<s>
Very	B-Algorithm
High	I-Algorithm
Frequency	I-Algorithm
Omnirange	O
Station	O
(	O
VOR	O
)	O
is	O
a	O
type	O
of	O
short-range	O
radio	O
navigation	O
system	O
for	O
aircraft	O
,	O
enabling	O
aircraft	O
with	O
a	O
receiving	O
unit	O
to	O
determine	O
its	O
position	O
and	O
stay	O
on	O
course	O
by	O
receiving	O
radio	O
signals	O
transmitted	O
by	O
a	O
network	O
of	O
fixed	O
ground	O
radio	O
beacons	O
.	O
</s>
<s>
It	O
uses	O
frequencies	O
in	O
the	O
very	B-Algorithm
high	I-Algorithm
frequency	I-Algorithm
(	O
VHF	B-Algorithm
)	O
band	O
from	O
108.00	O
to	O
117.95MHz	O
.	O
</s>
<s>
A	O
VOR	O
ground	O
station	O
uses	O
a	O
specialized	O
antenna	O
system	O
to	O
transmit	O
both	O
an	O
amplitude	B-Device
modulated	I-Device
and	O
a	O
frequency	B-Application
modulated	I-Application
signal	O
.	O
</s>
<s>
The	O
intersection	O
of	O
radials	O
from	O
two	O
different	O
VOR	O
stations	O
can	O
be	O
used	O
to	O
fix	O
the	O
position	O
of	O
the	O
aircraft	O
,	O
as	O
in	O
earlier	O
radio	B-Device
direction	I-Device
finding	I-Device
(	O
RDF	O
)	O
systems	O
.	O
</s>
<s>
Each	O
station	O
broadcasts	O
a	O
VHF	B-Algorithm
radio	O
composite	O
signal	O
including	O
the	O
mentioned	O
navigation	O
and	O
reference	O
signal	O
,	O
station	O
's	O
identifier	O
and	O
voice	O
,	O
if	O
so	O
equipped	O
.	O
</s>
<s>
A	O
VORTAC	B-Device
is	O
a	O
radio-based	O
navigational	O
aid	O
for	O
aircraft	O
pilots	O
consisting	O
of	O
a	O
co-located	O
VHF	B-Device
omnidirectional	I-Device
range	I-Device
and	O
a	O
tactical	O
air	O
navigation	O
system	O
(	O
TACAN	O
)	O
beacon	O
.	O
</s>
<s>
Most	O
VOR	O
installations	O
in	O
the	O
United	O
States	O
are	O
VORTACs	B-Device
.	O
</s>
<s>
VHF	B-Algorithm
radio	O
is	O
less	O
vulnerable	O
to	O
diffraction	O
(	O
course	O
bending	O
)	O
around	O
terrain	O
features	O
and	O
coastlines	O
.	O
</s>
<s>
VOR	O
stations	O
,	O
being	O
VHF	B-Algorithm
,	O
operate	O
on	O
"	O
line	O
of	O
sight	O
"	O
.	O
</s>
<s>
VORs	O
are	O
assigned	O
radio	O
channels	O
between	O
108.0	O
MHz	O
and	O
117.95MHz	O
(	O
with	O
50kHz	O
spacing	O
)	O
;	O
this	O
is	O
in	O
the	O
very	B-Algorithm
high	I-Algorithm
frequency	I-Algorithm
(	O
VHF	B-Algorithm
)	O
range	O
.	O
</s>
<s>
One	O
of	O
them	O
is	O
amplitude	B-Device
modulated	I-Device
,	O
and	O
one	O
is	O
frequency	B-Application
modulated	I-Application
.	O
</s>
<s>
On	O
conventional	O
VORs	O
(	O
CVOR	O
)	O
,	O
the	O
30Hz	O
reference	O
signal	O
is	O
frequency	B-Application
modulated	I-Application
(	O
FM	B-Application
)	O
on	O
a	O
9,960	O
Hz	O
subcarrier	O
.	O
</s>
<s>
On	O
these	O
VORs	O
,	O
the	O
amplitude	B-Device
modulation	I-Device
is	O
achieved	O
by	O
rotating	O
a	O
slightly	O
directional	O
antenna	O
exactly	O
in	O
phase	O
with	O
the	O
reference	O
signal	O
at	O
30	O
revolutions	O
per	O
second	O
.	O
</s>
<s>
Modern	O
installations	O
are	O
Doppler	B-Device
VORs	I-Device
(	O
DVOR	O
)	O
,	O
which	O
use	O
a	O
circular	O
array	O
of	O
typically	O
48	O
omni-directional	O
antennas	O
and	O
no	O
moving	O
parts	O
.	O
</s>
<s>
The	O
active	O
antenna	O
is	O
moved	O
around	O
the	O
circular	O
array	O
electronically	O
to	O
create	O
a	O
doppler	O
effect	O
,	O
resulting	O
in	O
frequency	B-Application
modulation	I-Application
.	O
</s>
<s>
The	O
amplitude	B-Device
modulation	I-Device
is	O
created	O
by	O
making	O
the	O
transmission	O
power	O
of	O
antennas	O
at	O
e.g.	O
</s>
<s>
The	O
role	O
of	O
amplitude	O
and	O
frequency	B-Application
modulation	I-Application
is	O
thus	O
swapped	O
in	O
this	O
type	O
of	O
VOR	O
.	O
</s>
<s>
Decoding	O
in	O
the	O
receiving	O
aircraft	O
happens	O
in	O
the	O
same	O
way	O
for	O
both	O
types	O
of	O
VORs	O
:	O
the	O
AM	O
and	O
FM	B-Application
30Hz	O
components	O
are	O
detected	B-Algorithm
and	O
then	O
compared	O
to	O
determine	O
the	O
phase	O
angle	O
between	O
them	O
.	O
</s>
<s>
The	O
VOR	O
signal	O
also	O
contains	O
a	O
modulated	O
continuous	O
wave	O
(	O
MCW	O
)	O
7wpm	O
Morse	O
code	O
station	O
identifier	O
,	O
and	O
usually	O
contains	O
an	O
amplitude	B-Device
modulated	I-Device
(	O
AM	O
)	O
voice	O
channel	O
.	O
</s>
<s>
A	O
co-located	O
VOR	O
and	O
TACAN	O
beacon	O
is	O
called	O
a	O
VORTAC	B-Device
.	O
</s>
<s>
VORTACs	B-Device
and	O
VOR-DMEs	O
use	O
a	O
standardized	O
scheme	O
of	O
VOR	O
frequency	O
to	O
TACAN/DME	O
channel	O
pairing	O
so	O
that	O
a	O
specific	O
VOR	O
frequency	O
is	O
always	O
paired	O
with	O
a	O
specific	O
co-located	O
TACAN	O
or	O
DME	O
channel	O
.	O
</s>
<s>
On	O
civilian	O
equipment	O
,	O
the	O
VHF	B-Algorithm
frequency	O
is	O
tuned	O
and	O
the	O
appropriate	O
TACAN/DME	O
channel	O
is	O
automatically	O
selected	O
.	O
</s>
<s>
The	O
composite	O
signal	O
is	O
detected	B-Algorithm
by	O
the	O
receiver	O
.	O
</s>
<s>
This	O
causes	O
the	O
effective	O
sideband	O
signal	O
to	O
be	O
amplitude	B-Device
modulated	I-Device
at	O
60Hz	O
as	O
far	O
as	O
the	O
aircraft	O
's	O
receiver	O
is	O
concerned	O
.	O
</s>
<s>
The	O
phase	O
of	O
this	O
modulation	O
can	O
affect	O
the	O
detected	B-Algorithm
phase	O
of	O
the	O
sub-carrier	O
.	O
</s>
<s>
This	O
distortion	O
is	O
itself	O
amplitude	B-Device
modulated	I-Device
with	O
a	O
60Hz	O
amplitude	B-Device
modulation	I-Device
(	O
also	O
some	O
30Hz	O
as	O
well	O
)	O
.	O
</s>
<s>
Note	O
that	O
ICAO	O
Annex	O
10	O
limits	O
the	O
worst	O
case	O
amplitude	B-Device
modulation	I-Device
of	O
the	O
sub-carrier	O
to	O
40%	O
.	O
</s>
<s>
A	O
Doppler	B-Device
VOR	I-Device
beacon	O
will	O
typically	O
change	O
over	O
or	O
shut	O
down	O
when	O
the	O
bearing	O
error	O
exceeds	O
1.0	O
°	O
.	O
</s>
<s>
For	O
instance	O
,	O
in	O
Australia	O
,	O
a	O
Primary	O
Alarm	O
limit	O
may	O
be	O
set	O
as	O
low	O
as	O
±	O
0.5	O
°	O
on	O
some	O
Doppler	B-Device
VOR	I-Device
beacons	O
.	O
</s>
<s>
Note	O
that	O
the	O
signals	O
received	O
by	O
these	O
antennas	O
,	O
in	O
a	O
Doppler	B-Device
VOR	I-Device
beacon	O
,	O
are	O
different	O
from	O
the	O
signals	O
received	O
by	O
an	O
aircraft	O
.	O
</s>
<s>
The	O
bearing	O
accuracy	O
specification	O
for	O
all	O
VOR	O
beacons	O
is	O
defined	O
in	O
the	O
International	B-General_Concept
Civil	I-General_Concept
Aviation	I-General_Concept
Organization	I-General_Concept
Convention	O
on	O
International	O
Civil	O
Aviation	O
Annex	O
10	O
,	O
Volume	O
1	O
.	O
</s>
<s>
A	O
Doppler	B-Device
VOR	I-Device
(	O
DVOR	O
)	O
is	O
required	O
to	O
be	O
±1°	O
.	O
</s>
<s>
Note	O
that	O
Doppler	B-Device
VOR	I-Device
beacons	O
utilise	O
an	O
elevated	O
groundplane	O
that	O
is	O
used	O
to	O
elevate	O
the	O
effective	O
antenna	O
pattern	O
.	O
</s>
<s>
Doppler	B-Device
VOR	I-Device
beacons	O
are	O
inherently	O
more	O
accurate	O
than	O
conventional	O
VORs	O
because	O
they	O
are	O
less	O
affected	O
by	O
reflections	O
from	O
hills	O
and	O
buildings	O
.	O
</s>
<s>
The	O
variable	O
signal	O
in	O
a	O
DVOR	O
is	O
the	O
30Hz	O
FM	B-Application
signal	O
;	O
in	O
a	O
CVOR	O
it	O
is	O
the	O
30Hz	O
AM	O
signal	O
.	O
</s>
<s>
In	O
a	O
DVOR	O
beacon	O
,	O
the	O
variable	O
signal	O
,	O
if	O
reflected	O
,	O
will	O
seem	O
to	O
be	O
two	O
FM	B-Application
signals	O
of	O
unequal	O
strengths	O
and	O
different	O
phases	O
.	O
</s>
