Definition
Ground-based stations equipped with direction-finding equipment that can determine the bearing of an aircraft's radio transmission and use that bearing information to provide navigational assistance, particularly to pilots in distress or those who are lost. A controller or specialist at the DF facility can give the pilot headings to steer in order to reach the airport or station.
Plain English
These are ground stations that can tell which direction an aircraft's radio signal is coming from. When a pilot keys the microphone, the station figures out the bearing to the aircraft and can then talk the pilot home by giving them headings to fly.
Context Anchor
Seen in radio navigation and ATC or flight service assistance discussions, especially when a pilot needs help finding position or direction.
Derivation
"Direction finder" describes exactly what the equipment does — it finds the direction from which a radio signal is arriving. The term dates back to early radio navigation, when bearings were taken by rotating a loop antenna until the signal was strongest or weakest.
Why Pilots Care
Provides a backup method for controllers to locate an aircraft whose exact position is unknown, especially during lost-communications or emergency situations.
Grounding Statement
When you transmit on the radio, DF equipment can sense the direction your signal is coming from.
Intuition Check
Do not read “facilities” here as just a building or office. DF facilities means the direction-finding equipment and the people or service using it; it is not GPS tracking and it only works where that service is available.
Example Sentence 1
When the pilot reported being unsure of his position, the controller suggested contacting one of the nearby DF facilities for steering assistance.
Example Sentence 2
DF facilities are listed in the Chart Supplement for airports that still maintain the service.