Radio Navigation: VOR, DME, ILS, RNAV and surveillance
Ground-based aids
VOR gives a radial from the station; the CDI shows deviation from the selected radial, set with the OBS. VOR is line-of-sight with a cone of confusion overhead. DME gives slant range, which exceeds ground distance - negligible at long range, large close-in at high altitude. NDB/ADF gives a relative bearing; the needle points at the station and the trap is confusing relative bearing with QDM/QDR.
ILS
Localiser on VHF (108–112 MHz) gives lateral guidance within a narrow sector and limited range; glideslope is UHF (329–335 MHz) at a nominal 3°. Full-scale deflection is about 2.5° for the localiser and 0.7° for the glideslope, so the glideslope is the more sensitive. Markers give range fixes. False glideslopes exist above the true path. Approach minima depend on facility and aircraft category: CAT I 200 ft, CAT II 100 ft, CAT III no decision height.
Area navigation and surveillance
RNAV and RNP (RNP adds on-board performance monitoring and alerting); GNSS with RAIM and integrity issues. Primary radar versus secondary radar; SSR modes A/C/S; ACAS/TCAS with traffic advisories and resolution advisories; EGPWS/TAWS.
Worked example
On a 3° glideslope, height above the threshold approximately equals range in nautical miles × 300 ft (close to range × 6 076 × tan 3°). At 4 nm DME from the threshold, expect about 1 200 ft; at 1 nm, about 300 ft. If high, do not chase the glideslope - level off or go around.
Traps
Mixing radials and bearings; treating DME as ground distance; assuming ADF is free of errors (thunderstorm and coastal refraction affect it).