Sky Wave Interference RNAV

During twilight hours (some answers have "dawn and dusk"), the ionosphere undergoes change as the sun is going down. The D layer of the ionosphere, the one responsible for attenuating waves, disappears. The effect of this is that waves that would ordinarily disappear (in the MFMF —Medium Frequency and LFLF —Low Frequency bands) remain strong and are refracted back to earth as skywaves. These refracted waves will almost certainly arrive in a different phase to the ground waves that have travelled along the earth, and so they interfere. This process is called fading/night effect, and is a particular problem with the NDBNDB —Non-Directional Beacon navaidnavaid —Navigation Aid, causing fluctuations of bearing on the instrument used (RBIRBI —Relative Bearing Indicator or RMIRMI —Radio Magnetic Indicator.)

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When does the phenomenon "fading" occur?
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