Part 1: Radio Direction Finding – The Foundation

Not all VDF bearings are equally accurate. Ground stations have different levels of precision, and the quality of a bearing depends on several factors: the distance from the station (closer is better), the antenna quality, and the strength of your signal. To set expectations, the ICAO defines three accuracy classes for radio bearings:

Class A: ±2° — The most accurate bearings, typically provided by modern direction-finding equipment at well-equipped aerodromes. A Class A bearing of 090° means the station operator is confident you're somewhere between 088° and 092° from their position.

Class B: ±5° — The standard for most VDF services at medium-sized aerodromes. Still very useful for navigation, but the margin of error is wider. A Class B bearing of 090° gives you a bearing line that could be as far off as 085° or 095°.

Class C: ±10° — Lower-accuracy bearings, sometimes provided by smaller aerodromes with older equipment, or when you're a long distance away from the station. Still helpful, but less precise. A Class C bearing of 090° might actually put you anywhere from 080° to 100°.

When you request a bearing, ask the station what accuracy class they can provide. If they're busy or the signal is weak, they might tell you: "Only Class C available due to range." That's honest and helpful. You now know the limitations of the information you've received.

The accuracy class should be announced before the bearing. You might hear: "Class A bearing, your QDM is two-seven-zero." Or: "Class B available, your QDR is zero-nine-zero."

After you've logged the bearing on your chart, you know that the true position lies somewhere within that ±accuracy band. When you get a second bearing from a different station (or a second bearing from the same station a few minutes later after you've flown further), you can plot both lines, and their intersection gives you a position fix. The smaller the intersection area, the more accurate your fix.