VOR

(Very High Frequency Omni directional Range)

The VOR system operates in the 108-118 MHz band with channels spaced at 50 kHz. This band
is shared with ILS Localizer the VOR being allocated to 160 of the 200 available channels. Of
these 160 channels 120 are allocated to VOR station intended for en route navigation while the
other forty are for terminal VOR station (TVOR).The output power of the en route station will
be about 200W providing a service up to 200 nautical miles. The TVOR will have the output
power of 50W providing a service up to about 25 nautical miles.

The crew of an appropriately equipped aircraft can tune into a VOR station within range and
read the bearing to the station and the relative bearing of the station. To obtain a position fix
from VOR one needs bearing to two separate stations. When used in this way VOR can be
considered a theta- theta system. If a VOR station is co- located with a DME station an aircraft
can obtain a fix using the pair as a rho- theta system.

BASIC PRINCIPLE:

In reality a VOR station radiates v.h.f energy modulated with a reference phase signal –the
Omni directional light- and a variable phase signal – the rotating beam. The bearing of the
aircraft depends on the phase difference between reference and variable phase. The radiation
from a conventional VOR (CVOR) station is a horizontally polarized v.h.f wave modulated as
follows:

1. 3o Hz a.m: the variable phase signal.

2. 9960 Hz a.m: this is a sub carrier frequency, modulated at 30 Hz with a deviation of 480
Hz.
3. 1020 Hz a.m: identification signal keyed to provide Morse code identification at least
three times each 30s.
4. Voice a.m: the VOR system can be used as a ground to air communication channel as
long as it does not interfere with the basic navigational function.

The 30 Hz variable phase is space modulated in that the necessary amplitude variation in
the received signal at the aircraft is achieved by radiating a cardioid pattern at 1800rpm.
The frequency modulated 9960 Hz sub- carrier amplitude modulates the r.f. at source
before radiation. It is arranged that the aircraft due north of the beacon will receive variable
and reference signal in phase, for aircraft at X degrees magnetic bearing from the station
the variable phase will lag the reference phase by X degrees.
Automatic VOR

The 30 Hz reference signal is phase compared with the variable signal, the difference in the
phase giving the bearing from the station. The actual reading presented to the pilot is the
bearing to the station rather than from.

An RMI is used to display the difference in the relative and magnetic bearing to the station
and the aircraft heading. In this instrument the card is driven by the compass, as normal, so
that the card reading at the lubber line is the aircraft heading. At the same time a pointer is
driven to a position determined by the difference between the bearing to the station and
the heading. A differential synchro or resolver is used to give the required angular
difference.

Manual VOR

The pilot selects the particular radial on which he wants to position his aircraft. The actual
radial on which the aircraft is flying is compared with the desired radial. If the two are
different the appropriate fly left or fly right signal are derived and presented to the pilot.
Further when the aircraft is on the course, i.e. the steering command is nulled, the aircraft
may be heading either towards or away from the station on the selected radial. A To /from
indication removes the ambiguity.

If the reference phase (R) is phase shifted by the selected course (C) and then compared
with the variable phase, a fly right indication will be given If R+C lags V while if R+C leads V
the command will be fly left.

Doppler VOR (DVOR)

The use of the CVOR leads to considerable site errors where the station is installed in the
vicinity of the obstruction or where aircraft are required to fly over mountainous terrain
while using the station. DVOR is relatively insensitive to siting effects which would render
CVOR unusable. In the DVOR the reference signal is 30 Hz a.m. while the variable signal is 30
Hz F.M. on a 9960 Hz sub- carrier.

In a double sideband DVOR the carrier, fc, with 30 Hz a.m. is radiated from an Omni
directional antenna. Two unmodulated r.f. sideband signals, one 9960 Hz above fc, the
other 9960 Hz below fc, are radiated from antennas diametrically opposite in a ring of about
fifty antennas.
 Installation of VOR:

Since VOR & ILS localizer share same band of frequencies therefore they share the same
receiver. The antenna may serve ILS as well as VOR but some aircraft may have separate
antennas. The VOR antenna employs horizontal polarization with an Omni directional
radiation pattern. A horizontal dipole is often used with the dipole element forming a V
shape to give a Omni directional pattern.

The VOR receiver contains a conventional superhet, a filter and a converter to provide the
required outputs which are:

1. Audio to AIS
2. Bearing information to two RMIs
3. Deviation from selected radials
4. TO/FROM signals
5. Flag or warning signal

The flag signal is of vital importance sine it gives warning of unreliable data from the VOR
receiver.

Simplified block diagram operation:

Received signals are selected, amplified and detected by a conventional single or double
superhetrodyne receiver. The reference phase channel (CVOR) consist of a 9960 Hz filter, a
discriminator to detect the 30 Hz F.M. the 30 Hz reference signal then undergoes various phase
shifts. The phase shifted signal (R) is now compared with variable phase signal. If they are in
phase or 180® out of phase there is no lateral movement of the deviation bar. In the absence of
either or both of the signals the flag will be in view.

TO/FROM information derived by comparing

the variable phase with the reference phase shifted by the OBS setting plus 180®. It follows that
if the if the reference phase has been shifted by 90® before feeding the deviation phase
comparator we only require a further 90® phase shift before feeding the To/From comparator
rather than 180® phase shift. For automatic VOR operation the reference channel is phase
shifted & compared with the variable phase. If the two inputs are in phase quadrature there is
no drive to the motor, otherwise the motor will turn, changing the amount by which the
reference phase is shifted until the phase quadrature is achieved.
 Points to Ponder

It is essential to remember the following points concerning the VOR and its operation:

􀁺A radial is a magnetic bearing from the VOR beacon

􀁺The Deviation Indicator can be centered on either of two bearings by the OBS.
These bearings will be 180° apart with the ambiguity automatically resolved
by the TO/FROM indicator

􀁺The VOR system, in practice, should be considered to be accurate only to the

order of ± 5°

􀁺As the VOR propagation is ‘line of sight’, the distance at which a VOR beacon
can be received increases with aircraft elevation above the beacon

􀁺The VOR beacon must be identified aurally before information derived from
the beacon is used

􀁺The Omni Bearing Indicator always indicates magnetic bearings TO or FROM

the VOR beacon—never relative bearings

􀁺Heading of the aircraft at any instant does not affect VOR bearings. The
bearing obtained depends on the aircraft location with relation to the VOR
beacon

􀁺Bearing selection by the OBS is by 1 degree intervals. When a bearing is

selected, full travel of the Deviation Indicator from one side to the other
represents 20° of azimuth that is 10° either side of the selected bearing.