E arcs — Page 248, Lesson 238

I want to walk you through the E arcs first, because they tie directly into why DME is so valuable. E arcs are the curved paths you fly at a constant distance from a VOR/DME station, and the excerpt lists five operational uses. Let me give you each one precisely. First, E arcs provide range and height checks when you're flying non-precision approach procedures — that means approaches like locator only and VOR let-downs, where you don't have a glideslope to guide you vertically. Second, when you're flying an ILS/DME procedure, the E arc indicates accurate ranges to the runway threshold, and heights for range — so you know your height for any given distance. Third, it facilitates radar identification when the pilot reports his VOR/DME position — the controller can positively identify you on radar based on that reported fix. Fourth, it facilitates the separation and control of aircraft in non-radar airspace, based upon a VOR/DME fix reported by individual aircraft — so even without radar, traffic can be safely spaced. Fifth, it's the basis for a simple Area Navigation (RNAV) system when the appropriate computerization is fitted. And sixth, it provides accurate range inputs into the more complex and accurate RNAV systems — and here's the key detail: twin, self-selecting DME/DME are used in those systems. Now let's move into the heart of the chapter — the Principle of Operation of DME itself. DME stands for Distance Measuring Equipment. I want you to think of it as a secondary radar system providing slant range by pulse technique. Slant range means the straight-line distance from the aircraft to the ground station, not the horizontal distance along the ground — that distinction matters because at altitude, slant range is always slightly greater than ground range. Here's the pulse technique in detail. The aircraft's interrogator transmits a stream of omni-directional pulses on the carrier frequency of the ground transponder. Omni-directional means the pulses go out in all directions, not in a narrow beam. Simultaneously, the interrogator's receiver starts a Range Search — it begins looking for the reply. At the transponder on the ground, the received interrogation pulses are re-transmitted after a delay of 50 microseconds, at a frequency that is plus or minus 63 MHz removed from the interrogation frequency. So the reply comes back on a different frequency, offset by 63 megahertz, and there's a fixed 50-microsecond delay built into the ground station's response. Now, the airborne system identifies its own unique stream of pulses and measures the time interval, electronically, between the start of the interrogation and the reception of the response from the transponder. That time measurement, and hence range, is very accurate, and it's based upon the speed of radio waves — that's 3 × 10⁸ metres per second. A modern DME is inherently accurate to plus or minus 0.2 nautical miles. Here's a critical point about how the system copes with multiple aircraft. In theory, up to 100 aircraft can interrogate a DME transponder. So each aircraft is receiving its own returning paired pulses plus those which result from other aircrafts' interrogations, because the pulses all have the same carrier frequency. The airborne equipment has to pick out its own unique stream from all that traffic — that's why the identification of its own pulses is so important. Now the pulse characteristics. The width of the interrogation pulses is 3.5 microseconds, which is 1050 metres. And they're transmitted in pairs. The interval between the individual pulses of a pair is 12 microseconds for an X channel and 36 microseconds for a Y channel. So the channel type — X or Y — is defined by that pulse spacing within each pair. Let me tie this together with the VORTAC figure I have here. shows the combined Doppler VOR/DME installation, and shows a conventional VOR installation surmounted by a DME antenna. The VORTAC ground station combines VOR for bearing and DME for distance — the airborne equipment selects a channel, like 70X, and gets both distance and bearing back. That's the complete picture: DME gives you the slant range by measuring that round-trip time, corrected for the 50-microsecond ground delay, and the X or Y channel tells you which pulse spacing to expect.