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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

Wikimedia Commons has

media related to: VHF

omnidirectional range

UK Navigation Aids Gallery & Photos

Navigation aid search from airnav.com

VOR Navigation Simulator

Macromedia Flash 8 Based VOR Navigation Simulator

Categories: Avionics | Aircraft instruments | Radio

navigation | Aviation terminology

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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