Radio Navigation
What this domain tests
Radio Navigation tests VOR/DME, ILS, NDB/ADF, GNSS/RNAV, radar and transponder/TCAS. Expect questions on radials, CDI indications, glideslope geometry and the meaning of RNP/RNAV values.
Core rules
- VOR gives a magnetic bearing from the station (radial). The CDI centres when the selected radial matches the aircraft position; TO/FROM depends on the bearing relative to the station.
- DME gives slant range, always greater than ground distance; the error is greatest directly overhead.
- ILS: localiser gives lateral guidance, glideslope vertical. At 3 degrees, the aircraft is about 318 ft above the threshold per nautical mile.
- NDB/ADF: the needle points to the station; it is affected by coastal refraction, night effect and thunderstorm static.
- GNSS/RNAV: RNAV 1 and RNP 1 define accuracy in nautical miles; RNP includes on-board performance monitoring and alerting, so it can be used in remote areas without ground aids.
- Transponder: 7700 emergency, 7600 radio failure, 7500 unlawful interference.
Worked example
You are 4 nm from the ILS threshold on a 3 degree glideslope. Height above threshold = 4 x 6,076 x tan 3 degrees, approximately 4 x 6,076 x 0.0524, about 1,273 ft. Add the threshold elevation to compare with the altimeter.
Traps
Reading a VOR radial as a heading; forgetting that DME is slant range; setting the wrong ILS frequency or course; assuming RNP and RNAV are interchangeable; forgetting that an ADF bearing is relative to aircraft heading unless the instrument has a slaved compass.
Key habit
For every radio-navigation question, identify what the receiver measures (bearing, range or angle) before doing arithmetic.