Automatic Direction Finder (ADF)

GeneralLimitationsAdvantagesNavigation

AIM COM 4.6 Non Directional Beacons

An older form of radio navigation is the Automatic direction Finder (ADF) or Non-directional beacon (NDB). The ADF can be used as a back up to VHM navigation or when line-of-sight transmission is not possible.  An ADF can be used to determine position, receive voice communication and for instrument approach procedures. 

The ADFs and NDBs operates on the low/medium frequency range (190–415kHz and 510–535kHz) and is classed based on its power output:

  • “H” power output is 2,000 W or more
  • “M” power output is from 50 W to less than 2,000 W
  • “L” power output is less than 50 W

NDB systems are flight checked to an accuracy of at least ±5˚ for an approach and ±10˚ for en route. However, much larger errors are possible due to propagation disturbances caused by sunrise or sunset, reflected signals from high terrain, refraction of signals crossing shorelines at less than 30˚ and electrical storms.

ADF Limitations

  • Twilight Effect Caused by the reflection of radio waves by the ionosphere.  It causes the ADF pointer to fluctuate and is most pronounced just before and after sunrise/sunset.  The effect is minimized by flying at a higher altitude or selecting an NDB operating at a lower frequency.
  • Terrain Effect Mountains and cliffs can reflect radio waves.  Furthermore, magnetic deposits on the slopes of mountains have been known to cause significant errors.  Pilots operating near mountainous areas should exercise caution and only use NDBs that provide a definition directional indication.
  • Shorelines Water can refract radio waves leading to erroneous signals.  Pilots should not use an NDB signal that crosses over a shoreline and towards the aircraft at an angle of less than 30° 
  • Lightning If flying near a thunderstorm, the ADF pointer will point to sources of lighting rather than the selected station 
  • Bank Error This type of error is present in all turns due to the configuration of the loop antenna.  It is a significant factor during NDB approaches

Advantages

NDBs are relatively inexpensive to install and maintain but most importantly they are not subject to line of sight limitations.