E arcs — Page 248, Lesson 238

Let’s start with the E arcs, because they’re the practical payoff of everything we’re about to build. An E arc is a curved flight path flown at a constant distance from a VOR/DME station, and it earns its keep in several specific jobs. First, it provides range and height checks when you’re flying non-precision approach procedures — think locator-only approaches and VOR let-downs. Second, when you’re on an ILS/DME procedure, the E arc gives you accurate ranges to the runway threshold, and heights for that range. Third, it facilitates radar identification when you report your VOR/DME position to ATC. Fourth, it helps with separation and control of aircraft in non-radar airspace, because each aircraft reports a VOR/DME fix and that becomes the basis for spacing. Fifth, the E arc is the foundation of a simple Area Navigation — RNAV — system, when the appropriate computerization is fitted. And sixth, it feeds accurate range inputs into the more complex, more accurate RNAV systems, which use twin, self-selecting DME/DME pairs. Now, the heart of it all — Distance Measuring Equipment, DME. I want you to think of DME as a secondary radar system that gives you slant range by pulse technique. Let me unpack that. The aircraft carries an interrogator. It transmits a stream of omni-directional pulses on the carrier frequency of the ground transponder. At the same instant, the interrogator’s receiver starts what we call a Range Search — it’s listening for its own reply. At the ground transponder, those received interrogation pulses are re-transmitted after a fixed delay of 50 microseconds, and they come back on a frequency that is plus or minus 63 MHz removed from the interrogation frequency. So the reply is shifted in frequency, which is how the airborne system separates its own signal from the noise. The airborne system identifies its own unique stream of pulses and measures the time interval electronically — from the start of the interrogation to the reception of the response. That time measurement is what gives you range, and it’s very accurate, because it’s based on the speed of radio waves, which is 3 × 10⁸ metres per second. A modern DME is inherently accurate to plus or minus 0.2 nautical miles. Now, here’s a subtlety. In theory, up to 100 aircraft can interrogate a single DME transponder. That means each aircraft is receiving its own returning paired pulses plus the pulses that result from other aircraft’s interrogations — because all those pulses share the same carrier frequency. So the system has to be clever enough to pick out its own reply from the crowd. Let me give you the pulse details, because they matter. The interrogation pulses are 3.5 microseconds wide — that’s 1050 metres in physical length. They’re transmitted in pairs, and the interval between the individual pulses of a pair depends on the channel. For an X channel, the interval is 12 microseconds; for a Y channel, it’s 36 microseconds. That pulse-pair spacing is how the system distinguishes X from Y channels. So, to tie it together: the E arc is your operational use of DME — constant range, used for approaches, identification, separation, and RNAV. And DME itself is the pulse-based secondary radar that measures slant range by timing the round trip, with a 50-microsecond transponder delay, a 63 MHz frequency shift, and pulse-pair spacing of 12 or 36 microseconds depending on channel. That’s the core. Now let’s look at how the ground station and airborne equipment fit together in the VORTAC picture.