AIM - COM 3.10 - VHF Direction Finding System

VHF Direction Finding System (VDF) equipment, capable of multi-channel operation,
has been established at a number of FSSs and airport control towers. VDF normally
operates on six preselected frequencies in the 115 to 144 MHz range. Information
displayed to the controller or FSS position on a numerical readout gives an accurate (± 2°)
visual indication of the bearing of an aircraft from the VDF site. This is based on the radio
transmission received from the aircraft, thus giving the VDF operator a means of
providing steering, bearing, or homing information to pilots requesting the service.

(a) Primary Services

        Directional guidance to the VDF and, if requested, a bearing from the VDF site.

(b) Additional Services

        Track out assistance, estimated times or distances from the site, or fixes when used
        in conjunction with another VDF site, a VOR radial or a bearing from an NDB.

(c) Emergency Service

        “No compass homing” will be provided when no other course of action is available
        provided the pilot declares an emergency, or accepts the service suggested by the
        VDF operator. Cloud breaking procedures will be provided where they exist.