Radio Navigation: Ground Aids, GNSS and Approach Systems
Ground-based aids
Understand how each aid produces a line of position. VOR gives a magnetic radial from the station; the CDI centres when the selected radial matches, and the TO/FROM flag tells you which side of the station you are on. DME gives slant range and pairs with VOR to give a fix. NDB/ADF gives a relative bearing; the ADF needle points to the station relative to the aircraft's heading, and you convert relative bearing plus heading to a magnetic bearing.
ILS and approach aids
Know the localiser (lateral guidance, usually 3 to 6 degrees wide) and the glideslope (typically 3 degrees). Understand the decision altitude and the difference between a precision and a non-precision approach. Know marker beacons and their distances.
GNSS
Understand the satellite constellation, RAIM (receiver autonomous integrity monitoring) and why integrity matters more than accuracy for approach use. Know the difference between a 2D and 3D fix, and the meaning of an alarm.
Traps
- Confusing a VOR radial with a bearing to the station.
- Forgetting that ADF relative bearing must be added to heading to get a magnetic bearing.
- Assuming DME gives ground distance - it gives slant range.
- Believing RAIM is about accuracy when it is about integrity.
Worked example
Heading 270 magnetic, ADF relative bearing 030: magnetic bearing to the station is 270 + 030 = 300. To fly to the station, turn to 300 magnetic.