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Quadrantal errors associated with aircraft Automatic Direction Finding (ADF) equipment are caused by:
  • A
    misalignment of the loop aerial.
  • B
    signal bending by the aircraft metallic surfaces.
  • C
    signal bending caused by electrical interference from aircraft wiring.
  • D
    skywave/groundwave contamination.

ADF Accuracy and Errors

ICAO requirement is an accuracy of ±6° with a signal-to-noise ratio no less than 3:1.
The ADF is subject to a number of potential errors:


All forms of static can affect accuracy of the ADF. In snow and freezing rain precipitation static reduces the accuracy and attenuation reduces the range of bearing information.


Thunderstorms in the vicinity act as radio beacons and can cause the needle to deviate in their direction.
In conditions like this and where heavy static is present VHF aids should be used in preference to ADF.

Night Effect

The principal propagation method of NDBs is the ground wave. However it is possible for weak sky waves to be returned at night when the ionosphere is less dense and attenuation is least. Returning sky waves take a longer propagation path than ground waves so they are often out of phase.
Night effect can be detected by listening for Fading on the carrier wave (BFO on) and by the instrument hunting. It is most likely at dawn or dusk.

Station Interference

The long ground waves of LF and MF signals mean that occasionally signals from stations on similar frequencies overlap. This will not cause errors in the daytime if the stations are only used within the protected range. At night, returning sky waves can cause rogue signals at considerable range producing the same problems as night effect.

Coastal Refraction

Speed of a surface wave is affected by the surface over which it travels (faster over water than land). This change of speed means the wave is refracted at low altitude as it passes over a coastline. Refraction is always towards the coast. An aircraft receiving a refracted wave would give a false indication of the beacon's position. It will place the aircraft nearer to the coast than it actually is. This effect is worse the further back from the coast the beacon is sited.

Quadrantal Error

The wave front from the NDB can be distorted by the aircraft's structure as it approaches the aerial (“signal bending by the aircraft metallic surafaces”) The error is called "quadrantal error" because the effect is worst for signals arriving from 45° and 135° left and right of the nose, the four "quadrants". Quadrantal error is small and predictable. It can be compensated during the installation of the receiver aerial and any residual errors can be shown on a quadrantal error card kept near the instrument. Modern receivers completely remove it.


Dip occurs when the receiver sense aerial is masked by the loop aerial. Dip gives large bearing errors, only occurs in a turn and is at its greatest when the NDB is on a relative bearing of 45° and 135° left and right of the nose.

Mountain Effect

At low altitude multipath signals reflected from terrain can cause erroneous readings This effect diminishes with height as hills are further from the line of sight and interfere less with the surface wave

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