Radio Navigation
Radio Navigation tests the principle, the displayed reading and the failure mode of each aid. Expect VOR/DME geometry, ILS tolerances and GNSS integrity concepts.
Ground-based aids
- VOR: 108-117.95 MHz, radials measured FROM the station, a line of position only, with a cone of confusion overhead. The TO/FROM flag and CDI give the radial; VOR gives no range.
- DME: slant range. The error is negligible at distance but large overhead - an aircraft at 6 nm altitude directly over the station reads 6 nm.
- NDB/ADF: the RBI shows relative bearing; true bearing = relative bearing + heading. Night effect, coastal refraction, static and quadrantal error degrade it.
ILS
- Localiser 108-112 MHz, 90 Hz left of centreline and 150 Hz right; glide path 329-335 MHz, 90 Hz below and 150 Hz above, nominally 3°.
- Decisions are made at DA/DH: CAT I 200 ft, CAT II 100 ft, CAT III no decision height with RVR minima. Outer marker about 4 nm, middle about 3 500 ft.
- Below the glide path means 'fly up'; the CDI sense is reversed compared with VOR.
Area navigation and GNSS
- RNAV, RNP and PBN define navigation performance; RNP requires on-board performance monitoring and alerting.
- GPS needs RAIM or an SBAS (EGNOS/WAAS) for integrity; GBAS gives local differential corrections; ADS-B transmits position.
- Transponder codes: 7700 emergency, 7600 radio failure, 7500 unlawful interference. Follow a TCAS RA and inform ATC.
Worked example
An aircraft at 30 000 ft (about 5 nm) is 3 nm ground distance from a DME. Slant range = √(3² + 5²) = 5.8 nm. The DME over-reads by 2.8 nm; treating it as ground distance would place the aircraft 2.8 nm further along than it is.
Traps
Reading a VOR radial as a bearing to the station; assuming DME gives ground distance; forgetting the localiser needle is 'fly towards'; and treating RAIM availability as a nicety rather than a requirement for an RNP approach.