Non-directional Beacon - Common Adverse Effects

Common Adverse Effects

Navigation using an ADF to track NDBs is subject to several common effects:

• Night effect: radio waves reflected back by the ionosphere can cause signal strength fluctuations 30 to 60 nautical miles (54 to 108 km) from the transmitter, especially just before sunrise and just after sunset (more common on frequencies above 350 kHz)
• Terrain effect: high terrain like mountains and cliffs can reflect radio waves, giving erroneous readings; magnetic deposits can also cause erroneous readings
• Electrical effect: electrical storms, and sometimes also electrical interference (from a ground-based source or from a source within the aircraft) can cause the ADF needle to deflect towards the electrical source
• Shoreline effect: low-frequency radio waves will refract or bend near a shoreline, especially if they are close to parallel to it
• Bank effect: when the aircraft is banked, the needle reading will be offset

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, 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.