Plotting — Page 478, Lesson 477

Let’s start with the big picture, because this chapter is all about knowing exactly where you are. If an aircraft is to be navigated safely from one position to another, you have to make frequent checks on its ground position, and make any required correction. That’s the whole reason we plot. Now, two terms you’ll hear constantly. A ‘fix’ is a position defined from radio aids. A ‘pinpoint’ is a position found by map-reading. So the difference is the source — radio versus your eyes on a map. Some fixes give you a position directly. For instance, a GPS fix, or a fix from an ATC ground radar. Those hand you the position straight away. But other fixes are made from a combination of position lines. And here’s the key definition: a position line is a line, somewhere along which an aircraft is known to be at a particular time. Not a point — a line. You know you’re on it, but you don’t know where on it. Examples of position lines you can get visually are long straight stretches of railways, roads, rivers, or coastlines. You take the position and direction of that line and transfer it to your plotting chart. Position lines can be visual, radio, or radar. They get plotted on charts, and they represent lines somewhere along which the aircraft is known to be at the time the position line was obtained. Now here’s the clever bit: any two position lines obtained at the same time will give you a fix. Why? Because the aircraft is known to be somewhere along each line, so it must be at the point where they cut each other. That intersection is your fix. Plotting visual position lines is quite straightforward. You simply draw them in on the plotting chart in the same position and direction as on the topographical map. But plotting radio position lines is not quite so straightforward, and that needs further explanation. So let’s get into radio position lines. First, a fundamental rule you must never forget: radio waves follow great circle paths, and all directions measured are great circle bearings. That’s the foundation. Now, bearings measured by ground stations are referred to by groups of code letters beginning with the letter Q. These come from the old ‘Q code’, most of which is now obsolete. But three of them you need to know cold. QTE — that’s the true bearing of an aircraft from a ground station. So the ground station looks at you, and gives you the true bearing from the station to you. QDM — the magnetic track from an aircraft to a ground station. Note the word ‘track’ — that’s the direction you’d fly to get to the station, in magnetic. QDR — the magnetic bearing from a ground station to an aircraft. And here’s the relationship to remember: QDR is the reciprocal of QDM. They’re 180 degrees apart. Now, how do you actually get these? QTEs are obtainable by an aircraft when a ground station measures the direction, in degrees true, from which a radio signal sent out by the aircraft approaches it. So you transmit, the station sees which direction your signal came from, and that direction is passed to you by RT — radio telephony. QDMs and QDRs are obtained in two ways. The first is VDF. Here’s how it works: an aircraft sends out a radio signal, and a ground station measures a QTE. Then the ground station converts that QTE into a QDR by applying variation at the ground station — the variation where the measuring is done. Then, by adding or subtracting 180 degrees to change it into the form of a direction from the aircraft to the ground station, a QDM is obtained. So you go from QTE, to QDR, to QDM, step by step. Let me show you that flow on the board, because it’s easy to get tangled. So to recap the whole picture: you fix your position by intersecting position lines. Visual ones are easy — just draw them as they appear on the map. Radio ones come through the Q codes, and the VDF method walks you from a measured QTE, through variation to get QDR, then through 180 degrees to get QDM. And remember, all of it rests on the fact that radio waves follow great circle paths, so every bearing you measure is a great circle bearing.