of the RMI/RBI to point for a moment to the bearing of the lightning. Shoreline effect Radio waves speed up over water, causing the wave front to bend away from its normal path and pull it towards the coast. Refraction is negligible perpendicular (90°) to the coast, but increases as the angle of incidence decreases. The effect can be minimised by flying higher or by using NDBs situated nearer the coast. Station interference Due to congestion of stations in the LF and MF bands, there is the possibility of interference from stations on or near the same frequency. This will cause bearing errors. By day, the use of an NDB within the DOC will normally afford protection from interference. However, at night one can expect interference even within the DOC because of skywave contamination from stations out of range by day. Therefore, positive identification of the NDB at night should always be carried out. Dip (bank) angle During banking turns in an aircraft, the horizontal part of the loop aerial will no longer be horizontal and detect a signal. This causes displacement of the null in a way similar to the night effect giving an erroneous reading on the indicator which means that the pilot should not obtain a bearing unless the aircraft is wings-level. While pilots study these effects during initial training, trying to compensate for them in flight is very difficult; instead, pilots generally simply choose a heading that seems to average out any fluctuations. Radio-navigation aids must keep a certain degree of accuracy, given by international standards, Federal Aviation Administration (FAA), ICAO, etc.; to assure this is the case, Flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify NDB precision. The ICAO minimum accuracy for NDBs is ±5° Monitoring NDBs Besides their use in aircraft navigation, NDBs are also popular with long-distance radio enthusiasts (DXers). Because NDBs are generally low-power (usually 25 watts, some can be up to 5 kW), they normally cannot be heard over long distances, but favorable conditions in the ionosphere can allow NDB signals to travel much farther than normal. Because of this, radio DXers interested in picking up distant signals enjoy listening to faraway NDBs. Also, since the band allocated to NDBs is free of broadcast stations and their associated interference, and because most NDBs do little more than transmit their Morse code callsign, they are