NDB Night Effect RNAV
Night effect is a phenomenon caused by the changes in the Earth's ionosphere by night. At night, the sun in no longer shining on that part of the ionosphere, meaning it decreases in intensity and the different layers rise to higher altitudes. This means that waves start to refract back to Earth later, meaning that the waves will travel further before they return. This is not necessarily a good thing, as waves from other NDBNDB —Non-Directional Beacons are travelling much further and potentially interfering with the signal from a nearer NDBNDB —Non-Directional Beacon.
Night effect is worst when the ionosphere is in a state of change – so around dawn and dusk, when the sun is rising or falling in the sky. The promulgated range describes the radius of a circle around the NDBNDB —Non-Directional Beacon where you are guaranteed reception from the NDBNDB —Non-Directional Beacon without interference from other NDBNDB —Non-Directional Beacons. Because of night effect, this value is valid during the day only.