VHF Direction Finder (VDF) Antenna

VHF Direction Finder (VDF) Antenna

Picture: Dated 1 March 2009 by JF.

I heard today from an extremely reliable source (Mike Grierson), that the VHF Direction Finder (VDF) Antenna in the centre of the airfield is still in use by NATS. It is the centre of the UK Auto – Triangulation system, apparently working on 121.5 and 243 MHz (the civilian and military distress frequencies) for position fixing.

I have to say I am very surprised in the day when WAAS GPS (FAA) and EGNOS GNSS systems that can fix your position accurately that the VDF hardware is being maintained as satellites can do this significantly cheaper.

A standard aircraft ELT (Emergency Locator Transmitter) fitted to the Gulfstream G650 as an example, transmits every fifty (50) seconds for 72 hours after automatic activation by “G” following a crash/ditching situation:

  • Two frequencies, 121.5 and 243.0 Mhz.
  • First 24 hours, transmission on additional satellite frequency 406.28 Mhz
  • After twenty-four (24) hours, transmission ceases in order to preserve battery power. The 406.028 MHz signal transmitted by the ELT is a digital signal with the serial number of the ELT or aircraft Identification (ID), country code, ID code and aircraft position (latitude and longitude) encoded in it. The 406.28 frequency transmitted by the ELT are monitored by the Cospas-Sarsat satellite system.


WAAS – Wide Area Augmentation System (SBAS)
EGNOS – European Global Navigation Satellite System Agency

Link to RAF Distress and Diversion explanation.
A link to NATS explaining the triangulation.
PDF Distress Diversion PDF document explaining all.

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