Plotting — Page 478, Lesson 477

Let's start with the core idea of this chapter: fixing position. When you're navigating an aircraft safely from one place to another, you have to make frequent checks on your ground position — where you actually are over the earth — and then make any corrections that are needed. That's the whole point of plotting. Now, two terms you need to know precisely. A 'fix' is a position defined from radio aids. A 'pinpoint' is a position found by map-reading. So if you're using radio equipment to determine where you are, that's a fix. If you're looking out the window and matching terrain to a map, that's a pinpoint. Some fixes give you a position directly — for instance, a GPS fix, or a fix from an ATC ground radar. Those hand you your position in one step. 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. You don't know exactly where on that line you are — you just know you're on it. Examples of position lines you can obtain visually are long straight stretches of railways, roads, rivers, or coastlines. You see one of those, you know you're somewhere along it. You then transfer the position and direction of that line onto your plotting chart. Position lines can be visual, radio, or radar. They all get plotted on charts, and they all represent lines somewhere along which the aircraft is known to be at the time the position line was obtained. Now here's the beautiful part: 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 — the intersection. That's your position. Plotting visual position lines is quite straightforward. You simply draw them 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 that. First, a fundamental point you must always remember: radio waves follow great circle paths, and all directions measured are great circle bearings. That's the basis for everything that follows. 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 these codes are still vital for you. Let me give you each one precisely. QTE is the true bearing of an aircraft from a ground station. So the ground station looks at you and measures the true bearing — the direction from the station to you, in degrees true. QDM is the magnetic track from an aircraft to a ground station. Note the direction — it's from you, the aircraft, to the station, and it's magnetic. QDR is the magnetic bearing from a ground station to an aircraft. And here's the relationship you must hold onto: QDR is the reciprocal of QDM. They're 180 degrees apart. Now, how are these obtained? 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. In other words, the station sees your signal coming from a certain true direction, measures that, and passes it to you by RT — radio telephony. QDMs and QDRs are obtained in two ways. The first is VDF — that stands for VHF direction finding. Here's the sequence: an aircraft sends out a radio signal, and a ground station measures a QTE. Then the ground station converts the QTE into a QDR by applying variation at the ground station 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 walk through that chain with me. The station measures the true bearing from itself to you — that's the QTE. It applies local variation to turn that true bearing into a magnetic bearing from the station to you — that's the QDR. Then it reverses it by 180 degrees to get the direction from you to the station — that's the QDM. Three codes, one continuous conversion chain. That's the heart of this section.