Some Air Traffic Control Units have VHF Direction Finding (VDF) equipment which shows the air traffic controller the direction a radio transmission is coming from.
A pilot can make use of this VDF equipment by making a normal radio transmission on the aircraft’s VHF radio and asking for one of the following codes:
Most commonly, a pilot will want a QDM – the magnetic track to the VDF station. Once given a QDM, the pilot can either home or track to the station:
VDF accuracy is divided into 4 classes:
Since VHF radio waves only travel in straight lines, you need to be in line of sight of the VDF station for it to work. Any terrain or obstacles in the way will block your VHF transmissions.
Full details of Air Traffic Control Units with VDF services, their radio frequencies, and hours of operation are found in the AIP.
Aerodromes with VDF services also have “VDF” and the appropriate frequency marked on the 1:500 000 aeronautical chart.