So let's set the scene before we dive into the technical detail — Page 241, Lesson 235

This is the start of Chapter 15, Distance Measuring Equipment — DME. So let's set the scene before we dive into the technical detail. DME is a radio navigation aid that tells you, quite literally, how far you are from a ground station. It's a slant range measurement — the straight-line distance from your aircraft to the beacon, not the distance along the ground. That distinction matters, and we'll come back to it. The chapter is laid out in a logical order, and I want you to see the roadmap before we start. It opens with an introduction, then moves to frequencies — the specific radio channels DME uses. After that, the uses of DME, then the principle of operation — how the system actually works. From there, twin pulses, which is a fundamental detail of the signal structure. Then range search, which is how the equipment finds your signal in the first place. Then beacon saturation — what happens when too many aircraft interrogate one station. Station identification, how you confirm you're talking to the right beacon. Then VOR/DME frequency pairing, which is how DME is often combined with VOR. Then DME range measurement for ILS — the instrument landing system. After that, range and coverage, accuracy, and finally a DME summary, followed by the chapter questions and answers. So the full picture is: what it is, how it works, how it's used, and its limitations. Let's start with the introduction. DME is a secondary radar system. That's a key concept. Primary radar — like the weather radar or the radar at air traffic control that paints a target — works by bouncing a signal off an object and listening for the echo. Secondary radar is different. The ground station doesn't listen for a reflection. Instead, your aircraft transmits a signal, the ground station receives it, and then the ground station transmits a reply back to you. So there's a question-and-answer exchange happening. Your aircraft asks, the ground station answers. That's the fundamental architecture of DME. Your aircraft is the interrogator — it sends the pulses. The ground station is the transponder — it receives and replies. And by measuring the time it takes for that round trip — your signal out, the reply back — the equipment calculates the distance. Now, the frequencies. DME operates in the UHF band — ultra high frequency. The aircraft interrogates on one frequency and the ground station replies on another. The two are always separated by a fixed amount, 63 MHz. So if your aircraft transmits on, say, 1041 MHz, the ground station replies on 978 MHz. That pairing is fixed and standard, and it's how the system keeps the interrogation and the reply from interfering with each other. The uses of DME are straightforward. Its primary job is to give you distance from the beacon. But it's also used in conjunction with VOR — VHF omnidirectional range — to give you a position fix. VOR gives you a bearing, DME gives you a distance, and together they give you a fix. DME is also used for the instrument landing system, providing distance information during an approach. So it's not just a standalone aid; it's a building block in a larger navigation picture. Let me pause there. We've covered the introduction, the frequencies, and the uses. The next piece is the principle of operation — how the round-trip timing actually works