ground delay, and the accuracy, the slant-versus-ground geometry, and the ground speed limitations all flow from that timing and geometry.