Radio Navigation
Radio Navigation tests the principles and in-flight use of ground-based and satellite aids: what each system measures, which errors matter, and how to turn a cockpit indication into a track, a distance or a height.
VOR, DME and NDB
- VOR gives a magnetic radial from the station; the indicator shows the selected course with a TO/FROM flag. Accuracy is about ±5°, range is line-of-sight, and overhead the station lies the cone of confusion.
- DME measures slant range, so it overreads most directly overhead: slant range = √(ground distance² + height²). Modern DME also displays ground speed and time to the station.
- ADF displays relative bearing. QDM = magnetic heading + relative bearing; QDR = QDM ± 180°. Night effect, coastal refraction and thunderstorms degrade ADF accuracy.
ILS and GNSS
- Localizer and glideslope use 90 Hz/150 Hz modulation. A 3° glideslope gives about 300 ft/NM, and the sensitive area must be protected.
- GNSS guidance for approach needs RAIM or SBAS; RAIM prediction depends on time and location.
Worked example
Heading 090°M, ADF relative bearing 030°, variation 10°W. QDM = 090 + 030 = 120°M. With variation west the magnetic bearing is larger than true, so true QDM = 120 − 10 = 110°T. The station lies 110°T from the aircraft, so the aircraft is on the 290°T radial, northwest of the beacon.
Traps
- Treating relative bearing as a bearing from north.
- Forgetting that DME slant error is greatest overhead, not at long range.
- Assuming ADF is reliable at night or over the coast.
- Using the published 3° glideslope when the chart gives a different angle.