Ground-Based Radio Aids: ADF, VOR, DME, ILS and Radar
ADF and NDB
ADF measures the relative bearing of an NDB; bearing from the station = heading + relative bearing (subtract 360 if needed). QDM is the magnetic bearing to the station, QDR the magnetic bearing from it, QUJ the true bearing to it and QTE the true bearing from it. Homing points the aircraft at the station and drifts downwind; tracking holds a constant QDM. Errors include night effect, coastal refraction, mountain effect, static and quadrantal error.
VOR and DME
A VOR radial is a magnetic bearing from the station, identified by phase comparison; the CDI centres with OBS set and the TO/FROM flag shows the sense. Line-of-sight reception limits range and produces a cone of confusion overhead. DME measures slant range, so indicated distance exceeds ground distance, most noticeably close in and at high altitude; modern DME also displays groundspeed and time to station.
ILS
Localiser (108–112 MHz) gives lateral guidance with 90 Hz left and 150 Hz right of the centreline; glide path (329–335 MHz) gives vertical guidance, normally about 3°, roughly 300 ft per nm. Markers or DME give range fixes, and decision height is referenced to the radio altimeter. False glide paths and incorrect localiser sensing are classic traps; always check the sense of the deviation.
Radar
Primary radar relies on skin return; secondary surveillance radar interrogates the transponder, which replies with identification and, in mode C, pressure altitude. Mode S adds selective addressing. Weather radar shows precipitation return only, not turbulence directly.
Worked example: heading 090° magnetic, relative bearing 030°, gives bearing from station 120°, so QDM = 120° and QDR = 300°.
Traps: mixing QDM with QDR, forgetting heading when converting relative bearing, and treating DME slant range as ground range.