1. VHF Omnidirectional Range (VOR)
Lecture 4:
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. Learning Outcomes
At the end of this lecture, the student should be able to:
Explain the basic workings of VOR
Describe the purpose of VOR
Explain advantages and disadvantages of VOR

3. VHF Omni-directional Range
The VOR system is present in three slightly differentnavigation aids (NAVAIDs): VOR, VOR/DME, and VORTAC. By itself it is known as a VOR, and it provides magnetic bearing information to and from the station. When DME is also installed with a VOR, the NAVAID is referred to as a VOR/DME. When military tactical air navigation. (TACAN) equipment is installed with a VOR, the NAVAID is known as a VORTAC. DME is always an integral part of aVORTAC. Regardless of the type of NAVAID utilized (VOR, VOR/DME or VORTAC), the VOR indicator behaves thesame.
The prefi x “omni-” means all, and an omnidirectional range is a VHF radio transmitting ground station that projects straight line courses (radials) from the station in all directions.
From a top view, it can be visualized as being similar to the spokes from the hub of a wheel. The distance VOR radials are projected depends upon the power output of the transmitter.
The course or radials projected from the station are referenced to magnetic north. Therefore, a radial is defined as a line of magnetic bearing extending outward from the VOR station.
Radials are identified by numbers beginning with 001, which is 1° east of magnetic north, and progress in sequence through all the degrees of a circle until reaching 360. To aid in orientation, a compass rose reference to magnetic north is superimposed on aeronautical charts at the station location.
VOR ground stations transmit within a VHF frequency band of 108.0– MHz Because the equipment is VHF, the signals transmitted are subject to line-of-sight restrictions. Therefore, its range varies in direct proportion to the altitude of receiving equipment.
Generally, the reception range of the signals at an altitude of 1,000 feet above ground level (AGL) is about 40 to 45 miles. This distance increases with altitude. For VOR radio navigation, there are two components required: ground transmitter and aircraft receiving equipment. The ground transmitter is located at a specific position on the ground and transmits on an assigned frequency.
The aircraft equipment includes a receiver with a tuning device and a VOR or omninavigation instrument.
The navigation instrument could be a course deviation indicator (CDI), horizontal situation indicator (HSI), or a radio magnetic indicator (RMI).
Each of these instruments indicates the course to the tuned VOR.
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.

4. Introduction
VOR, short for VHF Omni-directional Range, is a type of radio navigation system for aircraft.
VOR provide MAGNETIC BEARING information to and from the station.
“Omni-” means all and an Omni-directional range means VOR station transmits signal in all directions.
VOR function as marking for the BEGINNING, CENTER-LINE and the END of airways.
In short word, VOR guide an aircraft from point A to point B, from point B to point C.
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.

5. VOR antenna at vertical
VOR Equipment
VOR antenna at vertical
tail of aircraft
VOR aircraft equipment
-VOR antenna at
vertical tail of aircraft
-VOR receiver & indicator
inside cockpit
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.
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.
VOR Ground Station (antenna)

6. VOR station for broadcast the signal
VOR Ground Antenna
Stationary Antennas
Rotating Antennas
The VOR ground station is oriented to magnetic north.
Consists of :
Single Stationary Antenna at the centre
Rotating antennas
It produces 360° radials/tracks at 1° spacing.
These 360 bearings are known as RADIALS
VOR ground installations are strategically located along air routes and airport to ensure continuity of guidance.
VOR station for broadcast the signal

7. VOR ground antenna
The VOR ground antenna is oriented to magnetic north.
Consists of :
Single Stationary Antenna at the centre
Rotating antennas
It produces 360° radials/tracks at 1° spacing.
These 360 bearings are known as RADIALS
VOR ground installations are strategically located along air routes and airport to ensure continuity of guidance.

8. RADIALS 360 Magnetic North 045 315 135º 270 090 135 225 180
VOR receiver gives 1 LOP called a Radial
135
225
180

9. PRINCIPLE OPERATION OF VOR
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."

10. How VOR works
VOR receiver in the cockpit is tuning to the specific frequencies assigned for that VOR ‘s airport.
It is VHF frequency which is between MHz.
After entering the frequency, the volume control should be turned up in order to confirm that the three letter identification code (Morse Code) is correct.
For example, KLIA airport has a VOR known as VKL-Victor Kilo Lima
The VOR station on the ground transmits two signals at the same time; one signal is constant in all directions, while the other signal is rotated about a point.
One from stationary antenna, while the other from rotating antenna.
The VOR station on the ground transmits two signals at the same time; one signal is constant in all directions, while the other signal is rotated about a point. The VOR on-board equipment receives both signals, looks electronically at the phase difference between the two signals, and interprets the result as the radial from the station. The ground station is calibrated in a way that the two signals are “in phase” (match each other) towards magnetic north on the position where this station is located. So, the required radial from the ground station is in fact the phase difference in degrees between the two signals transmitted from the ground station.
The operation of the VOR on-board equipment starts with the tuning of the desired frequency.
After entering the frequency, the volume control should be turned up in order to confirm that the three letter identification code (Morse Code) is correct.
Identification process is very important and the VOR information received can not be trusted unless the ground station has been positively identified.
Stationary antenna transmits constant signal in all directions and contains VOR station's identifier information given in MORSE CODE.
For example, KLIA airport has a VOR known as VKL-Victor Kilo Lima

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

14. VOR indicator Omnibearing Selector (OBS).
Allows pilot to select desired course/radial to fly
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.

16. Advantages of VOR More accurate & precise flying: Reliable:
The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree.
Reliable:
Can be used day and night.
Multiple number of route :
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.
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

17. Disadvantages of VOR
Signal s can not be received at low altitudes (below 1000ft)
VORs are sensitive to the interference of terrain. The nearest mountains and buildings cause the VOR bearings to be stopped and interrupted.
Other disadvantages is VOR equipments are costly to maintain.

18. Question Bank
VHF Omni directional range (VOR) is the primary navigational aid used by civil aviation for en-route navigation.
List out VOR equipments (2M)
Explain how VOR works (8M) 2.
What are the advantages of VOR (6M)
What are the disadvantages of VOR (6M)