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This article is about the radio navigation aid, see VOR for other uses.
D-VOR (Doppler VOR) ground station, co-located with DME.
VOR, short for VHF omnidirectional radio range, is a type of radio
navigation system foraircraft. A VOR ground station broadcasts
a VHF radio composite signal including the station's identifier, voice (if
equipped), and navigation signal. The identifier is morse code. The voice
signal is usually station name, in-flight recorded advisories, or live flight
service broadcasts. The navigation signal allows the airborne receiving
equipment to determine a magnetic bearing from the station to the aircraft
(direction from the VOR station in relation to the Earth's magnetic North at the
time of installation). VOR stations in areas of magnetic compass unreliability
are oriented with respect toTrue North. This line of position is called the
"radial" from the VOR. The intersection of two radials from different VOR
stations on a chart provides the position of the aircraft.
Contents
[hide]
1 Description
o 1.1 History
o 1.2 Features
o 1.3 Operation
o 1.4 Service Volumes
o 1.5 VORs, Airways and the Enroute Structure
o 1.6 Future
2 Technical Specification
o 2.1 Constants
o 2.2 Variables
o 2.3 CVOR
o 2.4 DVOR
o 2.5 Accuracy and Reliability
3 Using a VOR
o 3.1 Testing
o 3.2 Intercepting VOR Radials
4 See also
5 References
6 External links
Description
History
VOR
VOR-DME
VORTAC
These symbols denote different types of VORs onaeronautical charts.
Developed from earlier Visual-Aural 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 feet) 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 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, shown below) or a Horizontal Situation
Indicator(HSI, a more sophisticated version of the VOR indicator) and keeping
a course pointer centered on the display.
Presently, due to advances in technology, many airports are replacing VOR
and NDB approaches with RNAV (GPS) approach procedures; however,
receiver and data update costs[1] are still significant enough that many small
general aviation aircraft are not equipped with a GPS certified for primary
navigation or approaches.
Features
VORs signals provide considerably greater accuracy and reliability than NDBs
due to a combination of factors. VHF radio is less vulnerable to diffraction
(course bending) around terrain features and coastlines. Phase encoding
suffers less interference from thunderstorms.
VOR signals offer a predictable accuracy of 90 meters, 2 sigma at 2 nm from
a pair of VOR beacons;[2] as compared to the accuracy of
unaugmented Global Positioning System (GPS) which is less than 13 meters,
95%.[2] Repeatable VOR accuracy is 23 meters, 2 sigma. VOR signals
originate from fixed ground stations, usually below the aircraft, often at landing
facilities. Low incidence angle reflection from ground and clouds above
enhances signal strength. Low frequency (30 Hz) suffers less timing distortion
by reflection. VOR stations fixed relative to landing facilities are usable for
approaches without the trigonometric precalculations Area
Navigation database required for GPS.
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
(andDME) 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 airway systems.
Operation
VORs are assigned radio channels between 108.0 MHz (megahertz) and
117.95 MHz (with 50 kHz spacing); this is in the VHF (very high frequency)
range. The first 4 MHz is shared with the ILS band (See Instrument landing
system). To leave channels for ILS, in the range 108.0 to 111.95MHz, the 100
kHz digit is always even, so 108.00, 108.05, 108.20, and so on are VOR
frequencies but 108.10, 108.15, 108.30, and so on, are reserved for ILS.
The VOR encodes azimuth (direction from the station) as
the phase relationship of a reference and a variable signal. The omni-
directional signal contains a modulated continuous wave (MCW) 7 wpm
Morse code station identifier, and usually contains an amplitude
modulated(AM) voice channel. The conventional 30 Hz reference signal is on
a 9960 Hz frequency modulated (FM) 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. When the signal is
received in the aircraft, the two 30 Hz signals are detected and then
compared to determine the phase angle between them. The phase angle by
which the AM signal lags the FM subcarrier signal is equal to the direction
from the station to the aircraft, in degrees from local magnetic north, and is
called the "radial."
This information is then fed to one of four common types of indicators:
1. An Omni-Bearing Indicator (OBI) is the typical light-airplane VOR
indicator[3] and is shown in the accompanying illustration. It consists
of a knob to rotate an "Omni Bearing Selector" (OBS), and the OBS
scale around the outside of the instrument, used to set the desired
course. A "course deviation indicator" (CDI) is centered 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.
2. A Horizontal Situation Indicator (HSI) 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 map.
3. A Radio Magnetic Indicator (RMI), developed previous to the HSI,
features a course arrow superimposed on a rotating card which
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 degrees different)
course to the station.
4. An Area Navigation (RNAV) system is an onboard computer, with
display, and up-to-date navigation database. At least two VOR
stations, or one VOR/DME station is required, for the computer to
plot aircraft position on a moving map, or display course deviation
relative to a waypoint (virtual VOR station).
D-VORTAC TGO (TANGO) Germany
In many cases, VOR stations have co-located DME (Distance Measuring
Equipment) or military TACAN (TACtical Air Navigation) — 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 co-located VOR and
TACAN beacon is called a VORTAC. A VOR co-located 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 co-located TACAN or DME channel. On civilian
equipment, the VHF frequency is tuned and the appropriate TACAN/DME
channel is automatically selected.
Service Volumes
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. Although
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).[4]
US Standard Service Volumes (excerpted from FAA AIM[5])
SSV Class Designator
Dimensions
T (Terminal)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 60,000 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.
VORs, Airways and the Enroute Structure
The Avenal VORTAC 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.
As you fly in a commercial airliner you 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
upon GPS, more and more airways have been defined by such points,
removing the need for some of the expensive ground-based VORs. A recent
development is that, in some airspace, the need for such points to be defined
with reference to VOR ground stations has been removed. This has led to
predictions that VORs will be obsolete within a decade or so. There are three
types of VORs: High Altitude, Low Altitude and Terminal. The range of the
three differ. Terminal VORs are accurate to 25 NM outward up to 12,000 ft.
In many countries there are two separate systems of airway at lower and
higher levels: the lowerAirways (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 toward one VOR station while
watching the second receiver to see when a certain radial from another VOR
station is crossed, essentially seeing when a particular fix is crossed.
Future
VORTAC located on Upper Table Rock inJackson County, Oregon
It's likely that space-based navigational systems such as the Global
Positioning System (GPS), which have a lower transmitter cost per customer,
will eventually replace VOR systems[6] and many other forms of aircraft radio
navigation currently in use. Low VOR receiver cost is likely to extend VOR
dominance in aircraft, until space receiver cost falls to a comparable level.
The VOR signal has the advantage of weather tolerance and static mapping
to local terrain. Future satellite navigation systems, such as the European
Union Galileo, and GPS augmentation systems are developing techniques to
eventually equal or exceed VOR signals. As of 2008 in the United States,
GPS-based approaches outnumber VOR-based approaches but VOR-
equipped IFR aircraft outnumber GPS-equipped IFR aircraft.[citation needed]
Technical Specification
The VOR signal encodes a morse code indentifer, optional voice, and a pair
of navigation tones. The radial azimuth is equal to the phase angle between
the lagging and leading navigation tone.
Constants
Standard[2] modulation modes, indices, and frequencies
Description Formula Notes Min Nom Max Units
ident
i(t)
on 1
off 0
Mi A3 modulation index 0.07
Fi A1 subcarrier frequency 1020 Hz
voice
a(t) -1 +1
Ma A3 modulation index 0.30
navigation Fn A0 tone frequency 30 Hz
variable Mn A3 modulation index 0.30
reference Md A3 modulation index 0.30
Fs F3 subcarrier frequency 9960 Hz
Fd F3 subcarrier deviation 480 Hz
channel
Fc A3 carrier frequency 108.00117.9
5MHz
carrier spacing 50 50 kHz
speed of light C 299.79 Mm/s
radial azimuth
A relative to magnetic north 0 359 deg
Variables
Symbols
Description Formula Notes
time signal left
t center transmitter
t+(A,t) higher frequency revolving transmitter
t-(A,t) lower frequency revolving transmitter
signal strength
c(t) isotropic
g(A,t) anisotropic
e(A,t) received
CVOR
Conventional VOR
red(F3-) green(F3) blue(F3+)
black(A3-) gray(A3) white(A3+)
The conventional signal encodes the station identifier, i(t), optional
voice a(t), and navigation reference signal in, c(t), the isotropic (i.e.
omnidirectional) component. The reference signal is encoded on an
F3 subcarrier (color). The navigation variable signal is encoded by
mechanically or electrically rotating a directional, g(A,t), antenna to
produce A3 modulation (grayscale). Receivers (paired color and
grayscale trace) in different directions from the station paint a
different alignment of F3 and A3 demodulated signal.
DVOR
Doppler VOR
red(F3-) green(F3) blue(F3+)
black(A3-) gray(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), and navigation variable signal in, c(t), an isotropic
(i.e. omnidirectional) component. The navigation variable signal
is A3 modulated (grayscale). The navigation reference signal is
delayed, t+, t-, by electrically revolving a pair of transmitters. The
cyclic blue shift, and corresponding red shift, as a transmitter
closes on and recedes from the receiver results in F3
modulation (color). 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 modulation, g(A,t).
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 KG, 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".
[citation needed]
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 aircraft. The
electronic operation of "Detection" effectively shifts the
carrier down to 0Hz, 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(phi)).
If phi was 180 degrees, then the airplane'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 center
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
coax cable past 50 (or 48) antenna feeds. As the coax
moved between two antenna feeds, it would couple signal
into both.
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 60Hz 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 mid-point. This
creates a half-sinusoidal 1500Hz amplitude distortion in the
case of a 50 antenna system, (1440Hz in a 48 antenna
system). This distortion is itself amplitude modulated with a
60Hz amplitude modulation(also some 30Hz as well). This
distortion can add or subtract with the above-mentioned
60Hz 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 60Hz components tend to null
one another. There is a 30Hz component, though, which
has some pernicious effects.
DVOR designs use all sorts of mechanisms to try and
compensate these effects. The methods chosen are major
selling points for each manufacturer, with each extolling the
benefits of their technique over their rivals.
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 requirement.
Accuracy and Reliability
The predictable 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
shutdown when the bearing accuracy exceeds 1.0°.
[2] 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 degrees on some Doppler VOR beacons.
ARINC 711 – 10 January 30, 2002 states that receiver
accuracy should be within 0.4 degrees with a statistical
probability of 95% under various conditions. Any receiver
compliant to this standard should meet or exceed 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 high.
The general philosophy followed is that no signal is better
than a bad signal.
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 1.5dB different (at 113 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 the 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
degrees. A Doppler VOR (DVOR) is required to be +/- 1
degree.
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 (NDB).
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 degrees which
complements the zero degree 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 approach path.
Doppler VOR beacons are inherently more accurate than
Conventional VORs because they are more immune to
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 30Hz 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
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 degrees) aligns with the
pointer (called the Primary Index) at the top of the dial.
When the aircraft intercepts the 90-degree 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-degree radial while the VOR indicates that the course
"to" the VOR station is 270 degrees. 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 degree 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 degrees (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
degree 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
degree 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 degree
radial. An interactive VOR simulator can be seen here.
Testing
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 centered. 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.[7]
Intercepting VOR Radials
Aircraft in NW quadrant with VOR indicator shading heading
from 360 to 090 degrees
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 airplane 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 airplane till 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 degrees and vice versa. The last step
is once the VOR needle is centered, turn the heading of the
airplane 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 centered.
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 center. 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.
A good example is this, an airplane is traveling in the
northwest quadrant in relation to the VOR. The exact VOR
radial the aircraft is on is 315 degrees. After tuning,
identifying and twisting the OBS knob to 360 degrees, the
needle deflects to the right. The needle shades the
numbers between 360 and 090. If the airplane turns to a
heading anywhere in this range, the airplane will intercept
the radial.
How is reverse sensing negated using this method? In the
previous exercise, if the airplane was flying a heading of
180 degrees, the needle will still deflect right showing the
correct headings to fly but from the pilot's perspective it
will seem to indicate a turn westerly. The pilot should turn
left even though the needle points right, as it is a shorter
turn to a heading of 045 degrees to intercept the radial.
Using this method will ensure quick understanding of how
an HSI works as the HSI visually shows what we are
mentally trying to do.
See also
TACAN
Direction finding (DF)
Instrument flight rules (IFR)
Instrument Landing System (ILS)
Non-directional beacon (NDB)
Distance Measuring Equipment (DME)
Global Positioning System (GPS)
Wide Area Augmentation System (WAAS)
Head-up display (HUD)
Airway (aviation) (Victor Airways)
References
1. ̂ Airplane Owners and Pilots Association (March 23,
2005). "Inexpensive GPS Databases". AOPA Online.
Airplane Owners and Pilots Association. Retrieved
December 5, 2009.
2. ^ a b c d Department of Transportation and
Department of Defense (March 25, 2002). "2001
Federal Radionavigation Systems" (PDF). Retrieved
November 27, 2005.
3. ̂ CASA. Operational Notes on VHF Omni Range
(VOR)
4. ̂ FAA Aeronautical Information Manual 1-1-8 (c)
5. ̂ Federal Aviation Administration (February 11,
2010). "Aeronautical Information Manual". FAA.
Retrieved May 5, 2010.
6. ̂ Department of Defense, Department of Homeland
Security and Department of Transportation (January
2009). "2008 Federal Radionavigation Plan" (PDF).
Retrieved June 10, 2009.
7. ̂ Wood, Charles (2008). "VOR Navigation".
Retrieved January 9, 2010.
External links
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media related to: VHF
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Navigation aid search from airnav.com
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Macromedia Flash 8 Based VOR Navigation Simulator
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VOR NavigationPart I
The VHF Omnidirectional Range navigation system, VOR, was probably the most significant aviation invention other than the jet engine. With it, a pilot can simply, accurately, and without ambiguity navigate from Point A to Point B.
The widespread introduction of VORs began in the early 1950s and 50 years later it remains the primary navigation system in the overwhelming majority of aircraft.
If you jumped to this point of the website without proceeding through the earlier sections, I strongly
recommend that you return to the Air Navigation section and review the sections on VFR Sectional Charts, IFR enroute low altitude charts, and the basics of plotting a course. Further, you should go to the NDB Approaches/Approach Platessection and read the basics of Instrument Approach Plates, now called Terminal Procedures.
The basic principle of operation of the VOR is very simple: the VOR facility transmits two signals at the same time. One signal is constant in all directions, while the other is rotated about the station. The airborne equipment receives both signals, looks (electronically) at the difference between the two signals, and interprets the result as aradial from the station.
The GPS, Global Positioning System, is making inroads onto the navigation scene and offers a flexibility unavailable with either NDB or VOR systems. However, it is supplementing these systems, not replacing them.
The RMI indicator used in the NDB navigation exercises is as close to a "hands-off" indicator as you will find. In an aircraft the RMI compass card must initially be aligned with the compass before a flight begins and then rechecked every fifteen minutes or so, and that's it.
With VOR, however, course information must be manually entered into the indicator. The VOR indicator below shows an aircraft heading toward, "TO," the Omni station.
NOTE this very important fact, with more info farther down. The radial signals of a VOR always point away from the station. The indicator below shows 345°, but since we are heading toward the VOR, see arrow D, we are actually on the reciprocal radial, or the 165° radial. This aircraft is south of the station. This
will become more clear in a moment.
See the text for details on the four components of the VOR Indicator.The digital indicator is a separate gauge used on the Nav Trainer Panel.
The VOR display has four elements:
A. A Rotating Course Card, calibrated from 0 to 360°, which indicates the VOR bearing chosen as the reference to fly TO or FROM. Here, the 345° radial has been set into the display. This VOR gauge also digitally displays the VOR bearing, which simplifies setting the desired navigation track.
B. The Omni Bearing Selector, or OBS knob, used to manually rotate the course card.C. The CDI, or Course Deviation Indicator. This needle swings left or right indicating the direction to turn to
return to course. When the needle is to the left, turn left and when the needle is to the right, turn right, When centered, the aircraft is on course. Each dot in the arc under the needle represents a 2° deviation from the desired course. This needle is more-frequently called the left-right needle, with the CDI term quickly forgotten after taking the FAA written exams. Here, the pilot is doing well, and is dead-on course—or maybe lazy and with the autopilot activated in the "NAV" mode.
D. The TO-FROM indicator. This arrow will point up, or towards the nose of the aircraft, when flying TO the VOR station. The arrow reverses direction, points downward, when flying away FROM the VOR station. A red flag replaces these TO-FROM arrows when the VOR is beyond reception range, has not been properly tuned in, or the VOR receiver is turned off. Similarly, the flag appears if the VOR station itself is inoperative, or down for maintenance. Here, the aircraft is flying TO the station.
Radials, Radials, Radials
To grasp the VOR system you must understanding that it is entirely based onradials away from the station.
In the Sandy Point VOR to the left, note first that the arrow on the 0° radial points away from the center of the compass rose. You'll remember that this radial points to the west of true north because of the west magnetic variation. North on a VOR is Magnetic North. So, if you overflew this VOR on the 0° radial, you would be flying away from the VOR.
Similarly, note the arrows by the 30°, 60°, 90° marks and the rest of the way around the compass rose. They all point away from the station. Radials are always away from the station.
There is only one line on the chart for each numbered radial for a particular VOR station.
Whether you are flying it outbound or inbound, or crossing it, a radial is always in the same place.
The only possible complication lies in the reciprocity of the numbers. Whenever you are proceeding outbound, your magnetic course (and heading when there is no wind) will be the same number as the radial. Turn around and fly inbound you must mentally reverse the numbers and physically reverse the OBS setting so that your course is now the reciprocal of the radial. But the radial you are flying on hasn't changed.
Some examples will cement this in your mind.
This aircraft is north of the Omni station, flying on the 345° radial away FROM the station. The left-right needle shows the aircraft on course and the FROM flag is present, pointing down, toward the station behind.
This aircraft is south of the Omni station. Its magnetic course is 345°. Walk through the steps below to understand the VOR reading.
1. The aircraft isn't on the 345° radial because that radial extends from the Omni to the northwest as shown by the arrow.
2. The aircraft is actually on the reciprocal radial, the radial pointing towards the plane. That reciprocal radial is 165°, away from the station like all radials.
3. If the 165° radial were set into the VOR, the FROM flag would properly show, because the aircraft is away from the Omni on that radial.
4. Here is the important point. If the OBS is rotated until the needle centers and the FROM flag shows, it will always show the correct radial from the Omni that the aircraft is on regardless of the aircraft heading.
5. To eliminate the confusion of location relative to an Omni, the magnetic course of the aircraft and the radial setting on the VOR should be the same.
6. Presumably the aircraft is flying in the desired course direction, so its heading will be approximately the same as the VOR setting, i.e., the magnetic course. The heading may differ slightly from the VOR because of the correction needed to correct for wind drift.
7. Thus, with the OBS set to 345° the left-right needle shows the aircraft on course and the TO flag is showing, pointing up, toward the station ahead.
Experiment with this on your FS98 or FS2K to see the effects of the OBS setting on the TO-FROM flag. Select any Omni, position the aircraft to be flying TO it, then rotate the OBS so that its reading centers the needle and the TO flag appears.
Next, rotate the OBS to the reciprocal of the course. The needle will again center, but the FROM flag will
appear.
A one-line recap: to know whether you are flying TO or FROM an Omni, the OBS setting must be approximately the same as the aircraft heading.
Where am I?
This illustration shows the confusion that can result, yes, that the VOR indicator can actually provide wrong information if the OBS isn't set properly.
Same example as before. The aircraft is south of the Omni, on the 165° radial. It is flying northwest. Observe the DG. The aircraft is heading 345° as desired. But the OBS was improperly set to 165° and the VOR is falsely informing the pilot, with a nicely centered needle, that he/she is flying away FROM the Omni. The aircraft, of course, is flying TO the Omni.
Hate to beat a dead horse, but again, the TO-FROM confusion disappears if the aircraft heading and the OBS setting are approximately the same which they weren't here. Pay attention to this and you will stay out of trouble.
This sort of error usually happens when the pilot rotates the OBS, watching only for a centered needle, not also paying attention that the setting should approximate the magnetic course, or aircraft heading.
Wandering off course?
This aircraft has drifted to the right of the desired course. To be "on course" the aircraft must be on the red line. Not paying attention to a crosswind (what other kind is there?), or simply letting the heading wander could do it. In any event, the VOR needle has swung to the left, indicating that the aircraft must move to the left to return to course. So a left turn is in order. Like the RMI, with the VOR a pilot always turns towards the needle to return to course, assuming that the OBS setting approximates the aircraft heading.
This aircraft is 4° off course. Each dot of the arc under the needle is a 2° deviation from the desired course. Don't confuse heading, the direction of the aircraft's nose, with course, the desired track along the ground. Only with no wind will heading and course be the same.
"The needle is centered, my flying is perfect"
Nice thought, but not necessarily. The VOR system operates in the VHF frequency band, from 108.0 to 117.95 MHz. Reception of VHF signals is a line-of-sight situation. Nominally, you must be 1000 ft AGL to pick up an Omni within its maximum low-altitude service range.
The VOR indicator is smart enough to know when a usable signal has not been received and displays an "OFF" flag, a red and white barber-pole striped flag in the gauge in the illustration to the left. So when you are flying to or from an Omni station and you're quite content at how stable the CDI needle has been, it's worth taking another glance at the gauge to see if the OFF flag is staring back at you.
The OFF flag also displays if the Nav receiver is tuned to the wrong frequency or, blush, if it's properly tuned but you neglected to turn on the power switch. If you're taking your check ride with an FAA
examiner for a real license, that oversight is likely to get you a quick return to terra firma. And, there's also the possibility of a popped circuit breaker interrupting power to the Nav receiver, a connector jiggled loose, etc.
VOR RangeAh, the oft asked and seldom answered question: how far away can I pick up a reliable signal from the Omni and what altitude need I be at? The FAA neatly skirts the answer by classifying Omnis by an altitude code, with the ranges vs. altitudes as shown in the table below.
Reception Range vs. Altitude of VORs
VOR ClassRangenm
within Altitudefeet
Terminal (T) 25 1000 – 12,000
Low Altitude (L) 40 1000 – 18,000
High Altitude (H)40100130
1000 – 14,50014,500 – 60,000,18,000 – 45,000
Data is from the Aeronautical Information Manual, AIM.
These ranges assume, please contain your laughter, that terrain plays no part in VOR ranges of reception. But terrain, of course, can greatly impact the reliable range of an Omni.
Consider the Bangor VOR, BGR, at Bangor (Maine) Int'l. Airport. Here are the comments in the Airport/Facility Directory:
"VOR unusable 342°—063° below 2500 ft."
Pretty significant terrain impact, wouldn't you say? So think of the FAA data in the table as a starting point that may be modified by terrain.
Checking VOR accuracy
The VOR is the most common navigation instrument presently on aircraft panels. We rely on it to accurately track VOR radials, whether flying between Omni stations, or locating intersections, or arriving and departing
from airports. We accept at face value that what it displays is accurate. Well, on FS98 and FS2000 it is always accurate. But in the real world, not only can the gauge be wrong, but the FAA requires that a pilot check the VOR for accuracy within 30 days of an IFR flight. Even if a pilot never flys IFR, it is prudent to regularly check the VOR for accuracy.
One acceptable way to formally check VOR accuracy is with a VOR Test Facility, more commonly called a VOT. A VOT is a low-power Omni station located on many of the mid-to-large size airports. A VOT differs from a standard Omni in that it transmits only a single radial, the 360° radial.
To calibrate a VOR, the pilot tunes in the VOT frequency while on the ground (in rare instances this check is performed in the air). Refer to the back of the Airport/Facility Directory for frequencies and whether it is a ground check (G) or an airborne check (A). See the Connecticut illustration below.
CONNECTICUTVOR TEST FACILITIES (VOT)
Facility (Arpt Name) Freq.Type VOT
Remarks
Bradley Int'l 111.4 G
Bridgeport (Sikorsky Mem) 109.25 G
Groton (Groton–New London)
110.25 G
Hartford (Hartford–Brainard)
108.2 A3 nm Radius 1200–5000 ft.
Data is from the Airport/Facility Directory.
Next, rotate the OBS until the to-from needle centers. Read the number from the Omni Bearing Indicator ring or digital display. To be legal, the gauge must be within 4° of either 180° with the TO flag showing or of 0° with the FROM flag showing.
Make note in the illustration above that the VOT at Bradley Int'l. airport is on 111.4 MHz. That information is important later while performing one of the VOR approach practice flights.
http://www.navfltsm.addr.com/vor-nav.htm