Operating instructions

AVIONICS SYSTEM
B-5
VOR
In the VOR mode the Test Set simulates the transmissions from VHF OMNIDIRECTIONAL
RADIORANGE beacons.
These beacons mark the ‘air corridors’ used by aircraft and are situated at strategic locations
throughout the world airways network.
The transmission from VOR beacons, provide information to determine the direction of the beacon
from the receiver installed in the aircraft. It is a complex signal radiated partially from a
horizontally polarized antenna rotating at 30 revolutions per second with a cardioid polar response
pattern, and partially from a fixed omnidirectional antenna.
The rotating antenna transmits an un-modulated carrier that will appear to the receiver to be
amplitude modulated at 30 Hz.
The signal from the omnidirectional antenna is at the same carrier frequency but is amplitude
modulated with a 9960 Hz sub-carrier. This is frequency modulated with a 30 Hz signal
synchronized with the rotating antenna, to be in phase only when received on a receiver located at
a compass position due north of the beacon. At any other location the phase difference between
the two 30 Hz signals is directly related to the receiver's position relative to the beacon.
The VOR signal can carry beacon identity and air traffic control information. Amplitude
modulation is used to apply this directly onto the carrier.
The simulated VOR signal
The signal produced by the Service Monitor is an RF carrier within the range 108.0 MHz to
118 MHz, with a 9.960 kHz AM sub-carrier. The sub-carrier is frequency modulated with a 30 Hz
reference tone. In addition to the sub-carrier, the RF signal is amplitude modulated with a 30 Hz
signal which is phase variable with the 30 Hz reference tone through 360°.
The beacon identity/voice signal is simulated by applying a tone of approximately 1020 Hz to the
carrier signal using amplitude modulation.
The Service Monitor VOR display screen and menu gives a graphic and digital readout of the
parameters relating to the simulated signal. The soft key menu allows the parameters to be
adjusted either directly or by using the data input keys or the variable control.
B5153
Fig. B-3 VOR display screen and menu