Aircraft Communication Topic 9 vhf omni range (vor)

FOR TRAINING PURPOSE ONLY

Subject Code: AFD31202

Malaysian Institute of Aviation Technology

Issue No : 000

Principles of VOR Navigation

The VOR System was introduce in 1945 with the introduction of VHF OmniRange (VOR) navigation stations.

VOR stations appear on the U.S. Aeronautical Charts as a compass circle centered over the station and oriented toward magnetic north.

Lines, known as radials, are drawn from the station out in the direction of a magnetic heading.

A VOR station broadcasts an infinite number of radials.

If the VOR system indicates that the aircraft is on a zero radial, this means that it is somewhere on a line drawn from the VOR station to magnetic north. – So VOR indication is independent of aircraft heading, whereas the ADF

indication changes with the aircraft heading as the ADF points to the station.




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VOR station shown on aeronautical chart




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The principle of VOR operation is based on the generation of radials, or The principle of VOR operation is based on the generation of radials, or magnetic bearings, by a ground station transmitter and their reception by an magnetic bearings, by a ground station transmitter and their reception by an airborne receiver. airborne receiver.

The receiver instrumentation unit determines which radial is passing through the aircraft position.

The determined radial is the angle between magnetic north and the aircraft as measured from the ground station.

The magnetic course from the aircraft when flying inbound to the station is the reciprocal of the radial




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VOR radials and magnetic courses




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VOR stations operate in the VHF frequency range from 108.00 MHz to 117.95 MHz– therefore limited to line-of-sight navigation– an aircraft flying at an altitude of 20,000 feet should be able to receive

VOR stations from as far as 200 miles away

The VOR stations are identified by the transmission of a three-letter Morse code group sent 10 times each minute.

In some cases, voice identification is added immediately following the Morse code signal.

A pilot determines the aircraft's bearing to or from a VOR station by: – first selecting the frequency of the desired station– then turning the omnibearing selector (OBS) until the "left/right' needle on

the course deviation indicator (CDI) centers.




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The reading on the OBS will indicate the radial of the aircraft is on regardless The reading on the OBS will indicate the radial of the aircraft is on regardless of its of its headingheading. . A "to/from" arrow is provided on the CDI for determining if the indicated A "to/from" arrow is provided on the CDI for determining if the indicated bearingbearing is to or from the station and a warning "flag" will appear if the is to or from the station and a warning "flag" will appear if the information is unusable. information is unusable. The VOR and the localizer portion of the instrument landing system operate in The VOR and the localizer portion of the instrument landing system operate in the same frequency rangethe same frequency range– one VHF navigation receiver is used for both functions. one VHF navigation receiver is used for both functions.

Usually, the VOR and localizer circuitry will be found in the same chassis as Usually, the VOR and localizer circuitry will be found in the same chassis as the VHF navigation receiver. the VHF navigation receiver.

If the VOR and localizer circuitry is contained within the instrument, the If the VOR and localizer circuitry is contained within the instrument, the combination is known as a combination is known as a converter-indicatorconverter-indicator. .

In light aircraft using panel-mounted equipment, the VHF navigation receiver In light aircraft using panel-mounted equipment, the VHF navigation receiver is sometimes located in the same chassis as the VHF communications is sometimes located in the same chassis as the VHF communications transceiver. transceiver.




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VOR Navigation Concepts

The concept of VOR navigation is based on: The concept of VOR navigation is based on: – detection two distinct signals contained within one carrier signal detection two distinct signals contained within one carrier signal

transmitted from a VOR ground station, andtransmitted from a VOR ground station, and– comparing the phase difference between them to derive the bearing from comparing the phase difference between them to derive the bearing from

the station the station This concept is analogous to a tower having a sharply focused beacon light This concept is analogous to a tower having a sharply focused beacon light which rotates at a constant speed, and a second light which flashes in all which rotates at a constant speed, and a second light which flashes in all directions (omni-directional) when the rotating beacon points to magnetic directions (omni-directional) when the rotating beacon points to magnetic north. north. The The bearingbearing from the tower can thus be determined by from the tower can thus be determined by multiplyingmultiplying the the speedspeed of the rotating beacon by the of the rotating beacon by the timetime required between when the fixed required between when the fixed (reference) light flashes and the rotating (variable) light passes by the (reference) light flashes and the rotating (variable) light passes by the observer. observer.




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VOR Navigation Concepts (cont’d)

A VOR station radiates a composite electromagnetic field from two ground-based antennas on the same carrier frequency.

The first antenna is omni-directional and radiates an amplitude-modulated reference signal. – The modulation frequency of the reference-phase signal varies from

9,480 Hz to 10,440 Hz at a rate of 30 times per second. – The resultant reference-phase signal then consists of a 9,960-Hz sub-

carrier, frequency-modulated at 30 Hz, that amplitude-modulates the RF carrier.




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The second antenna is a horizontal dipole which rotates at the rate of 1,800 The second antenna is a horizontal dipole which rotates at the rate of 1,800 revolutions per minute (30 revolutions per second) and produces a figure-revolutions per minute (30 revolutions per second) and produces a figure-eight electromagnetic field pattern. eight electromagnetic field pattern. – The RF field within one of the lobes is exactly in phase with the RF The RF field within one of the lobes is exactly in phase with the RF

radiated from the omni-directional field. radiated from the omni-directional field. – The in-phase lobe extends the omni-directional pattern on one side that The in-phase lobe extends the omni-directional pattern on one side that

result in a cardioids field pattern which rotates at the rate of 30 result in a cardioids field pattern which rotates at the rate of 30 revolutions per second. revolutions per second.




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The airborne VOR receiver detects the RF carrier whose amplitude is The airborne VOR receiver detects the RF carrier whose amplitude is varying at a rate of 30 Hz due to the rotation of the cardioids pattern. varying at a rate of 30 Hz due to the rotation of the cardioids pattern.

The carrier is also amplitude-modulated by the 9,960-Hz reference-phase The carrier is also amplitude-modulated by the 9,960-Hz reference-phase signal, which is frequency-modulated at 30 Hz on a subcarrier to distinguish signal, which is frequency-modulated at 30 Hz on a subcarrier to distinguish it from the 30-Hz variable-phase signal. it from the 30-Hz variable-phase signal.

The VOR airborne receiver detects the variable and reference-phase signals The VOR airborne receiver detects the variable and reference-phase signals and compares their phase difference to determine the aircraft's bearing. and compares their phase difference to determine the aircraft's bearing.

At magnetic north, both signals appear in phase. At magnetic north, both signals appear in phase.





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VOR System Operation

The VOR station produces the radial pattern by transmitting 30-Hz reference The VOR station produces the radial pattern by transmitting 30-Hz reference signals and 30-Hz variable-phase signals for comparison by the airborne signals and 30-Hz variable-phase signals for comparison by the airborne receiver. receiver. The 30-Hz variable-phase signal is an amplitude-modulated component of The 30-Hz variable-phase signal is an amplitude-modulated component of the VOR station RF signal. the VOR station RF signal. – This signal is generated by rotating the transmitting antenna pattern, This signal is generated by rotating the transmitting antenna pattern,

either mechanically or electronically, at 30 revolutions per second. either mechanically or electronically, at 30 revolutions per second. The station identification code and voice transmissions are also amplitude-The station identification code and voice transmissions are also amplitude-modulated components of the signal. modulated components of the signal. The sum of all modulation components from the station results in a The sum of all modulation components from the station results in a maximum of 90% modulation equally divided between the reference-phase maximum of 90% modulation equally divided between the reference-phase component, variable-phase component, and the remaining voice or code component, variable-phase component, and the remaining voice or code identification component. identification component.




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Magnetic north is the reference for Magnetic north is the reference for all VOR measurements. all VOR measurements. – At magnetic north, the 30-Hz At magnetic north, the 30-Hz

variable signal is in phase with variable signal is in phase with the 30-Hz reference signal. the 30-Hz reference signal.

At all other radials, the 30-Hz At all other radials, the 30-Hz variable signal lead or lag the 30-variable signal lead or lag the 30-Hz reference signal by the number Hz reference signal by the number of degrees from magnetic north to of degrees from magnetic north to the radial. the radial. For example, at 180For example, at 180oo from magnetic from magnetic north, the variable signal is 180north, the variable signal is 180oo out-of-phase with the reference out-of-phase with the reference signal.signal.

VOR System Operation (cont’d)

VOR phase relationships




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Figure on the right-side Figure on the right-side illustrates the phase relationship illustrates the phase relationship of the 30-Hz reference and of the 30-Hz reference and variable signals for an aircraft variable signals for an aircraft flying to a VOR station on the flying to a VOR station on the 260260oo radial. radial.

The 30-Hz reference component The 30-Hz reference component is applied to an FM detector that is applied to an FM detector that produces a 30-Hz reference produces a 30-Hz reference signal: signal: – positive when the subcarrier positive when the subcarrier

frequency is high (10,440 frequency is high (10,440 Hz) Hz)

– negative when the frequency negative when the frequency is low (9,480 Hz)is low (9,480 Hz) Phase relationships of a 260o VOR radial





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The 30-Hz variable component is applied to an AM detector that produces a 30-Hz variable-phase signal: – positive when the major

lobe of the radiated signal is toward the aircraft

When the 30-Hz variable and reference phases are compared, the difference is a direct measure of bearing from the VOR station to the aircraft (in this case, 260o).

Phase relationships of a 260o VOR radial





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The VOR circuitry derives The VOR circuitry derives to/fromto/from bearing information bearing information and and left/rightleft/right deviation deviation from the selected course: from the selected course: – by comparing the by comparing the

received bearing with received bearing with the setting of the the setting of the omnibearing selector. omnibearing selector.

An aircraft flying inbound An aircraft flying inbound to a VOR station on the to a VOR station on the 260260oo radial requires the radial requires the pilot to slew the OBS to pilot to slew the OBS to the 80the 80oo magnetic course magnetic course setting. setting.

Flying a VOR radial





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Since the 80o selected magnetic course is displaced by more than 90o from either side of the VOR radial, the to/from arrow points upwards indicating the aircraft is flying to the station.

The VOR circuitry produces left/right course deviation information in a manner similar to to/from deviation.

When the OBS is set at 80o, a deviation window of approx. plus or minus 10o (full scale on the left/right deviation bar on the CDI) is formed symmetrically around the 260o VOR radial.





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The VOR circuitry compares the selected OBS magnetic course with the The VOR circuitry compares the selected OBS magnetic course with the received VOR radial and positions the course deviation indicator bar to received VOR radial and positions the course deviation indicator bar to indicate the direction, indicate the direction, – the pilot must turn the aircraft to intercept the desired radial the pilot must turn the aircraft to intercept the desired radial

Thus, the deviation bar indicates where the selected VOR radial is located Thus, the deviation bar indicates where the selected VOR radial is located with respect to the aircraft's position. with respect to the aircraft's position.

The pilot keeps from deviating from the selected course by following the The pilot keeps from deviating from the selected course by following the command of the left/right deviation bar to maintain the bar in the center command of the left/right deviation bar to maintain the bar in the center position. position.





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VHF omnirange (VOR) is an electronic navigation system that enables a pilot VHF omnirange (VOR) is an electronic navigation system that enables a pilot to determine the bearings of the VOR transmitter from any position in its to determine the bearings of the VOR transmitter from any position in its service area. service area. – This is possible because the VOR ground station, or transmitter, This is possible because the VOR ground station, or transmitter,

continually broadcasts an infinite number of directional radio beams or continually broadcasts an infinite number of directional radio beams or radials. radials.

The VOR signal received in an airplane is used to operate a visual indicator The VOR signal received in an airplane is used to operate a visual indicator from which the pilot determines the bearings of the VOR station with respect from which the pilot determines the bearings of the VOR station with respect to the airplane. to the airplane.

VOR Transceiver System




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Omni bearing indicatorOmni bearing indicator




Issue No : 000 Block diagrams of VOR transmitter and receiver systems




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The diagram for the VOR ground station, or transmitter, shows a five-unit The diagram for the VOR ground station, or transmitter, shows a five-unit antenna array. antenna array.

The center loop of the antenna array continuously broadcasts the reference The center loop of the antenna array continuously broadcasts the reference phase signal, which is modulated at 30 Hz. phase signal, which is modulated at 30 Hz.

Two outputs are radiated by the diagonal pairs of comer antennas, Two outputs are radiated by the diagonal pairs of comer antennas, – the signals radiated from these pairs are modulated by 30 Hz and differ in the signals radiated from these pairs are modulated by 30 Hz and differ in

phase by 90phase by 90oo

Each pair of antennas radiates a figure-eight pattern, each pattern being Each pair of antennas radiates a figure-eight pattern, each pattern being displaced from the other by 90displaced from the other by 90oo both in space and in time phase. both in space and in time phase.

The resulting pattern is the sum of the two figure eight patterns and consists of The resulting pattern is the sum of the two figure eight patterns and consists of a rotating field turning at 1800 rpm, or 30 Hz. a rotating field turning at 1800 rpm, or 30 Hz.

VOR Transceiver System (cont’d)




Issue No : 000

The total effect of the radiation from the VOR transmitter is to produce two The total effect of the radiation from the VOR transmitter is to produce two signals whose phase characteristics vary in accordance with the direction signals whose phase characteristics vary in accordance with the direction (bearing) of the transmitter from the receiver. (bearing) of the transmitter from the receiver.

The two signals radiated due south (magnetic) of the transmitter are exactly in The two signals radiated due south (magnetic) of the transmitter are exactly in phase; phase; – hence an airplane flying magnetic north directly toward the VOR hence an airplane flying magnetic north directly toward the VOR

transmitter will show an indicated bearing of 0transmitter will show an indicated bearing of 0oo to the VOR station. to the VOR station.

The TO-FROM indicator will show that the airplane is flying to the station. The TO-FROM indicator will show that the airplane is flying to the station.

In a clockwise direction around the VOR station, the radiated signals become In a clockwise direction around the VOR station, the radiated signals become increasingly out of phase. increasingly out of phase.





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At 90At 90oo clockwise from the due south direction, the signals are 90 clockwise from the due south direction, the signals are 90oo out of out of phase, at 180phase, at 180oo they are 180 they are 180oo out of phase; at 270 out of phase; at 270oo they are 270 they are 270oo out of phase; out of phase; and at 360and at 360oo (0 (0oo) they are back in phase. ) they are back in phase. The phase difference of the two signals makes it possible for the receiver to The phase difference of the two signals makes it possible for the receiver to establish the bearings of the ground station. establish the bearings of the ground station. The directional bearings of VOR stations are set up in accordance with the The directional bearings of VOR stations are set up in accordance with the earth's magnetic field so that they may be compared directly with magnetic-earth's magnetic field so that they may be compared directly with magnetic-compass indications on the airplane. compass indications on the airplane. During the operation of VOR equipment on a particular heading, an airplane During the operation of VOR equipment on a particular heading, an airplane flying toward the VOR station will show a TO indication on the omni-indicator. flying toward the VOR station will show a TO indication on the omni-indicator. After the airplane passes the station, the indicator will show FROM, and the After the airplane passes the station, the indicator will show FROM, and the heading information will remain the same as it was. heading information will remain the same as it was.





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For example, if an airplane is flying toward a VOR station having a bearing of For example, if an airplane is flying toward a VOR station having a bearing of 200200oo the omnibearing indicator will show 200 the omnibearing indicator will show 200oo TO. TO. After the airplane passes over the VOR station, the indicator will show 200After the airplane passes over the VOR station, the indicator will show 200oo FROM. FROM. The carrier frequency of the VOR station is in the VHF range, between 112 and The carrier frequency of the VOR station is in the VHF range, between 112 and 118 MHz. 118 MHz. A modulation of 9960 Hz is placed on the carrier of the reference signal to A modulation of 9960 Hz is placed on the carrier of the reference signal to provide a subcarrier, which is modulated by a 30-Hz signal.provide a subcarrier, which is modulated by a 30-Hz signal.The 9960-Hz modulation on the original carrier wave is AM, and the 30-Hz The 9960-Hz modulation on the original carrier wave is AM, and the 30-Hz signal on the subcarrier is FM. signal on the subcarrier is FM. The carrier wave for the variable-phase signal is amplitude-modulated by a 30-The carrier wave for the variable-phase signal is amplitude-modulated by a 30-Hz signal. Hz signal.





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The VOR receiver mounted in an airplane may be an independent unit, or it The VOR receiver mounted in an airplane may be an independent unit, or it may operate in conjunction with the VHF communication radio. may operate in conjunction with the VHF communication radio.

Light aircraft typically use the combined unit, known as a VHF NAV/COM Light aircraft typically use the combined unit, known as a VHF NAV/COM radio. radio.

The VOR receiver receives both components of the VOR signal transmitted The VOR receiver receives both components of the VOR signal transmitted from the ground station and from these signals produces two 30-Hz signals: from the ground station and from these signals produces two 30-Hz signals: – one being the reference phaseone being the reference phase– the other being the variable phase the other being the variable phase

The angular distance between the two phases is applied to the omnirange The angular distance between the two phases is applied to the omnirange indicator, by which it is translated into usable heading information. indicator, by which it is translated into usable heading information.





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The onmirange indicator includes an azimuth dial, a LEFT-RIGHT deviation The onmirange indicator includes an azimuth dial, a LEFT-RIGHT deviation needle, and a TO-FROM indicator. needle, and a TO-FROM indicator.

When the VOR receiver on an airplane is tuned to a VOR ground station, the When the VOR receiver on an airplane is tuned to a VOR ground station, the LEFT-RIGHT indicating needle will be deflected either to the right or to the left LEFT-RIGHT indicating needle will be deflected either to the right or to the left unless the selected course on the omnirange indicator is in agreement with the unless the selected course on the omnirange indicator is in agreement with the bearing of the VOR ground station.bearing of the VOR ground station.

Once the pilot has tuned to the correct ground station frequency and selected Once the pilot has tuned to the correct ground station frequency and selected the correct course, the unit is ready for navigation. the correct course, the unit is ready for navigation.

For example, if the course-deviation indicator bar moves to the left, the pilot For example, if the course-deviation indicator bar moves to the left, the pilot knows the intended course is to the left of the aircraft. knows the intended course is to the left of the aircraft.

To correct the flight path, the pilot must turn the aircraft to the left. To correct the flight path, the pilot must turn the aircraft to the left.





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VOR Equipments system OperationVOR Equipments system Operation

A VHF navigation receiver operates in the same manner as a VHF A VHF navigation receiver operates in the same manner as a VHF communications receiver with the exception that the navigation receiver does communications receiver with the exception that the navigation receiver does not incorporate the use of a squelch circuit to disable the audio output when not incorporate the use of a squelch circuit to disable the audio output when the signal strength is below a certain threshold setting. the signal strength is below a certain threshold setting.

The audio output must be made available to the VOR converter or The audio output must be made available to the VOR converter or instrumentation unit at all times in order for it to derive navigational instrumentation unit at all times in order for it to derive navigational information from the composite VOR signal.information from the composite VOR signal.

The sensitivity of a typical superheterodyne navigation receiver will cause a The sensitivity of a typical superheterodyne navigation receiver will cause a three microvolt input signal modulated at 30% with a 1,000-Hz tone to three microvolt input signal modulated at 30% with a 1,000-Hz tone to produce 200 milliwatts output with a six dB signal-to-noise ratio. produce 200 milliwatts output with a six dB signal-to-noise ratio.

An automatic gain control is used to maintain not more than a three dB An automatic gain control is used to maintain not more than a three dB variation over a 5 millivolt to 50,000 millivolt input signal range. variation over a 5 millivolt to 50,000 millivolt input signal range.

Aircraft Communication Topic 9 vhf omni range (vor)

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Transcript of Aircraft Communication Topic 9 vhf omni range (vor)