VDF Flashcards
What is DF?
A ground based direction finding equipment
What is a VDF?
VHF direction finder and tends to be used by civil A/C
What freq band does a VDF use?
VHF 118 to 137 MHz
VDF is a class B non precision approach
What is a UDF?
UHF direction finder, used by military A/C
What kind of bearings can a VHF give?
˚M or ˚T at the station
Though one VDF fix wouldn’t give an accurate position, comparing 2 VDF points will.
What is equipment is required for VDF?
A transmitter and a receiver
How does a VDF work, and why is it so good at giving direction?
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.
What is QDM?
˚M bearing TO the ground DF station
What is QDR?
˚M bearing FROM the ground DF station
What is QUJ?
˚T bearing TO the ground DF station
What is QTE?
˚T bearing FROM the ground DF station
What are QDL and QGH?
Other services:
QDL: receiving steering instructions
QGH: DF controlled approach
What is heterogeneous noise?
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.
What errors can you experience with a VDF?
Heterodyne noise, multi path signals or reflections i.e. When signal refract off mountains or reflect off clouds.
What are the 4 ICAO defined DF bearing classes?
- 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.
