Distance Measuring Equipment (DME) — Page 248, Lesson 236

Let’s start with the big picture, because DME is one of those systems that sounds simple but has a lot of precision built into it. Distance Measuring Equipment, or DME, is a secondary radar system. That phrase matters. A primary radar just bounces a signal off a target and listens for the echo. A secondary radar works differently: the aircraft sends out a signal, the ground station receives it, and the ground station transponds — it replies with its own signal. So there are two signals involved, and the timing between them is what gives you your range. What DME actually gives the pilot is range from a ground station. And I want to be precise here: it gives you slant range, not horizontal distance. We’ll come back to why that distinction matters. Now, think about how DME fits into the navigation picture. You get accurate magnetic bearings from a VHF Omni-range beacon — that’s VOR. And you get accurate slant ranges from DME. The two facilities are normally co-sited — placed at the same location — to form the standard ICAO-approved RHO-THETA short-range, line-of-sight navigation aid. Rho is the Greek letter for range, Theta is the Greek letter for bearing. So RHO-THETA just means: range plus bearing. That combination gives you a fix. Let me show you what these installations look like. is a combined Doppler VOR/DME installation, and shows a conventional VOR installation with a DME antenna mounted on top of it. That’s the co-siting in practice — one site, two aids. Now let’s get into the technical side: frequencies and channels. DME has an emission code of P0N. That’s a technical classification of the signal type. It operates between 960 and 1215 MHz — that’s the UHF band — at 1 MHz spacing. That spacing gives us 252 spot frequencies, or channels. Here’s a key relationship: there is always a difference of ±63 MHz between the interrogation frequency and the transponding frequency. The aircraft interrogates on one frequency, the ground station transponds on another, and those two are always 63 MHz apart — either plus or minus. The channels are numbered 1 to 126X and 1 to 126Y. So you have 126 channels with an X suffix and 126 with a Y suffix — that’s where the 252 comes from. Now, who selects these channels? A TACAN — that stands for TACtical Air Navigation — equipped military aircraft. TACAN gives the military pilot both range and bearing. The military pilot selects a channel number directly. Civil aircraft are different. They have the cheaper VOR/DME equipment, and instead of selecting a channel number, the pilot selects the appropriate paired VHF frequency to get range from either a DME or a military TACAN facility. So the civil pilot doesn’t think in channels — they think in VHF frequencies, and the equipment does the pairing automatically. Let me give you the example from the table so you can see the numbers in action. Take MAZ TACAN. The aircraft interrogation frequency is 1131 MHz. The beacon transponds on 1194 MHz. Notice the difference: 1194 minus 1131 is 63 MHz. The military aircraft selects Channel 107X. The civil aircraft selects 116.0 MHz — a VHF frequency — and gets range from that same TACAN. Now take OX DME. Aircraft interrogation is 1148 MHz, beacon transponds on 1211 MHz — again, 63 MHz apart. Military selects Channel 124X, civil selects 117.7 MHz. So the pattern is clear: every DME or TACAN has a paired VHF frequency that the civil pilot uses, and a channel number that the military pilot uses. There’s one more pairing I want to cover: DME paired with the ILS localizer transmitter. DME is also frequency-paired with ILS localizer frequencies. These DMEs supplement or replace the range information provided by the Marker Beacons. And here’s the critical detail: the range information is zero referenced to the ILS runway threshold. That means when the DME reads zero, you are at the threshold of the runway — not at the DME ground station. The pilot gets this DME simply by selecting the ILS frequency. So on an ILS approach, your DME readout is telling you distance to the runway threshold, which is exactly what you want for approach planning. Now let’s talk about what a DME is actually used for. I’ll walk you through the list. First, it provides very accurate slant range — that gives you a circular position line. And in conjunction with another DME, or a co-sited VOR, you get two position line fixes. One range gives you a circle around the station; add a second range or a bearing, and the intersection fixes your position. Second, when the aircraft is fitted with an appropriate computer, the DME integrates the change of slant range into groundspeed and elapsed times. So the computer watches how fast the slant range is changing and derives groundspeed and time from it. Third, DME permits more accurate flying of holding patterns and DME arcs. A DME arc is a curved path at a constant range from the station, and DME gives you the continuous range readout to fly it precisely. Fourth, it provides range and height checks when flying non-precision approach procedures — for example, locator only approaches and VOR let-downs. On these approaches you don’t have a glideslope, so you use DME range to confirm your position and check your height against the published profile. And the list continues — the excerpt cuts off at “indicates,” but you get the picture: DME is a workhorse for range information across en-route, terminal, and approach phases. Let me make sure the slant range point is crystal clear, because it’s the heart of the system. Slant range is the straight-line distance from the aircraft to the ground station — the hypotenuse of the triangle formed by your altitude and your horizontal distance. When you’re high and close to the station, slant range reads larger than your horizontal distance. When you’re far away, the difference becomes negligible. That’s why DME is described as giving slant range, and why the zero reference on an ILS DME is the runway threshold — it’s all about knowing exactly what your readout means. That’s the foundation of DME. We’ve covered what it is, how the frequencies and channels work, how it pairs with VOR and ILS, and what pilots use it for.