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VOF-VHF Omnidirectional Range basics

VOR avionic system is a short range navigational aids and it stands for VHF Omnidirectional Range. VOR Provides Aircraft radial with respect to a ground station. In other words, the VOR system only informs us of the aircraft location as an entity seen by a ground VOR transmitter. However, we have no knowledge whatsoever on the heading of the aircraft. The radial of the aircraft is obtained by taking the phase difference of two signals reference Phase Signal (R) & Variable Phase Signal (V) transmitted by the ground station.

Following are applications of VOR:
•  V-Airway: Used to assign highways in the sky.
•  Track intercept: To be able to get back to the track due to cross wind. Simply fly toward the needle of the VOR display known as the course deviation indicator to ensure that the aircraft is going in the direction of the VOR ground station.
•  Triangulation Position Fix: To know where the aircraft is located, tune VOR receiver to at least two VOR ground stations and obtain the 2D position fix.

VOR,VHF Omnidirectional Range transmit system

Figure-1 depicts, VOR transmit system. On the ground VOR has following specifications.
•  Frequency of operation: VHF from 108 to 118 MHz
For Approach NAV: 108 to 112 MHz
For Short Range NAV: 112 to 118 MHz
Number of Channels: 100
•  Two signals are emitted at the same time with following characteristics:

Reference Phase Signal:
•  Designated as symbol 'R'
•  Modulation: FM
•  Rate: 30 Hz
•  Type: Non-directional

•  Variable Phase Signal:
•  Designated as Symbol 'V'
•  Modulation: AM
•  Rate: 30 revolutions per second (rps) equal to 30 Hz
•  Type: Rotational

•  The VOR transmitter sends a Morse ID code to the aircraft VOR receiver.
•  VOR transmitter is also used for two way voice communication.

VOR receive system

VOR receive system has following specifications:
•  Frequency of operation: VHF
•  The phase difference between 'R' and 'V' is calculated.
•  Aircraft Course Deviation Indicator(CDI) shows the VOR radial reading. If CDI becomes defective a red flag will appear.



Advantages

VOR is more efficient than ADF since its indicator point to the ground VOR transmit system. Whereas ADF provides a relative bearing corresponding to the offset between the aircraft longitudinal axis and the NDB, and hence does not point to the ground base beacon.

Disadvantages

•  Error is approx. about +/- 2 Degree.
•  It is limited to LOS due to VHF band operation.
•  VOR signals are either reflected or blocked or distorted due to buildings,Mountains,Fences, power lines etc.
•  At higher altitude interference may occur between two ground stations operated at the same frequency. •  VOR does not provide the aircraft heading, it only points to the ground station.

Though GPS is taking place of VOR, VOR is still in use as back up navigational aid system in case of satellite link failure.

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