Learning Objective 062.02.01.02.03: Explain that by using more than one ground station, the position of an aircraft can be determined and transmitted to the pilot.

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VDF (VHF Direction Finding) is a method of measuring the direction from which a VHF signal came. It has been used for many decades now, and is particularly useful as the transmitting aircraft only needs a standard VHF radio to ask for a VDF bearing from a suitably equipped ATSU. This can give them:

QDM - Magnetic bearing from the aircraft to the station.
QDR - Magnetic bearing from the station to the aircraft.
QUJ - True bearing from the aircraft to the station.
QTE - True bearing from the station to the aircraft.

With this information, the pilots can plot the location of their aircraft on a chart by drawing a true radial (QTE) outbound from each station that was used. 2 stations (and therefore 2 lines) can be enough to calculate the position of an aircraft, by finding the point at which they intercept. This is a process known as triangulation. It is important to get a large angle at the point of interception, as the larger the angle (optimum 90º), the better the precision of the fix. More than 2 VDF stations can also be used to get a more accurate fix.

Distress and Diversion (121.5 MHz) have access to an auto-triangulation feature which gives them an immediate readout of the position of any calling aircraft, due to the reception of its transmission at multiple different locations with VDF capability. The computer allows the triangulation to be done immediately.

Position is the only thing that can be gained from VHF direction finding though, so it is limited in that sense, but is very useful in the sense that only a VHF radio is required in the aircraft.