Automatic Direction Finder (ADF) — Page 99, Lesson 84

Let's pick up with the NDB instrument approach procedure, then move into the factors that affect ADF accuracy. Most aerodromes have NDB runway instrument approach procedures. The pilot flies the published procedure in order to position the aircraft in poor weather conditions for a visual landing. The NDB may also be used in conjunction with other runway approach aids for the same purpose. So the NDB isn't just a navigation aid for en-route flying — it's also a precision tool for getting you lined up with the runway when visibility is poor, down to the point where you can see the runway and land visually. Now, the factors affecting ADF accuracy. First is the Designated Operational Coverage, or DOC. The DOC of NDBs is based upon a daytime protection ratio — that's a signal-to-noise ratio of 3:1 — between wanted and unwanted signals that permits the required level of bearing accuracy. So the DOC is the area within which the NDB signal is strong enough relative to noise to give you accurate bearings. At ranges greater than those promulgated, bearing errors will increase. And adverse propagation conditions, particularly at night, will also increase bearing errors. So if you fly beyond the published DOC, or if conditions are bad, your bearings get less reliable. Next, static interference. There are two types that can affect ADF performance. The first is precipitation static, generated by the collision of water droplets and ice crystals with the aircraft. It causes a reduction in the signal-to-noise ratio, which affects the accuracy of the bearings and can, in extreme circumstances, completely mask the incoming signal. The indications on the RMI or RBI will be a wandering needle, and the audio will have a background hiss — which is also likely to be present on VHF frequencies. So if you hear that hiss and see the needle wandering, that's precipitation static. The second type is thunderstorms. They have very powerful discharges of static electricity across the electromagnetic spectrum, including LF and MF. These discharges cause bearing errors in the ADF. A static discharge in a cumulonimbus cloud — a Cb — will be heard as a loud crackle on the audio, and the needle will move rapidly to point to the Cb. When there are several active cells close together, it's possible for the needle to point to them for prolonged periods. Care must be taken in the use of ADF when Cb activity is forecast. It's been said that during Cb activity, the only sensible use of the ADF is to indicate where the active cells are. So the ADF can actually become a thunderstorm detector — the needle points to the storm, not the NDB. Finally, the night effect. By day, the D-region absorbs signals in the LF and MF bands. At night, the D-region disappears, allowing sky wave contamination of the surface wave. So during the day, the D-region of the ionosphere absorbs those low and medium frequency signals, protecting the surface wave. At night, that D-region is gone, and the sky wave — the signal that bounces off the ionosphere — contaminates the surface wave, which is the direct ground wave you're trying to track. That's why night-time bearings can be less accurate. So to sum up: the DOC defines where the NDB is reliable, precipitation static and thunderstorms degrade the signal in specific ways you can recognise, and the night effect is all about the D-region disappearing and letting sky wave contamination in.