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VHF Omnidirectional Range (VOR)

Introduction

The older radio navigation were based on low/medium frequency (LF/MF) transmissions, which had problems associated with it since they are propagated by either ground waves or sky waves.

Hence VHF frequency based system was developed which is know as VHF omnidirectional range (VOR), as VHF propagation uses sky wave, the problems associated with LF and MF transmissions are eliminated. 

Here, the word “range” in the title is carried over from the name “radio range” given to the old LF navigation system in the United States and is not related to distance, which the VOR on its own is unable to determine.

Basic VOR Terminology

In order to understand the basic principle of VOR navigation, certain terms are needs to be defined as follows

Aircraft Heading

The clockwise angle from the north pole to the nose of the aircraft is termed as heading of an aircraft. If the direction is taken from magnetic north it is termed as Magnetic Heading, If the direction is taken from true north it is termed as True Heading.

Bearing

The clockwise angle for the north pole to the radio station is termed as bearing to an station. If the angle is taken from the magnetic north it is termed as Magnetic Bearing and if the angle is taken from true north then it is termed as True Bearing. 

Relative Bearing

The angle between the bearing of an station and the heading of an aircraft is termed as relative bearing.

Relative Bearing.jpg

HEADING, BEARING AND RELATIVE BEARING

The bearing of the station is always calculated in the a clockwise direction, there can be two types of bearing 

Bearing TO the station and Bearing FROM the station. The TO and FROM bearing depends on the heading or the position of aircraft with respect to the station, whether it is flying towards (TO) the station or away (FROM) from the station.

Bearing TO and FROM the Station.jpg

BEARING TO AND FROM THE STATION

Bearing are also calculated with respect to the True or Magnetic North, the pilot communicates the same in terms of Q code which are as follows

QDM: Magnetic Bearing TO the station

The Clockwise angle of the radio station from the Magnetic North. 

QDR: Magnetic Bearing FROM the station

The Clockwise angle of the radio station from the Magnetic North.

QUJ: True Bearing TO the station

The Clockwise angle of the radio station from the True North.

QTE: True Bearing FROM the station

The Clockwise angle of the radio station from the True North.

Magnetic and True Bearings.jpg

MAGNETIC AND TRUE BEARING

Another very important concept which needs to be understood is the radial and course for the radio station. 

When the aircraft is flying towards the station the aircraft has to fly (heading) in the opposite radial of the radio station bearing as it has to fly to the station. When the aircraft is flying away from the station it has to fly (heading) is the same direction as the station bearing as it is going away from the station. 

VOR Radials.jpg

VHF Omnidirectional Range (VOR)

Very High Frequency (VHF) Omni-Directional Range (VOR) is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons.   

 

The emission characteristics are A9W:
A = main carrier amplitude modulated double side-band.
9 = composite system.

W = combination of telemetry, (telephony) and telegraphy.

Each VOR operates at a frequency in the range 108–117.95 MHz with a channel spacing of 50 kHz, the first 4 MHz is shared with the instrument landing system (ILS) band.

ILS frequencies are allocated to the odd tenths of each 0.5 MHz increment, e.g. 109.10 MHz, 109.15 MHz, 109.30 MHz, etc. VOR frequencies are allocated to even tenths of each 0.5 MHz increment, e.g. 109.20 MHz, 109.40 MHz, 109.60 MHz, etc.

Hence out of the total number of channels are 200, 160 channels are allocated to VOR and remaining 40 are allocated to Instrument Landing system (ILS).

Out of 160 Channels 120 are allocated to En-route VOR and 40 are allocated to Terminal VOR (TVOR).

En-route VOR have about 200W power with a range of 49 nautical miles. The frequency range is 112 MHz-117.95 MHz.

Terminal VOR (TVOR) have 50W power with a range of 25 nautical miles. The frequency range is 108 MHz-112 MHz. The Terminal VOR (TVOR) shares the frequency band with Instrument Landing System (ILS). 

VOR navigation aids are identified by unique three-letter codes. The code is modulated onto the carrier wave as a 1020 Hz tone that the crew can listen to as a Morse code signal. Some VOR navigation aids have an automatic voice identification announcement that provides the name of the station; this alternates with the Morse code signal.

 

The location of the VOR navigation aids (specified by latitude and longitude), together with their carrier wave frequencies, is provided on navigation charts.

 

VOR has the following uses:

  • Marking the beginning, the end and centre line of airways or sections of airways.

  • As a let-down aid at airfields using published procedures.

  • As a holding point for aircraft.

  • As a source of en route navigational position lines.

VOR Operation

The VOR station works on the lighthouse principle, and assume you're sitting north of the lighthouse and watching the beam go around, you see a bright flash only when the light points directly at you. At that moment, begin counting to see how much time it takes for the beam to flash again. 

Let's say the beam takes 40 seconds for one rotation, or 360 degrees, now you can convert the number of seconds into where the beam is aimed at any time. So by the time is 10 seconds from the flash the beam would be pointing east (90º), by time 20 seconds from the flash the beam would be pointing south (180º), and by counting 30 seconds from the flash, the beam will be pointing west (270º) and at time 40 sec from the flash the beam will be pointing North again (0º or 360º). 

LIght House principle.jpg

BASIC VOR WORKING

The VOR works on a similar principle to that of the light house, however VOR has two signals, which are 30 Hz sine waves modulated onto the VHF carrier, one is called the reference signal and other is called the variable signal.

The reference signal is omnidirectional signal with he same phase in all directions and the variable signal whose phase varies continuously around the circle from 0º to 360º relative to the reference signal, which depends on the position or bearing of the station from the north pole. 

Hence to calculate the position of the station with respect to the north the phase difference is calculated. The two signals are in phase along magnetic North, they are 90º out of phase in the East, they are 180º out of phase in the South and 270º out of phase in the West.

Basic VOR Principle.jpg

BASIC VOR PRINCIPLE

Types of VOR

The following types of VOR are in use

  • CVOR - Conventional VOR is used to define airways and for en-route navigation.

  • BVOR - A broadcast VOR which gives weather and airfield information between beacon identification.

  • DVOR - A Doppler VOR - this overcomes siting errors.

  • TVOR - Terminal VOR which has only low power; and is used at major airfields.

  • VOT - This is found at certain airfields and broadcasts a fixed omni-directional signal for a 360° test radial. This is not for navigation use but is used to test an aircraft’s equipment accuracy before IFR flight. More than +/-4° indicates that equipment needs servicing.

  • VORTAC - Co-located VOR and TACAN (DME) beacons.

 

There are two most commonly types used are VOR(CVOR) and the Doppler VOR (DVOR). They both use the same design of VHF antenna to generate the carrier, which is known as the Alford loop, It consists of a pair of resonant 90 dipoles arranged in a square and each fed in anti phase from the oscillator. The only difference being methods to produce the variable and reference 30 Hz modulation.

Conventional VOR (CVOR)

Conventional VOR (CVOR) stations radiate two horizontally polarized v.h.f. wave modulated signals:

  • Omnidirectional reference signal

  • Directional variable signal

 

The omnidirectional (or reference) signal is 30 Hz (frequency modulated) on a subcarrier of 9950 Hz (amplitude modulated) with a deviation of + 480 Hz. The directional (variable) signal is 30 Hz (Amplitude Modulated) radiated as a cardioid pattern, electronically rotating clockwise at 30 revolutions per second.

The station identification is morse coded and transmitted on a frequency of 1020 Hz (amplitude modulated) at least three time each 30 sec. Where a VOR & DME are collocated the identification transmission are synchronised.

When an aircraft is to the north of the beacon (radio station) it will receive variable and reference signals in phase, for an aircraft at X Degree magnetic bearing from the station variable phase will lag the reference phase by X degree.

The airborne equipment receives the composite signal radiated by the station to which the receiver is tuned. After detection the various modulating signals are separated by filters. The 30 Hz reference signal is phase compared with the variable signal, the difference in phase giving the bearing from the station.

The actual reading presented to the pilot is the bearing to the station rather than from, so if the difference in phase between variable and reference signal is 135° the 'to' bearing would be 135 + 180 = 315º.

Locations of conventional VOR (CVOR) ground stations have to be carefully planned to take into account local terrain and obstacles. Mountains and trees can cause multipath reflections, resulting in distortion (known as siting errors) of the radiated signal. These errors can be overcome with an enhanced second-generation system known as Doppler VOR (DVOR).

Doppler VOR (DVOR)

Due to the site errors in the CVOR, the Doppler VOR (DVOR) was developed. It works on the Doppler effect which can be summarised here as: ‘…the frequency of a wave apparently changes as its source moves closer to, or farther away from an observer’.

 

The transmission frequencies are the same, the transmitted bearing accuracy is improved as the transmissions are less sensitive to site error.

The transmission differences are:

  • The reference signal is AM.

  • The variable phase directional signal is FM.

  • The directional signal is anti-clockwise.

 

As a result the same airborne VOR equipment can be used with either CVOR or DVOR beacons.

Hence in DVOR the omnidirectional (or reference signal) is 30 Hz a.m., while the directional (or variable) signal is 30 Hz f.m. on a 9960 Hz sub-carrier and rotating in anti-clockwise direction at 30 rev per sec. Since the roles of the a.m. and f.m. are reversed with respect to CVOR the variable phase is arranged to lead the reference phase by X for an aircraft at X magnetic bearing from the station.

Since the FM variable signal is less prone to interference, DVOR performance is superior to CVOR.​

DVOR actually uses two unmodulated r.f. sideband signals rotating patterns one 9960 Hz above carrier, the other 9960 Hz below carrier, are radiated from antennas diametrically opposite in a ring of about fifty antennas.

If two aircrafts, one moving towards and other moving away from the DVOR station, will experience Doppler effect due to the two rotating signal; one aircraft will detect a decreased frequency, while other aircraft  will detect an increased frequency.

The diameter of the array is 13.5 m, the speed of pattern rotation is 30 rps, thus

 

the tangential speed at the periphery is π x13·5 X 30 = 1272 m.p.s.

 

At the centre frequency of the v.h.f. band, 113 MHz, one cycle occupies approximately 2·65 m, thus

 

the maximum Doppler shift is 1272/2·65 = 480 Hz.

Factors Affecting Operational Range of VOR

The higher the transmitter power, the greater the range. Thus en route VORs with a 200 watt transmitter will have about a 200 NM range, and a TVOR will normally transmit at 50 watts.

The transmitter and receiver height will also have an effect on the operational range of VOR as the transmissions give line of sight ranges, plus a slight increase due to atmospheric refraction.

 

This can be assessed by using the formula:

Maximum theoretical reception range (NM) = 1.23 × (√h1 + √h2)

where: h1 = Receiver height in feet AMSL, and

h2 = Transmitter height in feet AMSL.

Uneven terrain, intervening high ground, mountains, man-made structures etc., cause VOR bearings to be stopped (screened), reflected, or bent (scalloping), all of which give rise to bearing errors.

VOR Equipments

There are 4 main components of the VOR equipment in the aircraft, namely:

  • Aerial

  • Receiver

  • Control Panel

  • Indicator

VOR Antennas

The VOR antenna is a horizontally polarised, omnidirectional half-wave dipole, i.e. a single conductor with a physical length equal to half the wavelength of the VOR signals being received.

For slower aircraft the aerial is a whip type fitted on the fuselage and for high speed aircrafts two half-wave dipole  antennas are flush mounted on either side of the vertical fin.

 

The antennas are connected to the receivers via coaxial cables.

VOR Antenna.jpg

VOR ANTENNA LOCATION

VOR Receivers

VOR receivers is a box fitted in the avionics bay.

VOR receivers are based on the super- heterodyne principle with tuning from the control panel.

The received radio frequency signal is passed through an amplitude modulation and frequency modulation filter to separate the various signals and fed to the phase detector. A comparison of the phase angles of the variable and reference 30 Hz signals produces the VOR radial signal.

 

The analog VOR systems used synchro for the indications, where as digital systems uses data bus systems. Voice and Morse code tones are integrated with the audio system.

VOR Basic Block Diagram (4) (1).png

BASIC VOR RECEIVER BLOCK DIAGRAM

VOR Control panels

Control panels identified as ‘VHF NAV’ can be located on the glare shield or centre pedestal. This panel is used to select the desired course and VOR frequencies. 

Various aircrafts have different kinds of control panel, however the basic functioning of the buttons remains same.

 

The panel generally consists of the following

  • Two windows to display the frequencies selected the left or above window indicates active frequency or the right or the bottom indicates the standby frequency.

  • TRF button is used to toggle between the active and standby frequency. 

  • The frequency can be selected by a rotary switch or number depending on the type of control panel.

  • Test switch to test the VOR system on ground.

  • Mode Switch is used to toggle between VOR, ILS and ADF system as modern aicraft uses a combined control panel for all these systems.

VOR Control Panel.jpg

VOR CONTROL PANEL

VOR Indicators (CDI, HSI and RMI)

VOR indicators display the bearing information to the pilot which is determined by the relative phase of the 30 Hz reference and variable signals from the VOR.

Depending on the type of aircraft the following type of VOR indicators are used

  • Course Deviation Indicator (CDI)

  • Horizontal Situation Indicator (HSI)

  • Electronic Horizontal Situation Indicator (EHSI)

  • Radio Magnetic Indicator (RMI) 

Course Deviation Indicator (CDI)

This indicator is mostly found in the light aircrafts and is of electromechanical type. The phase difference is converted to an analog voltage and is used to drive the deflection of a needle labeled the course deviation indicator (CDI). The divisions on the CDI needle scale are 2º in VOR mode.

The omni bearing selector (OBS) knob is use to select the desired course in case of manual VOR flying. The CDI only indicates the selected course to the VOR station and is independent of the heading of the aircraft.
 

An TO and FROM indicator indicates weather the aircraft is flying towards or away from the selected course.

Course Deviation Indicator (CDI).jpg

COURSE DEVIATION INDICATOR (CDI)

Horizontal Situation Indicator (HSI)

One of the major drawbacks of using the Course Deviation Indicator (CDI) is that it only indicated the bearing to the station and is independent of the heading of the aircraft. Hence the pilot would need to refer two instruments one the CDI and other the heading indicator in order to determine the relative bearing to the station.

This can be a heavy workload, especially if the CDI is off-scale and the aircraft is flying a heading in a semicircle opposite to the one containing the required bearing.

 

Hence in order to determine the relationship between heading and bearing the CDI was combined with the direction indicator which indicated the relationship between the required bearing and heading, which is called as relative bearing.

 

The combined instrument is referred to as a horizontal situation indicator (HSI) and consist of the following

  • Lubber line which indicates the hearing of the aircraft

  • Aircraft Symbol which is fixed

  • Compass card which rotates with the aircraft heading

  • Course selected pointer which indicates the selected course

  • Course Select knob which is use to select the desired course radial.

  • Course deviation scale and Course deviation bar which indicate the deviation of the selected course

  • TO and FROM indication which indicates flying to the station or flying from the station

  • Heading select knob which is use to select a desired heading

  • Heading select bug which indicates the desired heading selected by the heading select knob

  • Glide Slope Indicator which indicate the glide slope indication when ILS mode is selected

  • NAV and HDG flag which indicates the loss of signal from the Navigation system selected and loss of heading signal respectively. 

Horizontal Situation Indicator (HSI).jpg

HORIZONTAL SITUATION INDICATOR (HSI)

Electronic Horizontal Situation Indicator (VOR mode)

In the modern aircrafts the HSI is a part of the Primary Flight Display (PFD), the functioning is and parts are similar to that of the traditional Horizontal Situation Indicator (HSI) with the following difference.

The EHSI can display two indicators together so bearing from two different VOR stations can be achieved or one needed can be use to indicate VOR and other can be used for Automatic Direction Finding (ADF) indication. 

Another difference is the VOR can be used in expanded mode to achieve more accuracy 

With a VOR frequency selected, the EHSI displays a full compass rose with the VOR source in the lower left and the frequency in the lower right.

Course selection is displayed by the magenta course needle, the tip pointing to the selected course (150). Course deviation is shown by the traditional deviation bar moving across a two dot left and two dot right scale.

A TO / FROM pointer is shown in addition to the TO /FROM annunciation. DME distance displayed in the top left corner.

Current heading is shown in the window and by the lubber line at the top of the compass rose (130), the current selection is Magnetic Heading as shown either side of the window.

Current track is shown by the white triangle on the inside edge of the compass rose.

Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose.

Wind speed and direction are shown in the lower left corner orientated to the display selection (Heading or Track, Magnetic or True).

Weather radar displays are not available.

With a VOR frequency selected, the EHSI displays about 90° of compass rose with the VOR source in the lower left and the frequency in the lower right.

The white triangle at the bottom of the display is the aircraft symbol.

Selected course (track) is displayed by the magenta course needle, the tip pointing to the selected course (150). The course selectors are usually on either side of the autoflight main control panel (one for the Captain and one for the First Officer). Course deviation is shown by the traditional deviation bar moving across a two dot left and two dot right scale.

A TO /FROM annunciation is shown.

DME distance is displayed in the top left corner.

Current heading is shown in the window and by the lubber line at the top of the compass rose (130), the current selection is Magnetic Heading as shown either side of the track window. Current track is shown by the white line from the tip of the aircraft symbol to the compass arc. Selected heading is shown by the magenta heading “bug” on the outer scale of the compass rose. Wind speed and direction are shown in the lower left corner orientated to the display selection (Heading or Track, Magnetic or True).

Weather radar displays are available; when selected “on”, range arcs are also visible. Weather radar shows three colours: green, yellow and red, green being the least turbulence, red being the worst. If turbulence mode is available, it is shown as magenta, the area of greatest activity in the cloud. The range of the display can be selected on the control panel, half scale range is displayed (10 NM) so this display is selected to 20 NM. The outer arc of the compass rose is the furthest range from the aircraft.

Electronic Horizontal Situation Indicato

ELECTRONIC HORIZONTAL SITUATION INDICATOR (EHSI) IN VOR MODE

Radio Magnetic Indicator (RMI)

The Radio Magnetic Indicator is used generally as an standby instrument and is connected with bus that can power it during an event of major power failure.

It contains the following

  • Lubber line use to indicate the heading of the aircraft

  • The compass card which rotates with the aircraft heading

  • Two indicator which indicates the bearing to selected station depending on which system is selected VOR or ADF

  • The selector knob which are use to select the VOR or ADF systems

The RMI does not contain an TO and FROM indication, and presents the Relative bearing to the pilot just like the Horizontal Situation Indicator (HSI).

Radio Magnetic Indicator (RMI).jpg

RADIO MAGNETIC INDICATOR (RMI)

Different VOR Indicator (CDI vs HSI vs RMI)

The VOR indicator vary according to the type of aircraft and it is important to understand the difference between them which is as follows

Course Deviation Indicator (CDI) vs Hori

COURSE DEVIATION INDICATOR (CDI) VS HORIZONTAL SITUATION INDICATOR (HSI) VS RADIO MAGNETIC INDICATOR (RMI)

Course Deviation Indicator (CDI) vs Horizontal Situation Indicator (HSI)

The CDI and HSI differ in the following ways

  • CDI indicates the selected course which can be selected using the Omni Bearing Selected (OBS) and has a fixed card i.e the card does not rotate with the heading of the aircraft. The indication will always point to the selected radial and is independent of the aircraft heading.

  • HSI has a rotating compass card which moves with the aircraft heading, the course can be selected with the help of course selector. Hence pilot gets all three information from the HSI the heading of the aircraft, the selected course and the relative bearing to the station. 

Course Deviation Indicator (CDI) vs Hori

COURSE DEVIATION INDICATOR (CDI) VS HORIZONTAL SITUATION INDICATOR (HSI)

Course Deviation Indicator (CDI) vs Radio Magnetic Indicator (RMI)

The CDI and RMI differ in the following ways

  • CDI indicates the selected course which can be selected using the Omni Bearing Selected (OBS) and has a fixed card i.e the card does not rotate with the heading of the aircraft. The indication will always point to the selected radial and is independent of the aircraft heading. The CDI also has TO and FROM indications. It also contains the deviation bar which indicate the FLY LEFT or FLY RIGHT commands

  • RMI has a rotating compass card hence is capable of indicating the heading of the aircraft. It consists of two pointer which can display indication from two different VOR station or one pointer can display indiation from VOR station while other indicator can display indication from ADF station which depends on the selector knob position. The RMI does not indicate TO and FROM indications or FLY LEFT or FLY RIGHT Commands. The RMI provides pilot with heading of the aircraft, the bearing to the station and the relative bearing.

Course Deviation Indicator (CDI) vs Radi

COURSE DEVIATION INDICATOR (CDI) VS RADIO MAGNETIC INDICATOR (RMI)

Horizontal Situation Indicator (HSI) vs Radio Magnetic Indicator (RMI)

The HSI and RMI differ in the following ways​

  • HSI contains the deviation bar and scale which indicate the Pilot FLY LEFT or FLY RIGHT commands. It also contains the TO and FROM indications.

  • The RMI does not indicate TO and FROM indications or FLY LEFT or FLY RIGHT Commands. The RMI is generally a stand by instrument.

Horizontal Situation Indicator (HSI) vs

HORIZONTAL SITUATION INDICATOR (HSI) VS RADIO MAGNETIC INDICATOR (RMI)

Navigation using VOR

There are two kinds of navigation using VOR one is termed as automatic navigation and other is termed as manual navigation.

In automatic the VOR station frequency is tuned and the indication are displayed on the respective indicator while in manual naivation the VOR radial is selected instead of VOR frequency and flying is carried out.

Before we move into details of navigation using VOR, it important to understand two types VOR indication

TO and FROM Indications

The TO indication are displayed when an aircraft is flying Towards the station and the FROM indications are displayed when flying away from the station. The deviation pointer will show zero deviation when the aircraft is flying on the desired radial.

In practical this does not happen due to the effect to cross winds, as the heading of the aircraft differs from the selected radial.

To and From Indications.jpg

VOR TO AND FROM INDICATIONS

Fly Left and Fly Right Commands

As we have seen there are two types of navigation using VOR, Automatic and Manual.

 

In 'automatic' VOR, the pilot need do no more than switch on and tune in to an in-range station in order to obtain bearing information.

 

In 'Manual' VOR requires the pilot to select a particular radial on which he wants to position his aircraft. The actual radial on which the aircraft is flying is compared with the desired radial. If the two are different the appropriate fly-left or fly-right signals are derived and presented to the pilot.

To do so the reference phase (R) is phase shifted by the selected course (C) and then compared with the variable phase, a fly-right indication will be given if R + C lags V, while if R + C leads V, the command will be fly-left.

 

If we now add 180 to the phase-shifted reference phase we have R + C + 180 which will, on addition, either cancel V, partially or completely, in which case a TO indication will be given, or reinforce V, partially or completely, in which case a FROM indication will be given.

Fly Left and Fly Right Commands.jpg

VOR FLY-LEFT AND FLY-RIGHT INDICATIONS

Navigation using CDI

In order to join and track a specific radial using this type of display, the card is rotated till the OBI reads the correct bearing and the needle then indicates whether to fly a heading to the right or left of the selected bearing to approach the required radial.

This type of indicator the heading of the aircraft does not matter.

VOR Navigations.jpg

FLYING USING A COURSE DEVIATION INDICATOR (CDI)

Navigation using HSI or EHSI

The OBI is moved around the same display as the direction indicator and always shows the bearing relative to the current heading and when the aircraft turns, the OBI rotates with the direction indicator. Thus, the direction the aircraft is going relative to the selected bearing is always clear and easy to read.

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