Chapter

6

VHF Direction Finder (VDF)

Introduction										75
Procedures										75
Principle of Operation . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	77
Range of VDF . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	. 78
Factors Affecting Accuracy . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	78
Determination of Position . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	78
VDF Summary . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	. 79
Questions . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	80
Answers . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	. 82

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6 VHF Direction Finder (VDF)

(VDF) Finder Direction VHF 6

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VHF Direction Finder (VDF) 6

Introduction

The VHF Direction Finder (VDF) is a means of providing a pilot with the direction to fly towards a ground station - a bearing. The bearing can be used to ‘home’ towards the ground station or, in conjunction with another bearing, or bearings, can be used to establish a fix position. VDF is rarely used because there are so many more sophisticated and more accurate systems available.

Bearings are provided, by voice, on an aircraft’s VHF Communications frequency; they are therefore available on 118.0 - 137 MHz (Emission Code A3E). Auto-triangulation (position automatically provided from a number of VDF bearings from different stations) is available, but solely on the VHF International Distress Frequency – 121.5 MHz. At present, UHF DF is limited to military use.

The Aeronautical Stations offering a VDF service are listed in the AD Section of the AIP. Some VDF stations stipulate that the service is not available for en route navigation purposes (except in emergency). VDF bearing information will only be given when conditions are satisfactory and radio bearings fall within the calibrated limits of the station. If the provision of a radio bearing is not possible the pilot will be told of the reason.

Procedures

A pilot may request a VDF bearing using the appropriate phrase or Q-Code to specify the service required (see Figure 6.1):

‘QDM QDM QDM OXFORD APPROACH G-DS REQUEST QDM, G-DS’

Figure 6.1 QDM/QDR/QTE.

VHF Direction FInder (VDF) 6

75

(VDF) Finder Direction VHF 6

6 VHF Direction Finder (VDF)

A VDF station will provide the following as requested:

•QDR Aircraft’s Magnetic Bearing from the station (Radial); used for en route navigation

•QDM Aircraft’s Magnetic Heading to steer (assuming no wind) to reach the VDF station; used mainly for station homing and let-downs using published procedures

•QTE Aircraft’s True Bearing from the station; used for en route navigation

•QUJ Aircraft’s True Track to the station; not generally used

Note: QDM is the reciprocal of QDR; QUJ is the reciprocal of QTE. QDR, QDM and QTE are most commonly used.

The direction-finding station will reply in the following manner:

•The appropriate phrase or Q code

•The bearing or heading in degrees in relation to the direction finding station

•The class of bearing

•The time of observation, if required

The accuracy of the observation is classified as follows:

•Class A – Accurate within ± 2°

•Class B – Accurate within ± 5°

•Class C – Accurate within ± 10°

•Class D – Accuracy less than Class C

Note: Normally, bearings no better than Class B will be available. The latest equipment uses Doppler principles to determine a high-resolution bearing that can be displayed as a digital read-out; bearings produced in this manner have an accuracy of ±0.5° (available on UHF and VHF systems).

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