VHF Direction Finding. Two types of approach: 1 where the pilot transmits for several seconds every 30 seconds or so and ATC provides him with QDMs to the station. By interpreting the bearings he receives he can follow an approach procedure to reach the airfield. 2. QGH.. where ATC asks the pilot to transmit about every 30 seconds... the controller obtains bearings on the aircraft and issues headings to fly to carry out an approach. A good QGH controller can be a Godsend.... but is it still used? It's 35 years since I last did one!
VDF can also be used by en route a/c calling a station for a bearing which may then be used for a nav plot.