Radio Navigation Aids: VOR, DME, NDB/ADF, ILS, GNSS
What each aid gives you
- VOR: 108.0 to 117.95 MHz, line of sight. It provides the magnetic radial FROM the station; the CDI and OBS show course deviation and TO/FROM sense. It gives no range and no wind.
- DME: UHF 960 to 1215 MHz, paired with a VOR or ILS. It measures slant range and can display groundspeed and time to station.
- NDB/ADF: LF/MF 190 to 535 kHz. The ADF shows relative bearing; magnetic bearing = relative bearing + magnetic heading. Susceptible to coastal refraction, night effect, and thunderstorm static.
- ILS: localizer in 108.10 to 111.95 MHz (VHF) gives lateral guidance; glideslope 329.15 to 335.00 MHz (UHF) gives a nominal 3 degree path, about 300 ft per nm. Fly toward the needle; follow the flight director rather than chasing.
- GNSS: gives position, track and groundspeed. Integrity depends on RAIM or SBAS; RAIM prediction is required for certain approaches.
Worked example
At FL300, an aircraft is 20 nm ground range from a DME. Height is about 5 nm. Slant range = square root of (20 squared + 5 squared) = 20.6 nm, so the DME over-reads by 0.6 nm, about 3 per cent. The error is largest close in and at high altitude, and shrinks to zero overhead; this is why DME groundspeed and time readouts are unreliable near the station.
Traps
Reading a VOR radial as a bearing TO the station; reverse sensing when the OBS is set to the reciprocal; treating DME as ground range; forgetting that ILS glideslope is UHF and localizer is VHF; and assuming GNSS integrity without RAIM or SBAS coverage.