Joined: 11 Dec 2005
|Posted: Sat Mar 30, 2013 11:35 am Post subject: Communications Related to Direction Finding
|Communications Related to Direction Finding
Some VHF ground radio stations are equipped with automatic direction finding (VHF DF).
Military stations may provide VHF DF or UHF DF.
Bearing information can be provided on request expressed as either a magnetic or true direction either to or from the station.
Bearings from two DF stations can provide a fix.
The Q codes are used to avoid confusion.
• QTE True bearing from the station
• QDR Magnetic bearing from the station
• QDM Magnetic bearing to the station
Bearings are classified by their quality.
• Class A bearings are accurate to ±2°
• Class B to ±5°. It is the most common and can be assumed if class is not given.
• Class C to ±10°
• Class D outside 10°
Ground DF stations can decline to give a bearing if accuracy is poor.
VDF bearings from a single station do not provide a position fix.
To obtain a fix several VDF stations must be operating on the same frequency at the same time.
This only occurs on the VHF Emergency Frequency 121.5 Mhz
On this frequency auto-triangulation may be available or (depending on height and position) manual triangulation may give a position.
Fixing accuracy may be described by class:
• Class A - accurate within 5 nm
• Class B - accurate within 20 nm
• Class C - accurate within 50 nm
• Class D - accuracy less than Class C