1. VHF Omnidirectional Range (VOR)
1. Introduction
Objectives of navigation:
Know your position
Efficient use of fuel
Maintain a flight schedule
Avoid other air traffic
Avoid ground-to-air missiles and anti-aircraft artillery (known sites)
Minimize exposure to enemy radar

2. Outlines The uses of a VOR VOR Equipment How VOR work? Advantages VOR
Disadvantages of VOR

3. Introduction
VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft.
VOR is the primary navigational aid (NAVAID) used by civil aviation in the National Airspace System (NAS).
VHF omnirange (VOR)
Do not be confused by its name, VOR stations provide bearing information relative to the aircraft position.
VOR stations operate in the MHz band with a channel spacing of 50 kHz or 100kHz.
Each station transmits its identification via a Morse code modulated tone.
A reference 30 Hz signal is FM modulated onto the carrier.
A secondary signal is sent by a directed (cardioid) antenna that spins at 30 rev/sec.

4. The uses of a VOR
Marking the BEGINNING, the END and CENTER-LINE of airways. In short word, VOR guide an aircraft from point A to point B.
As a let-down aid at airport based on procedures.
As a holding point for aircraft
As a source of en-route navigational position lines.

5. VOR Equipment

6. VOR Equipment Ground VOR transmitter Aircraft’s antenna:
For low-speed a/c the antenna is whip type fitted on the fuselage.
For high-speed aircraft it is a blade type on vertical tail.
Aircraft’s VOR receiver in the cockpit:

7. Ground VOR transmitter
VOR station for broadcast the signal

8. Ground VOR transmitter
The VOR ground station is oriented to magnetic north.
It produces 360° radials/tracks at 1° spacing.
These 360 bearings are known as RADIALS
When DME is installed with the VOR, it is referred to as a VOR/DME.
When military tactical air navigation (TACAN) equipment is installed with the VOR, it is known as a VORTAC.
VOR ground installations are strategically located along air routes and airport to ensure continuity of guidance.
It is essential to remember the following points concerning the VOR and its operation:
A radial is a magnetic bearing from the VOR beacon The Deviation Indicator can be centred on either of two bearings by the OBS.
These bearings will be 180° apart with the ambiguity automatically resolved by the TO/FROM indicator The VOR system, in practice, should be considered to be accurate only to the order of ± 5°
As the VOR propagation is ‘line of sight’, the distance at which a VOR beacon can be received increases with aircraft elevation above the beacon
The VOR beacon must be identified aurally before information derived from the beacon is used
The Omni Bearing Indicator always indicates magnetic bearings TO or FROM the VOR beacon—never relative bearings
Heading of the aircraft at any instant does not affect VOR bearings. The bearing obtained depends on the aircraft location with relation to the VOR beacon
Bearing selection by the OBS is by 1 degree intervals. When a bearing is selected, full travel of the Deviation Indicator from one side to the other represents 20° of azimuth, that is 10° either side of the selected bearing.
Magnetic north is used as the baseline for measuring the phase relationship between the reference and variable signals. The two signals are aligned so that at magnetic north they are exactly in phase. As can be seen in Figure 3, a phase difference, which is analogous to the time difference between the white flash and sighting the green light, exists at any other point of azimuth around the beacon. This phase difference is measured electronically and converted to degrees of angle by the aircraft airborne equipment thus identifying the aircraft position in azimuth around the beacon. The information is presented visually by an indicator on the instrument panel.
An identification signal of two or three Morse Code letters is transmitted by the VOR beacon every ten seconds. It is also possible to transmit voice identification or other information, that is, meteorological information.

9. PRINCIPLE OPERATION OF VOR

10. How VOR works
VOR station provide BEARING information relative to the aircraft position.
VOR stations broadcasts a VHF signal between MHz with adequate power to provide coverage.
The basic principle of operation of the VOR is very simple, VOR stations transmit 2 signals at the same time
First signal is transmitted by single antenna in the center, this signal is constant in all direction and contains station's identifier information given in MORSE CODE. For example, KLIA airport has a VOR known as VKL-Victor Kilo Lima.
While, the second signals are transmitted by other antennas that circling around the single antenna.
VHF omnirange (VOR)
Do not be confused by its name, VOR stations provide bearing information relative to the aircraft position.
VOR stations operate in the MHz band with a channel spacing of 50 kHz or 100kHz.
Each station transmits its identification via a Morse code modulated tone.
A reference 30 Hz signal is FM modulated onto the carrier.
A secondary signal is sent by a directed (cardioid) antenna that spins at 30 rev/sec.
The aircraft’s antenna received both signal and interprets the results as RADIALs (Bearing) from the station.
VOR is seldom used on its own but with a DME. Explain how VOR works and how DME works.
How VOR works
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 a radial from the station.
VORs operate within the to MHz frequency band and have a power output necessary to provide coverage within their assigned operational service volume.
They are subject to line-of-sight restrictions, and the range varies proportionally to the altitude of the receiving equipment.
The VOR system uses the phase relationship between a reference-phase and a rotating-phase signal to encode direction.
The carrier signal is omni-directional and contains the amplitude modulated (AM) station Morse code or voice identifier.
The reference 30 Hz signal is frequency modulated (FM) on a 9960 Hz sub-carrier.
A second, amplitude modulated (AM) 30 Hz signal is derived from the rotation of a directional antenna array 30 times a second.
Although older antennas were mechanically rotated, current installations are scanned electronically to achieve the same result with no moving parts.
When the signal is received in the aircraft, the FM signal is decoded from the sub carrier and the frequency extracted.
The two 30 Hz signals are then compared to determine the phase angle between them.
The phase angle is equal to the direction from the station to the airplane, in degrees from local magnetic north, and is called the "radial."

11. How VOR works
When aircraft receives these two signals, an aircraft VOR receiver will measure the phase angle different between these two signals.
This phase angle different is translated as the magnetic BEARING which tell the pilot the aircraft direction to the VOR station.
This bearing angle also known as RADIALS.

12. VOR, short for VHF Omni-directional Radio Range, is a type of radio navigation system for aircraft. A VOR ground station broadcasts a VHF radio composite signal including the station's identifier in morse code (and sometimes a voice identifier), and data that allows the airborne receiving equipment to derive 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 to True North. This line of position is called the "radial" from the VOR. The intersection of two radials from different VOR stations on a chart allows for a "fix" or approximate position of the aircraft.

13. The Principle of the VOR
360
Magnetic North
045
315
135º
270
090
VOR receiver gives 1 LOP called a Radial
135
225
180

14. The Principle of the VOR

16. Aircraft’s VOR receiver
VOR receiver in aircraft to receive and interpret the signal.
Show the bearing/direction of the aircraft from the VOR station.
The Receiver take the signals and measure the phase angle difference between the reference signal and the variable signal. As the phase angle difference is a definite fixed amount for each radial, it is therefore possible to determine the bearing of the aircraft from the VOR beacon, and this information can be presented visually.
Similarly, if the equipment can be adjusted to a desired bearing (or phase angle difference) and indicate the relationship of the aircraft to the bearing and when the aircraft has reached the bearing, it is possible to preset tracks and then fly to and continue along them.
The visual indicators comprise a manually operated Omni Bearing Selector, a Deviation Indicator, and a TO/FROM Indicator, and these are normally combined in one instrument known as an Omni Bearing Indicator.
The information available from the navigation circuits is presented on the Deviation Indicator and the TO/FROM Indicator with relation to the setting of the Omni Bearing Selector. Information derived from the VOR may also be presented on a Radio Magnetic Indicator.

17. VOR indicator Omnibearing Selector (OBS).
Course deviation indicator (CDI).
The needle centers when the aircraft is on the selected radial.
TO/FROM indicator.
Shows whether the selected course will take the aircraft TO or FROM the station.
Flags:
The device that indicates a usable or an unreliable signal may be an “OFF” flag.

18. VOR indicator Omni bearing selector Receiver Course Deviation
Indicator (CDI)
To/From indicator.
CDI – needle that moves across the face of the instrument to indicate aircraft position. Each dot is 2 degrees.
OBS – allows pilot to select desired course/radial to fly
- must align with DG, otherwise reverse sensing
Ambiguity Indicator – indicates if selected course will take aircraft to or from the station
- station passage
- cone of confusion
Omni bearing selector

19. Advantages of VOR
The VOR is the most common navigation instrument presently on aircraft panels.
Accurate:
The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree.
Reliable:
Space wave transmission, travels in straight line, not effect by interference, not affected by sky waves, can be used day and nighr.
Advantages of VOR
The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree.
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, it is always accurate.
Benefits
More accurate, precise flying
Reliable
Not susceptible to interference
Voice Capable
Errors/Negatives
Costly to maintain
Line-of-sight

20. Advantages of VOR
Provide multiple number of route ‘towards’ or away from each station.
These routes are like invisible highways , which the pilot can navigate away from any location.
VOR: Information
AZIMUTH in VOR is a clockwise angle between magnetic
north and the line connecting the VOR and the aircraft. The
indication is displayed on an “Omni Bearing Indicator” in the
aircraft.
The COURSE is the information whether aircraft is flying to
the left or right of, or exactly on the pre-selected course line.
The course information is displayed on a “Flight Path
Deviation Indicator”.
TO-FROM indication tells the pilot whether an aircraft is
approaching to or moving away from VOR stations. 

21. Advantages of VOR
The VOR enables a pilot to select, identify, and locate a line of position from a particular VOR beacon. The following information can be obtained:
The magnetic bearing of the aircraft from the VOR beacon
The position of the aircraft
Provides multiple number of courses (invisible highways).
Determine a fix. A pilot can tune two VOR stations and reading of their bearings.
VOR is position sensitive, pilot can fly a straight line without error.
Straight line is achieved by maintaining a line of constant bearing
The aircraft can accurately kept on course.
VOR frequencies are free of static interference and therefore give reliable indicatio

22. Disadvantages of VOR
The range of VOR signal is limited to ‘line of sight’ reception. Thus, the range depends on:
Aircraft’s Altitude - minimum 1000 ft to receive the VOR signal
The ground VOR transmitter height- The lower the transmitter the lower the range.
The nearest mountains and buildings cause VOR bearings to be stopped and interrupted.
Other disadvantages is VOR equipments are costly to maintain.

23. The VHF Omni-directional Radio Range, the abbreviations for which are ‘VOR’ and ‘Omni’, enables a pilot to determine the direction of his aircraft from any position to or from a VOR beacon, and, if necessary, track to or from the beacon on a selected bearing. VOR is a Very High Frequency (VHF) navigation aid which operates
Because it is a VHF aid, its ground to air range is limited to ‘line of sight’ reception which is typical of VHF transmission. The range achieved is dependent, therefore, on the siting of the VOR beacon with relation to surrounding terrain, and on the height at which the aircraft is flying. (See Figure 1).
As a VHF navigation aid, the VOR is static-free, and the information given by it is displayed visually on easily read and interpreted cockpit instruments. An infinite number of bearings can be obtained and they may be visualized as radiating from the beacon like spokes from the hub of a wheel. However, for practical purposes the number of bearings can be considered to be limited to 360, one degree apart, and these 360 bearings are known as radials. A Radial is identified by its magnetic bearing outbound from the VOR beacon
The basic principle on which a VOR operates can be understood from a study of a simple analogous optical system which is illustrated at Figure 2. Assume that there are two lights located at the same position, one of which is a rotating green light which is only seen when the beam is directed at an observer, and the other a white light which is visible from all directions. The green light rotates at 10 degrees per second, that is, it completes a revolution in 36 seconds, and, when it is directed to magnetic north, it trips a switch to cause the white light to flash momentarily. An observer with a stop watch can note the time interval between a white flash and the next green flash, and, knowing the angular rate of the rotating green light, can determine his bearing from the lights. For example, if the observer sees the green light 10 seconds after a white flash, then the observer is on a bearing of 100° magnetic from the lights, that is, the 100° radial. The observer’s bearing to the lights is, of course, the reciprocal of 100°, viz., 280°.
Figure 2. Analogous Optical System 􀃘
These notes are a reproduction of a booklet originally 2-1 published by

24. VOR Test (VOT) Facility
VOR accuracy must be calibrated/checked for every months.
One acceptable way to check VOR accuracy is with a VOR (VOT) Test Facility
A VOT is a low-power Omni station and it transmits only a single radial, the 360° radial.
To calibrate a VOR, the pilot tunes in the VOT frequency while on the ground.

28. Exercise Define VOR and give two uses of VOR. Explain how VOR works
Explain 2 advantages of VOR
Explain 2 disadvantages of VOR