Avionics News March 2023 - 49

N or
FIGURE 2:
The reference signal is synchronized with the directional signal rotation.
When compared to the reference, the phase of the variable signal will be
different, depending on where the aircraft is in relation to the station.
REFERENCE HZ
S or
E or
W or
N
There is a fact about GPS that has led the Federal Aviation
Administration to maintain something called a minimum
operational network, or MON. GPS is under the control
of the Department of Defense. For reasons of national
security, the DOD may stop GPS service entirely or to
certain regions. If the system is shut down, pilots flying
aircraft using GPS as a sole means of navigation will be
left without a navigation system. The FAA has decided
to maintain a minimum operational network consisting
of VOR stations so pilots can navigate across the country
regardless of the status of the GPS.
The name of the VOR system explains a lot about how it
works. Prior to VOR, pilots could use a navigation system
called a four-course range. Think of these four courses as
lines over the ground. The four-course range was limited,
obviously, by having only four courses. In addition,
the pilot had to listen to a tone, which must have been
maddening. Finally, like the ADF, this system operated in
the medium frequency band, which is extremely noisy.
Engineers developed VOR as an improvement to the fourcourse
range system. First, they moved the frequencies into
the VHF range. Specifically, frequencies from 108 to 117.95
megahertz. This band of frequencies is divided into 200
channels, each separated by 50 kHz. Forty of those channels
(the channels with odd numbers in the tenths column, below
112 MHz) are reserved for the instrument landing system,
which leaves 160 available for cross-country navigation.
The other feature of the VOR is the ability of the pilot to
select any course from 0 to 359.999 degrees. Thus, this
system is a range system, which is omnidirectional. Put all
this together, and you get a VHF omnirange.
An advantage of any range station over ADF is the fact
the indications are relative to an imaginary line on the
ground. As a result, pilots do not need to do calculations
as they fly. Instead, they can keep the indication centered,
and the offset between heading and course required for
crosswind compensation will take care of itself.
The VOR system determines its position in relation to
the VOR station by comparing the phase of two 30-hertz
signals. One 30Hz waveform, called the reference, is
synchronized with a rotating directional radio frequency
signal broadcast from the VOR station. The other 30Hz
waveform is created in the aircraft, as the rotating signal
changes signal strength. As shown in Figure 1, the signal
rotating toward and away from the aircraft will create a
virtual amplitude modulated signal envelope, which the
VOR radio receiver demodulates into a 30Hz waveform
called the variable signal.
VARIABLE HZ
AIRCRAFT NORTH
W
E
S
N
VARIABLE HZ
AIRCRAFT EAST
W
E
S
N
VARIABLE HZ
AIRCRAFT SOUTH
W
E
S
N
VARIABLE HZ
AIRCRAFT WEST
W
E
S
At this point, some readers may already have discovered
a problem with this system as described so far. Somehow,
the 30Hz reference needs to be transmitted to the aircraft
without getting confused with the variable 30Hz signal.
To keep the two signals separate, the system needs to use
something called a subcarrier.
Our main carrier is the radio signal on some channel between
108 and 117.95 MHz. The subcarrier is a much lower frequency.
In this case, 9.96 kHz, or referred to in our industry as 9960
reference. The VOR amplitude modulates the radio carrier
with the 9960 subcarrier. From the aircraft's point of view, the
received signal envelope got more complicated, because it is
varying at a 30Hz rate due to the rotation of the radio field, and
it's varying at a 9960Hz rate due to the subcarrier.
To ensure the two 30Hz waveforms don't get mixed
together, the 30Hz reference is used to frequency modulate
the 9960. In the aircraft, a receiver will pick up the radio
signal and provide complex-looking VOR composite
avionics news * march 2023 49
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Avionics News March 2023

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