Radio Navigation Aids (VOR, DME, NDB/ADF, ILS)
VOR and DME
VOR transmits on 108.0–117.95 MHz and gives a magnetic radial from the station. The CDI and OBS select the radial; TO/FROM shows which side of the station the aircraft is on; the cone of confusion gives unstable indications overhead. Accuracy is about ±5°. DME on 960–1215 MHz measures slant range, accurate to ±0.5 nm or 3% of range, and provides groundspeed and time to station. Co-located VOR/DME gives a rho-theta fix.
NDB and ADF
NDB transmissions in the 200–535 kHz band are received by the ADF, which displays relative bearing. QDM (magnetic bearing to the station) = heading + relative bearing; QDR is the reciprocal. Homing points the nose at the station and drifts downwind; tracking needs a wind correction. Errors include night effect, coastal refraction, precipitation static, mountain effect and quadrantal error.
ILS
Localizer on 108–112 MHz (odd tenths), glide slope on 328.6–335.4 MHz. The localizer compares 90 Hz and 150 Hz modulation, with full-scale deflection at 0.45 DDM. A standard glide path is 3°; decision heights are 200 ft (CAT I), 100 ft (CAT II) and below 100 ft (CAT III). Marker beacons transmit on 75 MHz, and DME may replace the outer marker.
Worked example
Heading 090°, ADF relative bearing 030°. QDM = 090 + 030 = 120°. With no wind, turn right to 120° to track to the station. If the needle stayed at 030° after the turn, the aircraft would be homing, not tracking.
Traps
Confusing QDM with QDR; homing instead of tracking; reverse sensing on a back course; false glide-slope captures above the true path; and treating DME slant range as ground distance close to the station.