Radio Navigation Aids and Systems
What is examined
Radio navigation tests the operating principles, displays and failure modes of VOR, DME, NDB/ADF, ILS, GNSS and area navigation (RNAV/RNP). Questions often ask what the receiver measures, what the cockpit display means, and which limitation applies.
Core systems
- VOR gives bearing (radial) from the station; the CDI shows course deviation, not aircraft heading. TO/FROM tells whether the selected course leads to or from the station.
- DME measures slant range; at high altitude and close range, DME reads greater than ground distance.
- NDB/ADF gives relative bearing; the ADF needle points to the station, and homing does not correct drift.
- ILS: localizer gives lateral guidance, glideslope gives vertical guidance; decision altitude/height depends on the approach category and lighting.
- GNSS provides position, velocity and time; RAIM/fault detection and RNP containment are the integrity issues. RNAV is navigation by any sensor; RNP adds on-board performance monitoring and alerting.
Worked example
An aircraft flies an ILS at 140 kt ground speed on a 3.0° glideslope. Descent rate = ground speed in ft/min × tan 3°. 140 kt = 140 × 6076/60 = 14,177 ft/min along track; × 0.0524 = 743 ft/min. A rule of thumb is groundspeed × 5.3. At 140 kt, expect about 740 ft/min. If the aircraft flies 160 kt, the required rate rises to about 850 ft/min; failing to adjust leads to glideslope deviation.
Traps
Confusing a VOR radial with heading; reverse sensing when the OBS is set to the reciprocal; treating DME as ground distance; chasing false glideslope; assuming GNSS integrity without RAIM/RNP checks.