VDF is a ground based equipment used in Air Traffic Control to find the bearing of aircrafts
Bearing accuracy of VDF is up to half a degree however range information is not available
VDF works in the VHF band with frequencies range between 118 to 137 MHz
Loop antenna principle is used to find the direction of transmission where the transmission from an aircraft from a perpendicular direction to the loop ariel gives zero phase difference
Rotating loop is replaced by electronically rotated fixed vertical elements
Adcock ariel is an advanced form of electronic rotating ariel which uses doppler effect to resolve the bearing of transmission
Introduction to Q Codes
A VDF station can provide QDR, QDM, QTE and QUJ on request from aircraft
QDR or Radial is the aircraft’s Magnetic bearing from the station
QDM is the reciprocal of QDR which is is the magnetic heading to steer to reach over the VDF in nil wind
QTE is the true bearing from the station while QUJ is the true bearing to the station
Range and Accuracy of VDF
Maximum range of VDF is governed by line of sight formula
VDF range is further constrained by terrain, meteorological conditions and power of transmission
VDF readings are graded as Class A, B, C and D based on accuracies
Accuracies of A, B, C, D are 2, 5, 10 and greater than 10 degrees
VDF accuracies reduce due to Site Error, Synchronous Transmission, Attitude of aircraft and Multi Path Fading
Site error occurs due to reflection from buildings while Synchronous Transmission is due to two or more aircrafts transmitting in close proximity
Aircrafts flying at other straight and level attitude like climb, descend or during turns results in erroneous readings
Flight close to overhead the VDF ground facility may result in error due to Multi-path fading
Position Fix using Triangulation of VDF Bearings
Triangulation by a network of two or more VDF ground stations is provided to aircraft using distress frequency of 121.5 MHz
ATC equipped with VDF coordinate to triangulate and provide position fix for aircraft in distress or lost
Distress aircrafts are provided with Magnetic tracks to station (QDM) or calculated headings and height to fly (QGH)