Right, let's get into the night-time behaviour of the NDB and ADF — Page 99, Lesson 86

Right, let's get into the night-time behaviour of the NDB and ADF. We've already covered how the D-region absorbs signals in the LF and MF bands during the day. Now, the key change: at night, the D-region disappears. That single fact is what causes all the night-time problems we're about to discuss. With the D-region gone, the sky wave—the signal that goes up to the ionosphere and bounces back down—is no longer absorbed. It now reaches the aircraft and contaminates the surface wave, which is the one we're actually trying to use for navigation. This contamination arises for two distinct reasons. First, phase interference: the sky wave and the surface wave travel different paths, so they arrive at the aerial at different times and with different phases. They can reinforce or cancel each other, distorting the bearing. Second, the sky wave induces currents in the horizontal elements of the loop aerial. Remember, the loop aerial is what senses the direction of the incoming signal, so any current induced in its horizontal parts corrupts that directional information. Now, the aerial design tries to reduce this. It uses very short vertical elements, and it screens the aerial above and below. But I want to be clear: this reduces the effect, it does not eliminate it. The contamination is still there. The effect first becomes significant at a range of 70 to 100 nautical miles from the NDB. How do you notice it? The audio signal fades, and the needle 'hunts'—it swings back and forth instead of settling on a steady bearing. And it's worst around dawn and dusk, when the ionosphere is in transition, changing from its day to night state. So, if you're going to use ADF at night, there's a strict set of procedures. You must positively identify the NDB call sign. You must continue to check the tuning and the identification throughout. You must avoid using the equipment within 1 hour of sunrise or sunset. You must use NDBs within their promulgated range—and note that this range is valid during daytime only. You must treat bearings with caution if the needle wanders and the signal fades. And you must cross-check the NDB bearing information against other navigation aids. That last one is your safety net. Let's move on to station interference. The LF and MF bands are congested—there are many stations, and some are on or near the same frequency. That creates the possibility of interference, which causes bearing errors. By day, using an NDB within its DOC—that's the Documented Operating Coverage, the range within which the NDB is guaranteed to work—normally protects you from interference. But at night, you can expect interference even within the DOC. Why? Because of sky wave contamination from stations that are out of range by day. Their sky waves travel further at night and reach you. So positive identification of the NDB at night is not optional—it should always be carried out. Next, the mountain effect. Mountainous areas cause reflections and diffraction of the transmitted radio waves. Both of these produce errors in ADF systems. The errors increase at low altitude, and you can minimize them by flying higher. So if you're in the mountains, climb. Finally, coastal refraction. This is a big one. Radio waves speed up over water because there's reduced absorption of energy—reduced attenuation—compared to over land. When the wave speeds up, the wave front bends, or refracts, away from its normal path and pulls it towards the coast. The amount of refraction is negligible when the wave crosses the coast at 90°, but it increases as the angle of incidence increases—the more oblique the crossing, the greater the error. Here's the practical consequence: for an aircraft flying over the sea, the error puts the aircraft position closer to the coast than its actual position. So you think you're further out to sea than you really are. You can minimise this effect three ways. Use NDBs that are on or near the coast. Fly higher. And use signals that cross the coast at or near 90°. So to tie it together: night brings sky wave contamination, which you manage with identification and caution. Mountains bend the signal, so climb. And coasts refract the signal, so pick your NDB and your crossing angle carefully.