VHF Direction Finder (VDF) — Page 80, Lesson 67

I want to walk you through the VHF Direction Finder — VDF — and I'll start with what it actually is. VDF is a ground-based system that gives you, the pilot, a bearing — that is, a direction to fly — towards a ground station. That bearing has two main uses. You can use it to home towards the station, which means flying directly to it, or you can combine it with another bearing, or several bearings, to establish a fix — a known position on the map. Now, I want to be honest with you about its place in the modern world. VDF is rarely used, because there are far more sophisticated and more accurate systems available. But you still need to understand it, because it's a tool you can call on, and it appears in your procedures. Here's the key operational detail. Bearings are provided by voice, on the aircraft's VHF communications frequency. That means the service lives on 118.0 to 137 MHz, with an emission code of A3E — that's the standard amplitude-modulated voice. So you're talking to the ground station on your normal VHF comms, and they talk back to you with a bearing. There's a special case worth noting. Auto-triangulation — where your position is automatically provided from a number of VDF bearings taken from different stations — is available, but only on one frequency: the VHF International Distress Frequency, 121.5 MHz. And at present, UHF direction finding is limited to military use. Where do you find these stations? The Aeronautical Stations offering a VDF service are listed in the AD Section of the AIP — that's the Aeronautical Information Publication. And there's an important limitation to watch for: some VDF stations stipulate that the service is not available for en route navigation purposes, except in emergency. So you can't always rely on it for normal navigation. There's also a condition on when bearings are given at all. VDF bearing information will only be provided when conditions are satisfactory and the radio bearings fall within the calibrated limits of the station. And if a bearing can't be provided, the pilot will be told the reason. So the service has built-in safeguards. Now let's look at the procedures — how you actually request this. You request a VDF bearing using the appropriate phrase or Q-Code to specify the service required. Here's an example of the call: 'QDM QDM QDM OXFORD APPROACH G-DS REQUEST QDM, G-DS.' You say the Q-code three times, then the station name, then your callsign, then the request, then your callsign again. That figure shows you the geometry of the Q-codes, and I want to define each one precisely, because these are the heart of the system. First, QDR. That's the aircraft's Magnetic Bearing from the station — in other words, the radial. It's used for en route navigation. Second, QDM. That's the aircraft's Magnetic Heading to steer, assuming no wind, to reach the VDF station. It's used mainly for station homing and for let-downs using published procedures. So QDM is the heading you'd fly to get to the station if there were no wind. Third, QTE. That's the aircraft's True Bearing from the station. Like QDR, it's used for en route navigation, but it's in true, not magnetic, terms. Fourth, QUJ. That's the aircraft's True Track to the station. And I'll tell you plainly — it's not generally used. Now, here's a relationship you must commit to memory. QDM is the reciprocal of QDR — they're 180 degrees apart. And QUJ is the reciprocal of QTE. The three you'll see most commonly are QDR, QDM, and QTE. So when you request one of these, how does the station reply? The direction-finding station will respond with four things. First, the appropriate phrase or Q-code. Second, the bearing or heading in degrees, in relation to the direction-finding station. Third, the class of bearing. And fourth, the time of observation, if required. That class of bearing is critical, because it tells you how much you can trust the number. The accuracy of the observation is classified as follows. Class A — accurate within plus or minus 2 degrees. Class B — accurate within plus or minus 5 degrees. Class C — accurate within plus or minus 10 degrees. And Class D — accuracy less than Class C. Here's the practical reality: normally, bearings no better than Class B will be available. So in day-to-day operations, you should expect at best a plus-or-minus 5-degree accuracy. But there's a modern exception. The latest equipment uses Doppler principles to determine a high-resolution bearing that can be displayed as a digital read-out. Bearings produced this way have an accuracy of plus or minus 0.5 degrees, and this is available on both UHF and VHF systems. So the older classified system — A, B, C, D — applies to the traditional equipment, while the Doppler-based digital systems give you that much finer half-degree accuracy. So to tie it all together: VDF gives you a voice-delivered bearing on VHF comms, you request it with a Q-code, you get back a bearing with a class that tells you its accuracy, and you use QDM to home, QDR or QTE for en route navigation. That's the complete picture of the system.