Radio Navigation
Core concepts
Radio navigation covers ground-based and satellite systems: VOR, DME, NDB/ADF, ILS, GNSS, and radar. You must understand how each system works, its accuracy and limitations, and how to interpret the cockpit indications. Know the VOR radials, the cone of confusion, DME slant range versus ground distance, the ADF relative bearing and its errors, and the ILS localiser and glideslope.
Traps to watch
A classic trap is the "from/to" ambiguity on the VOR: the indicated radial is the bearing from the station, and the course you fly depends on the OBS setting. Another is DME slant range: directly overhead the station, the DME reads the aircraft's altitude, not zero. ADF questions test the relative bearing and the effect of coastal refraction and night effect. ILS questions test the localiser (lateral) and glideslope (vertical) and the meaning of the marker beacons.
Worked example
Overhead a DME station at 6 NM altitude, the DME reads 6 NM even though ground distance is zero. At 8 NM ground distance and 6 NM altitude, slant range = √(8² + 6²) = 10 NM.
Rule of thumb
DME reads slant range, never less than altitude. VOR radials are "from" the station.