Radio Navigation and GNSS
What the exam tests
This domain tests the practical use of VOR, DME, NDB, ILS and GNSS, including indications, limitations and failure modes relevant to PPL navigation.
Core rules
- VOR provides magnetic radials from the station. The OBS selects the radial or course; TO/FROM indicates whether the selected course leads to or from the station. Line-of-sight reception limits range at low altitude.
- DME measures slant range. Overhead a station at 6,000 ft, DME indicates about 1 NM even though ground distance is zero; at longer range the error becomes small.
- NDB/ADF gives a relative bearing; the pilot must apply aircraft heading to obtain the magnetic bearing to the station.
- ILS consists of a localiser for lateral guidance and a glide slope, normally 3°, for vertical guidance. False glide slopes and interference require cross-checking.
- GNSS provides position, track and waypoint navigation. RAIM detects integrity failures; without RAIM or with jamming, GNSS may not be usable for navigation.
Worked example
An aircraft passes directly over a DME station at 6,000 ft. Six thousand feet is about 1 NM, so slant range is the square root of 0 squared plus 1 squared, or 1 NM. The DME reads 1.0 NM at the overhead point, which is correct slant range, not an error.
Traps
Treating DME as ground distance; forgetting VOR is line-of-sight; using a GNSS position without RAIM when required; and misreading TO/FROM on a selected VOR course.