13.4(a) Navigation systems (ATA 34) — VOR, ADF, ILS, MLS, DME, RNAV, FMS, GNSS

Scope

Level 3 (B2, B2L COM/NAV rating). The EASA list: VOR, ADF, ILS, MLS, flight director systems, DME, RNAV, FMS, GNSS (GPS, GBAS, SBAS such as EGNOS and WAAS), data link. INS is covered here for completeness (the syllabus lists it in 13.4(b)).

VOR — VHF omnidirectional range

• 108.00–117.95 MHz, horizontally polarised; identification in Morse.
• Principle: the station radiates a reference 30 Hz signal (same phase in all directions) and a variable 30 Hz signal whose phase depends on the bearing from the station. The aircraft receiver measures the phase difference = magnetic bearing from the station (the radial).
  • Conventional VOR: reference 30 Hz is FM on a 9,960 Hz sub-carrier, variable 30 Hz is AM from a rotating pattern.
  • Doppler VOR (DVOR): roles reversed using a ring of antennas — more accurate, less site error.
• Displays: CDI/HSI (deviation from the selected course, TO/FROM flag), RMI (needle pointing to the station).
• Limitations: line of sight; cone of confusion overhead the station; site errors.
• Test: VOT (test transmitter) or ramp test set; accuracy check by radials.

ADF — automatic direction finder

• Receives NDBs (LF/MF, NDBs typically 190–535 kHz) and AM broadcast stations.
• Loop antenna (directional, figure-of-eight pattern with sharp nulls) combined with a sense antenna (omnidirectional) to resolve the 180° ambiguity → cardioid pattern. Modern units use fixed crossed loops with a goniometer/electronic processing.
• Displays: relative bearing on an RBI or magnetic bearing on an RMI (heading input added).
• Errors: night effect (sky-wave contamination at dusk/dawn), coastal refraction, quadrantal error (airframe re-radiation, corrected at installation), mountain effect, thunderstorm effect (needle points to the storm).