Radio Navigation
Radio Navigation
Radio navigation questions test the principles, indications and failure modes of ground and satellite systems used by ATC and pilots. Expect VOR/DME, ILS, NDB, radar, transponder and GNSS.
Core rules
- VOR gives bearing from the station; DME gives slant range. A DME reading at altitude is greater than ground distance.
- ILS localizer gives lateral guidance; glideslope gives vertical guidance, normally 3°. Decision altitude or height depends on approach category.
- NDB/ADF gives relative bearing; it is susceptible to thunderstorm and terrain errors.
- Primary radar detects skin return; secondary surveillance radar relies on transponder replies. Mode C is pressure altitude; Mode S has unique aircraft addressing.
- GNSS provides position; RAIM monitors integrity. RNAV and RNP use on-board navigation, with RNP adding alerting.
Worked example
An aircraft is on the 270 radial from a VOR, tracking inbound to the station. The 270 radial lies west of the station, so the aircraft is west of the station and must track east, 090° magnetic (ignoring variation). To use the CDI for inbound tracking, set OBS 090; the CDI should centre with TO. If OBS is left at 270, the CDI centres with FROM and the sensing is reversed. A radial is always from the station.
Traps
Do not confuse a radial with a bearing to the station; a radial is from the station. Do not use DME as exact ground distance. Do not assume an ILS is usable without checking the correct localizer and glideslope indications. Do not treat transponder altitude as QNH altitude.