Radio Navigation: ADF, VOR, DME, ILS, Radar and GNSS
Basic radio theory
Radio waves travel at about 300,000 km/s; frequency × wavelength = speed of light. Lower frequencies (NDB, ADF) follow the surface and are less line-of-sight limited but more affected by static and night effect; VHF and above are essentially line-of-sight, so range depends on altitude. The radio horizon is roughly 1.23 × √(height in feet) NM for an aircraft, plus the same for the ground station.
ADF, VOR, DME, ILS
ADF indicates the relative bearing of an NDB; relative bearing + heading = magnetic bearing to the station. Traps: reciprocal errors, and the fact that ADF bearings are affected by coastal refraction, night effect and thunderstorms. VOR gives magnetic bearing from the station (radial); the CDI shows whether you are on the selected radial, and TO/FROM indicates the direction. DME gives slant range, which reads higher than ground distance and is unusable directly overhead. ILS: localiser gives lateral guidance, glide path vertical guidance; the localiser is sensitive and must be intercepted at a shallow angle. Marker beacons give range checks. Radar: primary uses echoes; secondary uses transponder codes. GNSS: position from satellites, with RAIM monitoring integrity.
Worked example
Heading 030°M, ADF relative bearing 090°. Magnetic bearing to the NDB = 030 + 090 = 120°M. To fly to the station, turn to 120°M (no wind).
Rule of thumb
"Relative bearing + heading = magnetic bearing to station." For VOR, the radial is the magnetic bearing FROM the station.