Radio navigation aids and GNSS
This domain tests practical use of VOR, NDB/ADF, DME and GNSS for VFR navigation. Expect radial/bearing questions, TO/FROM interpretation, slant-range error, and the limitations of GNSS.
Core rules
- A VOR radial is the magnetic bearing from the station. The OBS selects the course you want; when the CDI is centred, TO means the selected course takes you to the station, FROM means it takes you away. With OBS 240 and TO, the station bears 240°M and you are on the 060 radial.
- ADF points to the NDB. Relative bearing plus heading gives magnetic bearing to the station (QDM); the reciprocal is QDR. Track mentally and correct for drift; do not chase the needle.
- DME reads slant range, not ground distance. Error is greatest overhead and at high altitude; at 12,000 ft, 12,000 ft = 2 NM.
- GNSS gives waypoint track, distance and ETA, but you must still navigate: compare GPS track with chart track, allow for wind, check RAIM and database currency, and keep a DR fallback.
Worked example
The OBS is 090 with TO and the CDI centred. The station bears 090°M, so the aircraft is on the 270 radial. If DME reads 10 NM at 12,000 ft, slant range exceeds ground distance by the height component: height = 2 NM, ground distance = sqrt(10² − 2²) = 9.8 NM. The error is small here but becomes significant closer in.
Traps
Confusing radial with bearing; reversing TO/FROM; setting OBS to heading rather than desired course; treating DME as ground distance; using an expired GPS database; and assuming GNSS is unaffected by RAIM or jamming.