VDF Flashcards

1
Q

What is DF?

A

A ground based direction finding equipment

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2
Q

What is a VDF?

A

VHF direction finder and tends to be used by civil A/C

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3
Q

What freq band does a VDF use?

A

VHF 118 to 137 MHz

VDF is a class B non precision approach

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4
Q

What is a UDF?

A

UHF direction finder, used by military A/C

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5
Q

What kind of bearings can a VHF give?

A

˚M or ˚T at the station

Though one VDF fix wouldn’t give an accurate position, comparing 2 VDF points will.

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6
Q

What is equipment is required for VDF?

A

A transmitter and a receiver

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7
Q

How does a VDF work, and why is it so good at giving direction?

A

When the pilot presses the press to transmit (PTT) button, the A/C transmitter sends out a carrier wave. As VDF uses VHF, these are space waves and so operate on line of sight. As VHF is line of sight then the signal propagates in a straight line.

The nearest antenna of the receiver, the DF on the ground will pick up this wave via its different aerial elements. As each aerial element receives a different phase, the DF then is able to work out you’re position.

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8
Q

What is QDM?

A

˚M bearing TO the ground DF station

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9
Q

What is QDR?

A

˚M bearing FROM the ground DF station

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10
Q

What is QUJ?

A

˚T bearing TO the ground DF station

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11
Q

What is QTE?

A

˚T bearing FROM the ground DF station

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12
Q

What are QDL and QGH?

A

Other services:

QDL: receiving steering instructions
QGH: DF controlled approach

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13
Q

What is heterogeneous noise?

A

Heterogeneous noise is an erroneous error, therefore the bearing given cannot be reliable.

This is when 2 DF stations transmit at the same time and/or simultaneously thus distorting the carrier wave.

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14
Q

What errors can you experience with a VDF?

A

Heterodyne noise, multi path signals or reflections i.e. When signal refract off mountains or reflect off clouds.

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15
Q

What are the 4 ICAO defined DF bearing classes?

A
  • Class A accurate within +/- 2˚
  • Class B “ “ +/- 5˚
  • Class C “ “ +/- 10˚
  • Class D accurate to less than class C, i.e. >10˚

The more accurate, the better/more you can use in bad Wx.

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16
Q

How does a VDF position fix?

A

While one VDF signal can give an approximate line of position, by fixing on 2 or more VDF stations, the different position lines will be able to triangulate and fix on your position.