Radio Navigation Principles and Position Fixing
Basic Principles
Radio aids give a line of position (LOP). A VOR gives a magnetic bearing (radial) from the station; a DME gives slant range. An NDB/ADF gives a relative bearing to the station, converted to magnetic bearing using heading and deviation/variation. Two or more LOPs intersecting give a fix.
Fix Quality
The angle of cut between LOPs determines fix accuracy; near 90° is best, and shallow cuts magnify error. A position can also be fixed by range–range (two DME), bearing–range (VOR/DME) or by radar. Slant range exceeds ground distance, with the error largest close in and at high altitude.
Instrument Approach Aids
ILS gives lateral guidance (localiser) and vertical guidance (glide path); markers or DME give range. ADF relative bearing = magnetic bearing − heading; magnetic bearing = relative bearing + heading.
Traps
Questions mix relative and magnetic bearings, or ask for the radial from a station versus the bearing to it (reciprocal). Slant-range error questions require recognising the geometry, and deviation applies when converting ADF bearings.
Worked Example
Heading 060°M, ADF relative bearing 030° → magnetic bearing to station = 090°M; the radial on which the aircraft sits is the reciprocal, 270°.
Rule of Thumb
Best fix angle of cut is 90°; avoid cuts below about 30°.