FINAL REPORT - Stakeholders - Ofcom

VOR beacons transmit in the 108 – 117.975 MHz band using two antennas (one omnidirectional
and one loop) to generate a polar diagram called a Limacon. In conventional
beacons, the loop aerial is then mechanically rotated at 30 Hz so that the polar diagram
rotates. An aircraft, receives this signal which varies in amplitude at 30 Hz and represents
the aircraft’s bearing from the beacon; however, to decode the bearing, a reference is
required and this is transmitted by the beacon as a fixed 30 Hz Frequency Modulated
signal (it actually uses a sub-carrier at 480 Hz from the primary frequency to avoid
interference with the AM modulations received at the aircraft). As shown below, the
reference signal is calibrated such that the two 30 Hz signals (one AM and one FM) are
identical for an aircraft on a bearing of magnetic North from the beacon [see Figure 3-17
– top]. Elsewhere, the measured phase difference (θ) between the AM and FM signals
represent the magnetic bearing of the aircraft from the beacon [see Figure 3-17 –
bottom].
VOR Beacon
Limacon A
North (M)
θ
Θ = Phase Difference measured
at the aircraft
Figure 3-17 – Operation of a VOR System
Even if there is some minor misalignment at the beacon, aircraft on the same bearing
from the beacon will receive the same information; this was not necessarily the case
using the DF/NDB bearing system, which usually suffered from different direction finding
errors even with two receivers in the same aircraft. Other advantages over NDB bearing
systems are that the VOR receiver is relatively simple needing only to demodulate the FM
signal and compare its phase with that of the AM signal; no moving parts are involved and
B
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