VHF Omni Directional Radio Range (VOR)

Very High Frequency Omni-directional Radio Range

VHF Radio Range (VOR) is an ICAO approved short range navigational aid that emits Radials
Magnetic Bearings with a spacing of 1 degree which are called Radials
VOR is free from Static and Night effect making it fit for air navigation during day and night
VOR operates on the principle of Bearing by Phase Comparison between simultaneously transmitted Reference and Variable signals
Airborne equipment consists of a receiver with a dipole antenna and its indications are presented in RMI as well as CDI
VOR emits a horizontally polarised waves in the frequency range from 108 to 117.95 MHz in the VHF Band
Even first number after decimal point (108.6) are allotted to VOR while the odd number after decimal is allotted to ILS Localiser
VOR identification consists of a three letter morse code or station name which is broadcasted at least 7 times per minute
Present generation VOR stations transmit Automatic Terminal Information Service (ATIS) instead of identification code
Emission designator of VOR is A9W which means amplitude modulated, double side band carrier wave, composite system transmitting telemetry information or a transmission of measured quantity

VOR operates on a principle of Bearing by phase comparison of Reference and Variable waves
Reference wave is a 30 Hz frequency modulated constant phase omni-directional wave
Variable wave is a 30 Hz amplitude modulated variable phase wave
A Rotating Limacon is the polar diagram obtained by combining the two waves
VOR Receiver measures the phase difference between two signals and converts then to magnetic bearing
At Magnetic North, the reference and variable signals are in phase or the radial as well as phase difference is zero
In all the other directions, the value of phase difference between the reference and variable signals provides radial information

Doppler VOR avoids Site Error caused due to scalloped rays from uneven terrain at higher range in low altitudes
Doppler VOR has a wide base of 13.4 meters with 48 transmitters in a circle
Doppler effect is created by frequency modulation of the variable signal since FM has a lesser interference than AM
Phase shift is obtained by electronically rotating the antenna anti-clockwise at 30 revolutions per second
The airborne equipment finds the phase shift to provide radials similar to conventional VOR

Designated operational coverage of a VOR is a zone where the signals are protected from interference from other ground stations
DOC is expressed as distance and height in sequence like DOC 50/25 which means a DOC of 50 NM up to 2500 feet
High altitude, low altitude and Terminal VOR equipments have different Designated Operational Coverage

Combined accuracy levels of the Ground Beacon and Airborne equipment would determine the overall accuracy of VOR
Proper selection of site restricts beacon accuracy to less than one deg which can be further reduced by using Doppler VOR
Airborne equipment accuracy is restricted to less than three degrees
Total equipment error which is a combination of both airborne and ground equipment is less than five degrees

VOR Ground based monitor would remove the ident of ground station if the error exceeds permissible limits
Permissible limits include radials error of one degree or transmission power reduction by 15%
Failure of monitoring device would also trigger an error displaying a red or yellow warning flag

Cone of airspace vertically above the VOR is called its Cone of Ambiguity or Confusion where the indications are unreliable
ICAO allows cone of ambiguity to be up to 40 degree but normally does not exceed 80 degree elevation angle

An air-route is designated as an airway only if aircrafts obtain VOR radials within acceptable accuracy limits throughout the roure
Accuracy of VOR is 5 degrees and width of an airway is 5 NM on either side of the centre line
As per 1 in 60 rule, inaccuracy of 5 degrees translate to 5 NM in 60 NM therefore airways should have a VOR at every 120 NM

VOR are classified as CVOR, BVOR, DVOR, TVOR, VOT and VORTAC
CVOR is a Conventional VOR and BVOR is a Broadcast VOR which gives weather and airfield information
DVOR is a Doppler VOR and TVOR is a Terminal VOR with low power for use in airfield vicinity
VOT is a Test VOR which must indicate 360 (+ or – 4 degrees) in the aircraft indicators
VORTAC is a VOR co-located with TACAN while DBVORTAC refers to a combination of DVOR, BVOR and VORTAC

Radio Magnetic Indicator (RMI) indicates magnetic bearing from both NDB and VOR on a Compass card
Head of the pointer in an RMI displays QDM while the Tail of the pointer displays QDR
VOR pointer of RMI obtains the radials from VOR and displays it on a dial
RMI Pointer selected to ADF obtains relative bearings from ADF and converts it to magnetic bearing for display

CDI or OBI provides VOR indications as a deviation from a radial selected on Omni Bearing Selector (OBS)
Horizontal deviation bar of OBI indicates the action required to intercept the selected radial
TO – FROM indicator shows whether the aircraft is flying towards or from the VOR
CDI in VOR mode has an accuracy of 10 degrees on each side which would translate to 2 degrees per dot on a 5 dot indicator
Heading of the aircraft is ignored while providing the Fly left or right indication

HSI provides Turn Left or Right to intercept a selected radial based on the Heading and Selected Radial
Electronic HSI displays Terrain and Weather from Airborne Weather Radar (AWR) in addition to turn guidance
Distance inputs to EHSI are provided by Distance Measuring Equipment (DME)
Proximity Warning detected by Traffic Collision Avoidance System (TCAS) are also displayed on the EHSI