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What is the effect of multipath signals (coming from the same aircraft) at the Ground VHF Direction Finder station?
  • A
    They may result in bearing errors.
  • B
    Regardless the difference in distance travelled by these signals, it results in their extinction of the signals at the Ground VHF Direction Finder station.
  • C
    It may result in an increase of the distance at which the Ground VHF Direction Finder station receives signals from the aircraft, if the Ground Station is situated in the skip zone.
  • D
    It reduces the range at which the Ground VHF Direction Finder station receives signals from the aircraft.

The VHF Direction Finder (VDF) is a means of providing a pilot with the direction to fly towards a ground station by means of a bearing. The bearing can be used to navigate to or from the station or to help establish a fixed position.

VDF bearing information will only be given when conditions are satisfactory and radio bearings fall within the calibrated limits of the station.

There are 4 factors that affect the accuracy of a VDF:

  • Propagation error and site error cause by aircraft’s transmissions being reflected from terrain as they travel to the site, or being reflected from buildings at the site.

  • Aircraft attitude, the VDF system and VHF communications are vertically polarized, therefore the most accurate is when the aircraft flies straight and level.

  • Poor accuracy is likely in the overhead of a VDF receiver. The reception of both direct wave and ground reflected wave can cause signal fading or loss. Together with other multi path signals this gives rise to bearing errors.

  • Transmissions done at the same time by two or more aircraft will cause momentary errors in bearings.

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