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Instructions
Avoid navigating by NDBs that are shadowed or blocked by mountains. Mountains or cliffs exert a terrain effect on radio energy from NDBs, reflecting the waves and producing false or inaccurate direction indications. Mountains that contain a high degree of iron ore may also distort radio waves and introduce inaccuracies.
Disregard fluctuating ADF readings during electrical storms. The burst of electromagnetic energy in lightning flashes may causes the ADF needle to deflect away from the NDB signal and point toward the source of the lightning instead.
Expect variations along shorelines and in coastal areas. Radio ground waves are bent slightly in passing from land to water or from water to land at certain angles. Don't rely on an NDB signal that passes over the shoreline at an angle that's less than 30 degrees. NDBs at angles greater than 30 degrees in coastal areas are generally accurate.
Pay attention to sunrise and sunset times. At other times of day, ADFs receive the stronger ground-wave radio signal from the NDB. A "twilight effect" occurs when the ionosphere reflects the NDB's weaker sky wave back to Earth just before and after sunrise or sunset. This competing signal on the same frequency may cause the ADF needle to fluctuate between the true NDB direction and a false direction 30 to 60 miles away. This effect may be minimized by climbing to a higher altitude or by averaging a course between the fluctuating indications of the needle. The twilight effect resolves itself soon after sunrise or sunset.