Radio Navigation
Radio navigation questions test what each system measures and how it fails. Organise your knowledge by system type: ground-based, satellite-based and on-board.
Key concepts: NDB/ADF (relative bearing, ADF errors, coastal refraction, night effect); VOR (radial, phase comparison, line-of-sight, cone of confusion); DME (slant range, not ground range); ILS (localiser, glide path, marker beacons, categories and their minima); radar (primary and secondary, transponder codes); GNSS (GPS, augmentation, RAIM, integrity); and area navigation (RNAV, RNP and their accuracy requirements). Understand line-of-sight limitations and the effect of terrain and altitude on reception.
Traps: treating DME slant range as ground range (they differ most at high altitude and short range); confusing VOR radial with the bearing to the station; assuming an ILS category guarantees autoland - it does not; and forgetting that RAIM is a GNSS integrity check, not a positioning system.
Worked example: an aircraft at 30 000 ft (about 5 nm vertically) reads 10 nm DME. Ground range = √(10² − 5²) = √75 ≈ 8.7 nm. Rule of thumb: DME reads high near the station - the error is largest at short range and high altitude.