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. Based on a 95% probability, the system accuracy for DME used for navigation — that's the designation DME/N — should give a total system error not exceeding plus or minus 0.25 nautical miles, plus 1.25% of the range. So the error budget grows slightly with distance. Precision systems, designated DME/P, are accurate to plus or minus 100 feet on Final Approach. That's a much tighter tolerance, and it's why DME/P is used for precision approach work. Now, where does that total system error come from? The total system limits include errors from causes such as those arising from airborne equipment, ground equipment, propagation, and random pulse interference effects. So it's a combined budget — your own transceiver, the ground station, the way the signal travels through the atmosphere, and stray pulses all contribute. Next, the difference between slant range and ground range. This is a big one. The difference between computed slant range and actual ground distance increases the higher and closer an aircraft gets in relation to the DME. As a general rule, the difference becomes significant when the aircraft is at a range which is less than 3 times its height. So if you're at 10,000 feet, the difference matters when you're within 30,000 feet of the beacon horizontally. Here's the extreme case: when the aircraft is directly over the DME — that's 0 NM ground distance — it will indicate the aircraft's height in nautical miles. So you're reading your own altitude, not your position. There is a small cone of confusion over a DME, plus range indications continue to be computed as the equipment has a 10 second memory circuit. So even as you pass overhead, the unit holds the last valid range for ten seconds rather than dropping out instantly. Let me walk you through the worked example. Aircraft at 36,840 feet. The slant range is the square root of the ground range squared plus the height squared. Height in nautical miles: 36,840 divided by 6,080 feet per nautical mile gives 6 NM. So if the slant range is 10 NM, then ground range squared is 10 squared minus 6 squared, which is 100 minus 36, giving 64. The square root of 64 is 8 NM ground range. So at 10 NM slant range and 6 NM height, your true ground distance is 8 NM. That's the geometry in action. Now, ground speed computation. The equipment's indicated ground speed, which is computed from the rate of change of slant range, becomes more inaccurate and under-reads the actual ground speed, the closer and higher an aircraft is in relation to the DME beacon. So it reads low, and the error worsens near and high above the station. Here's the classic trap: an aircraft circling a DME beacon at a constant range will have an indicated computed ground speed of zero knots. Because your slant range isn't changing — you're orbiting — the rate of change is zero, so it shows zero. A ground speed is only valid when an aircraft is homing to, or flying directly away from, a VOR/DME or TACAN. So only use that readout when you're on a radial inbound or outbound. Let me give you the summary now, because it ties everything together. Frequency: UHF band, 962 to 1213 MHz, with 1 MHz spacing, giving 252 channels. There's a plus or minus 63 MHz difference between transmitted and received frequencies. Selection is by paired VHF frequency — that's the VOR or ILS frequency you tune. When DME is paired with ILS, the range is zero-referenced to the ILS runway threshold, so you get accurate ranges to the threshold. Uses: a circular position line, ground speed and time to/from station, DME arcs, range and height checks during let-downs, accurate ranges to threshold, and RNAV. Principle of operation: the aircraft interrogator and receiver transmits pairs of pulses at random intervals, omni-directionally. The ground station transponder re-transmits all pulses at plus or minus 63 MHz after a delay of 50 microseconds. The aircraft 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 — that's the random spacing — are used to identify your own pulses. And frequency change prevents the aircraft locking on to reflections. Finally, range search. Pulse rate is initially 150 pulse pairs per second, and it's reduced to 60 pulse pairs per second after 15,000 pulse pairs. That's how the system manages its interrogation load. That's the complete DME picture — accuracy, slant range geometry, ground speed limitations, and the full system summary.