VHF omni directional radio range (VOR) is a type of short-range radio
navigation system for aircraft, enabling aircraft with a receiving
unit to determine their position and stay on course by receiving radio
signals transmitted by a network of fixed ground radio beacons. It
uses frequencies in the very high frequency (VHF) band from 108.00 to
117.95 MHz. Developed in the
United States beginning in 1937 and
deployed by 1946, VOR is the standard air navigational system in the
world, used by both commercial and general aviation. By 2000
there were about 3,000 VOR stations around the world including 1,033
in the US, reduced to 967 by 2013 with more stations being
decommissioned with the widespread adoption of GPS.
A VOR ground station sends out an omnidirectional master signal, and a
highly directional second signal is propagated by a phased antenna
array and rotates clockwise in space 30 times a second. This signal is
timed so that its phase (compared to the master) varies as the
secondary signal rotates, and this phase difference is the same as the
angular direction of the 'spinning' signal, (so that when the signal
is being sent 90 degrees clockwise from north, the signal is 90
degrees out of phase with the master). By comparing the phase of the
secondary signal with the master, the angle (bearing) to the aircraft
from the station can be determined. This line of position is called
the "radial" from the VOR. The intersection of radials from two
different VOR stations can be used to fix the position of the
aircraft, as in earlier radio direction finding (RDF) systems.
VOR stations are fairly short range: the signals are line of sight
between transmitter and receiver and are useful for up to 200 miles.
Each station broadcasts a
VHF radio composite signal including the
navigation signal, station's identifier and voice, if so equipped. The
navigation signal allows the airborne receiving equipment to determine
a bearing from the station to the aircraft (direction from the VOR
station in relation to Magnetic North). The station's identifier is
typically a three-letter string in Morse code. The voice signal, if
used, is usually the station name, in-flight recorded advisories, or
live flight service broadcasts. At some locations, this voice signal
is a continuous recorded broadcast of Hazardous Inflight Weather
Advisory Service or HIWAS.
1.4 Service volumes
1.5 VORs, airways and the en route structure
2 Technical specification
2.5 Accuracy and reliability
3 Using a VOR
3.2 Intercepting VOR radials
4 See also
6 External links
Developed from earlier Visual Aural
Radio Range (VAR) systems, the VOR
was designed to provide 360 courses to and from the station,
selectable by the pilot. Early vacuum tube transmitters with
mechanically-rotated antennas were widely installed in the 1950s, and
began to be replaced with fully solid-state units in the early 1960s.
They became the major radio navigation system in the 1960s, when they
took over from the older radio beacon and four-course (low/medium
frequency range) system. Some of the older range stations survived,
with the four-course directional features removed, as non-directional
low or medium frequency radiobeacons (NDBs).
A worldwide land-based network of "air highways", known in the US as
Victor airways (below 18,000 ft (5,500 m)) and "jetways" (at
and above 18,000 feet), was set up linking VORs. An aircraft can
follow a specific path from station to station by tuning into the
successive stations on the VOR receiver, and then either following the
desired course on a
Radio Magnetic Indicator, or setting it on a
course deviation indicator (CDI) or a horizontal situation indicator
(HSI, a more sophisticated version of the VOR indicator) and keeping a
course pointer centred on the display.
As of 2005, due to advances in technology, many airports are replacing
VOR and NDB approaches with
RNAV (GPS) approach procedures; however,
receiver and data update costs are still significant enough that
many small general aviation aircraft are not equipped with a GPS
certified for primary navigation or approaches.
VOR signals provide considerably greater accuracy and reliability than
NDBs due to a combination of factors. Most significant is that VOR
provides a bearing from the station to the aircraft which does not
vary with wind or orientation of the aircraft.
VHF radio is less
vulnerable to diffraction (course bending) around terrain features and
coastlines. Phase encoding suffers less interference from
VOR signals offer a predictable accuracy of 90 m (300 ft), 2
sigma at 2 NM from a pair of VOR beacons; as compared to the
accuracy of unaugmented
Global Positioning System
Global Positioning System (GPS) which is less
than 13 meters, 95%.
VOR stations rely on "line of sight" because they operate in the VHF
band—if the transmitting antenna cannot be seen on a perfectly clear
day from the receiving antenna, a useful signal cannot be received.
This limits VOR (and DME) range to the horizon—or closer if
mountains intervene. Although the modern solid state transmitting
equipment requires much less maintenance than the older units, an
extensive network of stations, needed to provide reasonable coverage
along main air routes, is a significant cost in operating current
VORs are assigned radio channels between 108.0
MHz and 117.95 MHz
(with 50 kHz spacing); this is in the very high frequency (VHF)
range. The first 4
MHz is shared with the instrument landing system
(ILS) band. To leave channels for ILS, in the range 108.0 to
111.95 MHz, the 100 kHz digit is always even, so 108.00,
108.05, 108.20, 108.25, and so on are VOR frequencies but 108.10,
108.15, 108.30, 108.35 and so on, are reserved for ILS in the US.
The VOR encodes azimuth (direction from the station) as the phase
relationship between a reference signal and a variable signal. The
omnidirectional signal contains a modulated continuous wave (MCW)
Morse code station identifier, and usually contains an
amplitude modulated (AM) voice channel. The conventional 30 Hz
reference signal is frequency modulated (FM) on a 9,960 Hz
subcarrier. The variable amplitude modulated (AM) signal is
conventionally derived from the lighthouse-like rotation of a
directional antenna array 30 times per second. Although older antennas
were mechanically rotated, current installations scan electronically
to achieve an equivalent result with no moving parts. This is achieved
by a circular array of typically 60 directional antennas, the signal
to each one being amplitude modulated by the 30 Hz reference
signal delayed in phase to match the azimuthal position of each
individual antenna. When the composite signal is received in the
aircraft, the AM and FM 30 Hz components are detected and then
compared to determine the phase angle between them.
This information is then fed over an analog or digital interface to
one of four common types of indicators:
A typical light-airplane VOR indicator, sometimes called an
"omni-bearing indicator" or OBI is shown in the illustration at the
top of this entry. It consists of a knob to rotate an "Omni Bearing
Selector" (OBS), the OBS scale around the outside of the instrument,
and a vertical course deviation indicator or (CDI) pointer. The OBS is
used to set the desired course, and the CDI is centred when the
aircraft is on the selected course, or gives left/right steering
commands to return to the course. An "ambiguity" (TO-FROM) indicator
shows whether following the selected course would take the aircraft
to, or away from the station. The indicator may also include a
glideslope pointer for use when receiving full ILS signals.
A radio magnetic indicator (RMI) features a course arrow superimposed
on a rotating card that shows the aircraft's current heading at the
top of the dial. The "tail" of the course arrow points at the current
radial from the station and the "head" of the arrow points at the
reciprocal (180° different) course to the station. An RMI may present
information from more than one VOR or ADF receiver simultaneously.
A horizontal situation indicator (HSI), developed subsequently to the
RMI, is considerably more expensive and complex than a standard VOR
indicator but combines heading information with the navigation display
in a much more user-friendly format, approximating a simplified moving
An area navigation (RNAV) system is an onboard computer with display
and may include an up-to-date navigation database. At least one
VOR/DME station is required for the computer to plot aircraft position
on a moving map or to display course deviation and distance relative
to a waypoint (virtual VOR station).
RNAV type systems have also been
made to use two VORs or two DMEs to define a waypoint; these are
typically referred to by other names such as "distance computing
equipment" for the dual-VOR type or "DME-DME" for the type using more
than one DME signal.
VORTAC TGO (TANGO) Germany
In many cases, VOR stations have collocated distance measuring
equipment (DME) or military Tactical Air Navigation (TACAN) — the
latter includes both the DME distance feature and a separate TACAN
azimuth feature that provides military pilots data similar to the
civilian VOR. A collocated VOR and
TACAN beacon is called a VORTAC. A
VOR collocated only with DME is called a VOR-DME. A VOR radial with a
DME distance allows a one-station position fix. Both VOR-DMEs and
TACANs share the same DME system.
VORTACs and VOR-DMEs use a standardized scheme of VOR frequency to
TACAN/DME channel pairing so that a specific VOR frequency is
always paired with a specific collocated
TACAN or DME channel. On
civilian equipment, the
VHF frequency is tuned and the appropriate
TACAN/DME channel is automatically selected.
While the operating principles are different, VORs share some
characteristics with the localizer portion of ILS and the same
antenna, receiving equipment and indicator is used in the cockpit for
both. When a VOR station is selected, the OBS is functional and allows
the pilot to select the desired radial to use for navigation. When a
localizer frequency is selected, the OBS is not functional and the
indicator is driven by a localizer converter, typically built into the
receiver or indicator.
A VOR station serves a volume of airspace called its Service Volume.
Some VORs have a relatively small geographic area protected from
interference by other stations on the same frequency—called
"terminal" or T-VORs. Other stations may have protection out to 130
nautical miles (NM) or more. It is popularly thought that there is a
standard difference in power output between T-VORs and other stations,
in fact the stations' power output is set to provide adequate signal
strength in the specific site's service volume.
In the United States, there are three standard service volumes (SSV):
terminal, low, and high (standard service volumes do not apply to
published instrument flight rules (IFR) routes).
US standard service volumes (from FAA AIM)
SSV class designator
From 1,000 feet above ground level (AGL) up to and including 12,000
feet AGL at radial distances out to 25 NM.
L (low altitude)
From 1,000 feet AGL up to and including 18,000 feet AGL at radial
distances out to 40 NM.
H (high altitude)
From 1,000 feet AGL up to and including 14,500 feet AGL at radial
distances out to 40 NM. From 14,500 AGL up to and including 18,000
feet at radial distances out to 100 NM. From 18,000 feet AGL up
to and including 45,000 feet AGL at radial distances out to 130 NM.
From 45,000 feet AGL up to and including 60,000 feet at radial
distances out to 100 NM.
VORs, airways and the en route structure
VORTAC (at 35.646999,-119.978996) shown on a sectional
aeronautical chart. Notice the light blue Victor Airways radiating
from the VORTAC. (click to enlarge)
VOR and the older NDB stations were traditionally used as
intersections along airways. A typical airway will hop from station to
station in straight lines. When flying in a commercial airliner, an
observer will notice that the aircraft flies in straight lines
occasionally broken by a turn to a new course. These turns are often
made as the aircraft passes over a VOR station or at an intersection
in the air defined by one or more VORs. Navigational reference points
can also be defined by the point at which two radials from different
VOR stations intersect, or by a VOR radial and a DME distance. This is
the basic form of
RNAV and allows navigation to points located away
from VOR stations. As
RNAV systems have become more common, in
particular those based on GPS, more and more airways have been defined
by such points, removing the need for some of the expensive
In many countries there are two separate systems of airway at lower
and higher levels: the lower Airways (known in the US as Victor
Airways) and Upper Air Routes (known in the US as Jet routes).
Most aircraft equipped for instrument flight (IFR) have at least two
VOR receivers. As well as providing a backup to the primary receiver,
the second receiver allows the pilot to easily follow a radial to or
from one VOR station while watching the second receiver to see when a
certain radial from another VOR station is crossed, allowing the
aircraft's exact position at that moment to be determined, and giving
the pilot the option of changing to the new radial if they wish.
VORTAC located on Upper Table Rock in Jackson County, Oregon
As of 2008[update], space-based
GNSS navigational systems such as the
Global Positioning System
Global Positioning System (GPS) are increasingly replacing VOR and
other ground-based systems.
GNSS systems have a lower transmitter cost per customer and provide
distance and altitude data. Future satellite navigation systems, such
as the European Union Galileo, and
GPS augmentation systems are
developing techniques to eventually equal or exceed VOR accuracy.
However, low VOR receiver cost, broad installed base and commonality
of receiver equipment with ILS are likely to extend VOR dominance in
aircraft until space receiver cost falls to a comparable level. As of
2008 in the United States, GPS-based approaches outnumbered VOR-based
approaches but VOR-equipped IFR aircraft outnumber GPS-equipped IFR
There is some concern that
GNSS navigation is subject to interference
or sabotage, leading in many countries to the retention of VOR
stations for use as a backup. The VOR signal has the advantage of
static mapping to local terrain.
The US FAA plans by 2020 to decommission roughly half of the
967 VOR stations in the US, retaining a "Minimum Operational
Network" to provide coverage to all aircraft more than 5,000 feet
above the ground. Most of the decommissioned stations will be east of
the Rocky Mountains, where there is more overlap in coverage between
them. On July 27, 2016 a final policy statement was
released specifying stations to be decommissioned by 2025. A total
of 74 stations are to be decommissioned in Phase 1 (2016-2020), and
234 more stations are scheduled to be taken out of service in Phase 2
In the UK, 19 VOR transmitters are to be kept operational until at
least 2020. Those at Cranfield and Dean Cross were decommissioned in
2014, with the remaining 25 to be assessed between 2015 and
2020. Similar efforts are underway in Australia, and
The VOR signal encodes a morse code identifier, optional voice, and a
pair of navigation tones. The radial azimuth is equal to the phase
angle between the lagging and leading navigation tone.
Standard modulation modes, indices, and frequencies
A3 modulation index
A1 subcarrier frequency
A3 modulation index
A0 tone frequency
A3 modulation index
A3 modulation index
F3 subcarrier frequency
F3 subcarrier deviation
A3 carrier frequency
speed of light
relative to magnetic north
time signal left
higher frequency revolving transmitter
lower frequency revolving transmitter
red(F3-) green(F3) blue(F3+)
black(A3-) grey(A3) white(A3+)
The conventional signal encodes the station identifier, i(t), optional
voice a(t), navigation reference signal in c(t), and the isotropic
(i.e. omnidirectional) component. The reference signal is encoded on
an F3 subcarrier (colour). The navigation variable signal is encoded
by mechanically or electrically rotating a directional, g(A,t),
antenna to produce A3 modulation (grey-scale). Receivers (paired
colour and grey-scale trace) in different directions from the station
paint a different alignment of F3 and A3 demodulated signal.
displaystyle begin array rcl e(A,t)&=&cos(2pi F_ c
t)(1+c(t)+g(A,t))\c(t)&=&M_ i cos(2pi F_ i
t)~i(t)\&+&M_ a ~a(t)\&+&M_ d cos(2pi int _ 0 ^ t (F_
s +F_ d cos(2pi F_ n t))dt)\g(A,t)&=&M_ n cos(2pi F_ n
red(F3-) green(F3) blue(F3+)
black(A3-) grey(A3) white(A3+)
USB transmitter offset is exaggerated
LSB transmitter is not shown
The doppler signal encodes the station identifier, i(t), optional
voice, a(t), navigation variable signal in c(t), and the isotropic
(i.e. omnidirectional) component. The navigation variable signal is A3
modulated (greyscale). The navigation reference signal is delayed, t+,
t−, by electrically revolving a pair of transmitters. The cyclic
doppler blue shift, and corresponding doppler red shift, as a
transmitter closes on and recedes from the receiver results in F3
modulation (colour). The pairing of transmitters offset equally high
and low of the isotropic carrier frequency produce the upper and lower
sidebands. Closing and receding equally on opposite sides of the same
circle around the isotropic transmitter produce F3 subcarrier
displaystyle begin array rcl t&=&t_ +
(A,t)-(R/C)sin(2pi F_ n t_ + (A,t)+A)\t&=&t_ -
(A,t)+(R/C)sin(2pi F_ n t_ - (A,t)+A)\e(A,t)&=&cos(2pi F_ c
t)(1+c(t))\&+&g(A,t)\c(t)&=&M_ i cos(2pi F_ i
t)~i(t)\&+&M_ a ~a(t)\&+&M_ n cos(2pi F_ n
t)\g(A,t)&=&(M_ d /2)cos(2pi (F_ c +F_ s )t_ +
(A,t))\&+&(M_ d /2)cos(2pi (F_ c -F_ s )t_ - (A,t))\end array
where the revolution radius R = Fd C / (2 π Fn Fc ) is 6.76 ± 0.3 m
The transmitter acceleration 4 π2 Fn2 R (24,000 g) makes
mechanical revolution impractical, and halves (gravitational redshift)
the frequency change ratio compared to transmitters in free-fall.
The mathematics to describe the operation of a DVOR is far more
complex than indicated above. The reference to "electronically
rotated" is a vast simplification. The primary complication relates to
a process that is called "blending".
Another complication is that the phase of the upper and lower sideband
signals have to be locked to each other. The composite signal is
detected by the receiver. The electronic operation of detection
effectively shifts the carrier down to 0 Hz, folding the signals
with frequencies below the Carrier, on top of the frequencies above
the carrier. Thus the upper and lower sidebands are summed. If there
is a phase shift between these two, then the combination will have a
relative amplitude of (1 + cos φ). If φ was 180°, then the
aircraft's receiver would not detect any sub-carrier (signal A3).
"Blending" describes the process by which a sideband signal is
switched from one antenna to the next. The switching is not
discontinuous. The amplitude of the next antenna rises as the
amplitude of the current antenna falls. When one antenna reaches its
peak amplitude, the next and previous antennas have zero amplitude.
By radiating from two antennas, the effective phase centre becomes a
point between the two. Thus the phase reference is swept continuously
around the ring – not stepped as would be the case with antenna to
antenna discontinuous switching.
In the electromechanical antenna switching systems employed before
solid state antenna switching systems were introduced, the blending
was a by-product of the way the motorized switches worked. These
switches brushed a coaxial cable past 50 (or 48) antenna feeds. As the
cable moved between two antenna feeds, it would couple signal into
But blending accentuates another complication of a DVOR.
Each antenna in a DVOR uses an omnidirectional antenna. These are
usually Alford Loop antennas (see Andrew Alford). Unfortunately, the
sideband antennas are very close together, so that approximately 55%
of the energy radiated is absorbed by the adjacent antennas. Half of
that is re-radiated, and half is sent back along the antenna feeds of
the adjacent antennas. The result is an antenna pattern that is no
longer omnidirectional. This causes the effective sideband signal to
be amplitude modulated at 60 Hz as far as the aircraft's receiver
is concerned. The phase of this modulation can affect the detected
phase of the sub-carrier. This effect is called "coupling".
Blending complicates this effect. It does this because when two
adjacent antennas radiate a signal, they create a composite antenna.
Imagine two antennas that are separated by their wavelength/3. In the
transverse direction the two signals will sum, but in the tangential
direction they will cancel. Thus as the signal "moves" from one
antenna to the next, the distortion in the antenna pattern will
increase and then decrease. The peak distortion occurs at the
midpoint. This creates a half-sinusoidal 1500 Hz amplitude
distortion in the case of a 50 antenna system, (1,440 Hz in a 48
antenna system). This distortion is itself amplitude modulated with a
60 Hz amplitude modulation (also some 30 Hz as well). This
distortion can add or subtract with the above-mentioned 60 Hz
distortion depending on the carrier phase. In fact one can add an
offset to the carrier phase (relative to the sideband phases) so that
the 60 Hz components tend to null one another. There is a
30 Hz component, though, which has some pernicious effects.
DVOR designs use all sorts of mechanisms to try to compensate these
effects. The methods chosen are major selling points for each
manufacturer, with each extolling the benefits of their technique over
Note that ICAO Annex 10 limits the worst case amplitude modulation of
the sub-carrier to 40%. A DVOR that didn't employ some technique(s) to
compensate for coupling and blending effects would not meet this
Accuracy and reliability
The predicted accuracy of the VOR system is ±1.4°. However, test
data indicate that 99.94% of the time a VOR system has less than
±0.35° of error. Internal monitoring of a VOR station will shut it
down, or change over to a standby system if the station error exceeds
some limit. A Doppler VOR beacon will typically change over or shut
down when the bearing error exceeds 1.0°. National air space
authorities may often set tighter limits. For instance, in Australia,
a Primary Alarm limit may be set as low as ±0.5° on some Doppler VOR
ARINC 711 - 10 January 30, 2002 states that receiver accuracy should
be within 0.4° with a statistical probability of 95% under various
conditions. Any receiver compliant with this standard can be expected
to perform within these tolerances.
All radio navigation beacons are required to monitor their own output.
Most have redundant systems, so that the failure of one system will
cause automatic change-over to one or more standby systems. The
monitoring and redundancy requirements in some instrument landing
systems (ILS) can be very strict.
The general philosophy followed is that no signal is preferable to a
VOR beacons monitor themselves by having one or more receiving
antennas located away from the beacon. The signals from these antennas
are processed to monitor many aspects of the signals. The signals
monitored are defined in various US and European standards. The
principal standard is European Organisation for Civil Aviation
Equipment (EuroCAE) Standard ED-52. The five main parameters monitored
are the bearing accuracy, the reference and variable signal modulation
indices, the signal level, and the presence of notches (caused by
individual antenna failures).
Note that the signals received by these antennas, in a Doppler VOR
beacon, are different from the signals received by an aircraft. This
is because the antennas are close to the transmitter and are affected
by proximity effects. For example, the free space path loss from
nearby sideband antennas will be 1.5 dB different (at
MHz and at a distance of 80 m) from the signals received
from the far side sideband antennas. For a distant aircraft there will
be no measurable difference. Similarly the peak rate of phase change
seen by a receiver is from the tangential antennas. For the aircraft
these tangential paths will be almost parallel, but this is not the
case for an antenna near the DVOR.
The bearing accuracy specification for all VOR beacons is defined in
International Civil Aviation Organisation
International Civil Aviation Organisation Convention on
International Civil Aviation Annex 10, Volume 1.
This document sets the worst case bearing accuracy performance on a
Conventional VOR (CVOR) to be ±4°. A Doppler VOR (DVOR) is required
to be ±1°.
All radio-navigation beacons are checked periodically to ensure that
they are performing to the appropriate International and National
standards. This includes VOR beacons, distance measuring equipment
(DME), instrument landing systems (ILS), and non-directional beacons
Their performance is measured by aircraft fitted with test equipment.
The VOR test procedure is to fly around the beacon in circles at
defined distances and altitudes, and also along several radials. These
aircraft measure signal strength, the modulation indices of the
reference and variable signals, and the bearing error. They will also
measure other selected parameters, as requested by local/national
airspace authorities. Note that the same procedure is used (often in
the same flight test) to check distance measuring equipment (DME).
In practice, bearing errors can often exceed those defined in Annex
10, in some directions. This is usually due to terrain effects,
buildings near the VOR, or, in the case of a DVOR, some counterpoise
effects. Note that Doppler VOR beacons utilise an elevated groundplane
that is used to elevate the effective antenna pattern. It creates a
strong lobe at an elevation angle of 30° which complements the 0°
lobe of the antennas themselves. This groundplane is called a
counterpoise. A counterpoise though, rarely works exactly as one would
hope. For example, the edge of the counterpoise can absorb and
re-radiate signals from the antennas, and it may tend to do this
differently in some directions than others.
National air space authorities will accept these bearing errors when
they occur along directions that are not the defined air traffic
routes. For example, in mountainous areas, the VOR may only provide
sufficient signal strength and bearing accuracy along one runway
Doppler VOR beacons are inherently more accurate than conventional
VORs because they are less affected by reflections from hills and
buildings. The variable signal in a DVOR is the 30 Hz FM signal;
in a CVOR it is the 30 Hz AM signal. If the AM signal from a CVOR
beacon bounces off a building or hill, the aircraft will see a phase
that appears to be at the phase centre of the main signal and the
reflected signal, and this phase centre will move as the beam rotates.
In a DVOR beacon, the variable signal, if reflected, will seem to be
two FM signals of unequal strengths and different phases. Twice per
30 Hz cycle, the instantaneous deviation of the two signals will
be the same, and the phase locked loop will get (briefly) confused. As
the two instantaneous deviations drift apart again, the phase locked
loop will follow the signal with the greatest strength, which will be
the line-of-sight signal. If the phase separation of the two
deviations is small, however, the phase locked loop will become less
likely to lock on to the true signal for a larger percentage of the
30 Hz cycle (this will depend on the bandwidth of the output of
the phase comparator in the aircraft). In general, some reflections
can cause minor problems, but these are usually about an order of
magnitude less than in a CVOR beacon.
Using a VOR
A mechanical cockpit VOR indicator
VORTAC in California
If a pilot wants to approach the VOR station from due east then the
aircraft will have to fly due west to reach the station. The pilot
will use the OBS to rotate the compass dial until the number 27
(270°) aligns with the pointer (called the primary index) at the top
of the dial. When the aircraft intercepts the 90° radial (due east of
the VOR station) the needle will be centered and the To/From indicator
will show "To". Notice that the pilot sets the VOR to indicate the
reciprocal; the aircraft will follow the 90° radial while the VOR
indicates that the course "to" the VOR station is 270°. This is
called "proceeding inbound on the 090 radial." The pilot needs only to
keep the needle centered to follow the course to the VOR station. If
the needle drifts off-center the aircraft would be turned towards the
needle until it is centered again. After the aircraft passes over the
VOR station the To/From indicator will indicate "From" and the
aircraft is then proceeding outbound on the 270° radial. The CDI
needle may oscillate or go to full scale in the "cone of confusion"
directly over the station but will recenter once the aircraft has
flown a short distance beyond the station.
In the illustration on the right, notice that the heading ring is set
with 360° (north) at the primary index, the needle is centred and the
To/From indicator is showing "TO". The VOR is indicating that the
aircraft is on the 360° course (north) to the VOR station (i.e. the
aircraft is south of the VOR station). If the To/From indicator were
showing "From" it would mean the aircraft was on the 360° radial from
the VOR station (i.e. the aircraft is north of the VOR). Note that
there is absolutely no indication of what direction the aircraft is
flying. The aircraft could be flying due West and this snapshot of the
VOR could be the moment when it crossed the 360° radial. An
interactive VOR simulator can be seen here.
Before using a VOR indicator for the first time, it can be tested and
calibrated at an airport with a VOR test facility, or VOT. A VOT
differs from a VOR in that it replaces the variable directional signal
with another omnidirectional signal, in a sense transmitting a 360°
radial in all directions. The NAV receiver is tuned to the VOT
frequency, then the OBS is rotated until the needle is centred. If the
indicator reads within four degrees of 000 with the FROM flag visible
or 180 with the TO flag visible, it is considered usable for
navigation. The FAA requires testing and calibration of a VOR
indicator no more than 30 days before any flight under IFR.
Intercepting VOR radials
On the course deviation indicator we select the radial and together
the needle and TO/FR flag shows our position.
There are many methods available to determine what heading to fly to
intercept a radial from the station or a course to the station. The
most common method involves the acronym T-I-T-P-I-T. The acronym
stands for Tune – Identify – Twist – Parallel – Intercept –
Track. Each of these steps are quite important to ensure the aircraft
is headed where it is being directed. First, tune the desired VOR
frequency into the navigation radio, second and most important,
Identify the correct VOR station by verifying the
Morse code heard
with the sectional chart. Third, twist the VOR OBS knob to the desired
radial (FROM) or course (TO) the station. Fourth, bank the aircraft
until the heading indicator indicates the radial or course set in the
VOR. The fifth step is to fly towards the needle. If the needle is to
the left, turn left by 30–45° and vice versa. The last step is once
the VOR needle is centred, turn the heading of the aircraft back to
the radial or course to track down the radial or course flown. If
there is wind, a wind correction angle will be necessary to maintain
the VOR needle centred.
Aircraft in NW quadrant with VOR indicator shading heading from 360 to
Another method to intercept a VOR radial exists and more closely
aligns itself with the operation of an HSI (Horizontal Situation
Indicator). The first three steps above are the same; tune, identify
and twist. At this point, the VOR needle should be displaced to either
the left or the right. Looking at the VOR indicator, the numbers on
the same side as the needle will always be the headings needed to
return the needle back to centre. The aircraft heading should then be
turned to align itself with one of those shaded headings. If done
properly, this method will never produce reverse sensing. Using this
method will ensure quick understanding of how an HSI works as the HSI
visually shows what we are mentally trying to do.
In the adjacent diagram, an aircraft is flying a heading of 180°
while located at a bearing of 315° from the VOR. After twisting the
OBS knob to 360°, the needle deflects to the right. The needle shades
the numbers between 360 and 090. If the aircraft turns to a heading
anywhere in this range, the aircraft will intercept the radial.
Although the needle deflects to the right, the shortest way of turning
to the shaded range is a turn to the left.
Airway (aviation) (Victor Airways)
Direction finding (DF)
Distance measuring equipment
Distance measuring equipment (DME)
Global Positioning System
Global Positioning System (GPS)
Hazardous Inflight Weather Advisory Service (HIWAS)
Head-up display (HUD)
Instrument flight rules
Instrument flight rules (IFR)
Instrument landing system
Instrument landing system (ILS)
Non-directional beacon (NDB)
Transponder landing system
Transponder landing system (TLS)
Wide Area Augmentation System
Wide Area Augmentation System (WAAS)
VHF Omnidirectional Range, Aviation Tutorial –
^ Kayton, Myron; Fried, Walter R. (1997). Avionics navigation systems,
2nd Ed (2nd ed.). USA: John Wiley & Sons. p. 122.
^ Airplane Owners and Pilots Association (March 23, 2005).
GPS Databases". AOPA Online. Airplane Owners and Pilots
Association. Retrieved December 5, 2009.
^ a b c d Department of Transportation and Department of Defense
(March 25, 2002). "2001 Federal Radionavigation Systems" (PDF).
Retrieved November 27, 2005.
^ a b http://www.ntia.doc.gov/legacy/osmhome/redbook/4d.pdf
^ CASA. Operational Notes on
VHF Omni Range (VOR)
^ FAA Aeronautical Information Manual 1-1-8 (c)
^ Federal Aviation Administration (April 3, 2014). "Aeronautical
Information Manual" (PDF). FAA. Retrieved Jun 29, 2015.
^ Department of Defense, Department of Homeland Security and
Department of Transportation (January 2009). "2008 Federal
Radionavigation Plan" (PDF). Retrieved June 10, 2009.
^ "Provision of Navigation Services for the Next Generation Air
Transportation System (NextGen) Transition to Performance-Based
Navigation (PBN) (Plan for Establishing a VOR Minimum Operational
Network)". 26 July 2016.
^ "Page not found - UK Civil Aviation Authority" (PDF).
^ Clued Up, Autumn/Winter 2014. CAA.
^ permissions, Industry (15 November 2012). "CNS-ATM Navigation
frequently asked questions". www.casa.gov.au.
^ Wood, Charles (2008). "VOR Navigation". Retrieved January 9,
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