VHF Direction Finder (VDF) 6
Principle of Operation
The only equipment required to obtain a VDF bearing is a VHF radio; some specialist equipment is required on the ground: a suitable aerial and a display.
A VHF voice communications radio produces a vertically polarized signal; therefore, the ground antenna is vertically polarized and has an array of vertical elements arranged in a circle. See
Figure 6.2.
VHF Direction FInder (VDF) 6
Figure 6.2: Obtaining a VDF Bearing
The equipment resolves the bearing from transmissions received at each element within the array. The bearing is then displayed on the display. The bearing can be displayed relative to either True or Magnetic North (at the station).
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6 VHF Direction Finder (VDF)
(VDF) Finder Direction VHF 6
Range of VDF
•As VDF utilizes the VHF Band (or UHF as required) the range will obey the line of sight formula: the higher the transmitters the greater the reception range
Line of sight Range (MTR) = 1.23 × (√hTX + √hRX)
•Intervening high ground will limit range, especially for low flying aircraft in hilly terrain.
•The power of airborne and ground transmitters will limit ranges.
•Gradients of temperature and humidity can give greater than line of sight range.
Factors Affecting Accuracy
•Propagation error and site error caused by the 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, best reception and results will be obtained when the aircraft flies straight and level.
•Poor accuracy is likely in the overhead of a VDF receiver, particularly with the latest Doppler systems. The reception of both Direct Wave and Ground Reflected Wave can cause signal fading or loss; the phenomenon is usually short-lived. Together with other multi path signals this gives rise to bearing errors.
•Synchronous transmissions by two or more aircraft will cause momentary errors in bearings.
Determination of Position
If there are sufficient ground stations, linked to an ATCC, the aircraft’s position can be fixed using auto-triangulation and the position transmitted to the pilot. This facility may be available to Distress and Diversion Cells, but can not be guaranteed.
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VHF Direction Finder (VDF) |
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VDF Summary |
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Bearings: |
QDM |
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Mag TO Station |
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QDR |
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Mag FROM Station |
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QUJ |
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True TO Station |
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QTE |
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True FROM Station |
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Uses: |
Track Check |
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Position Line |
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Homing |
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6 |
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Let-downs |
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(VDF) |
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Class: |
A = ± 2° |
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FInder |
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B = ± 5° |
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C = ± 10° |
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Direction |
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D = > 10° |
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Principle: |
Ground Equipment – Direction Finding Aerial |
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VHF |
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CRT Display |
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Range: |
Line of Sight |
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Power of Transmitters |
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Intervening High Ground |
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Atmospheric Conditions (Ducting) |
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Accuracy: |
Propagation Error |
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Site Error |
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Aircraft Attitude |
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Overhead |
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Fading Due to Multi-path Signals |
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Position Service: |
Position Fixing by Auto-triangulation |
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Figure 6.3 VDF Summary
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6 Questions
Questions 6
Questions
1.An aircraft has to communicate with a VHF station at a range of 300 NM, if the ground station is situated 2500 ft AMSL which of the following is the lowest altitude at which contact is likely to be made?
a.190 ft
b.1 378 ft
c.36 100 ft
d.84 100 ft
2.Class ‘B’ VHF DF bearings are accurate to within:
a.± 1°
b.± 5°
c.± 2°
d.± 10°
3.A VDF QDM given without an accuracy classification may be assumed to be accurate to within:
a.2 degrees
b.5 degrees
c.7.5 degrees
d.10 degrees
4.An aircraft at altitude 9000 ft wishes to communicate with a VHF/DF station that is situated at 400 ft AMSL. What is the maximum range at which contact is likely to be made?
a.115 NM
b.400 NM
c.143 NM
d.63.5 NM
5.An aircraft is passed a true bearing from a VDF station of 353°. If variation is 8°E and the bearing is classified as ‘B’ then the:
a.QDM is 345° ± 5°
b.QDR is 345° ± 2°
c.QTE is 353° ± 5°
d.QUJ is 353° ± 2°
6.An aircraft at 19 000 ft wishes to communicate with a VDF station at 1400 ft AMSL. What is the maximum range at which contact is likely ?
a.175 NM
b.400.0 NM
c.62.5 NM
d.219 NM
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Questions 6
Questions 6
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