VERY HIGH FREQUENCY OMNIDIRECTIONAL RANGE (VOR)

Very high frequency Omni-directional Range equipment (VOR)

The VOR (Very high frequency Omni-directional Range) automatically gives the pilot the direction of the aircraft with respect to magnetic north.

The VOR system consists of one transmitting station on the ground and a VOR receiver in the air craft. The VOR ground stations are either situated on the air field or more often along the air ways in order to provide en-route navigation.

The VOR operates on principle that the phase difference between two signals can be employed as a means of determining azimuth (direction and bearing) if one of the signals maintain a fixed phase through 360 degrees (so that it may be used as a reference) and the phase of the other is made to vary as a direct function of azimuth. The phase difference between these two signals will indicate the azimuth of the aircraft with respect to the ground station. In practice the VOR determines direction of the aircraft from the VOR station by comparing the phase between a reference signal (REF) and a variable signal (VAR). 

The conventional 30 Hz REF signal is on a 9960 Hz frequency modulated (FM) subcarrier. The amplitude modulated (AM) VAR signal is conventionally derived from a directional antenna array rotating electronically at 30 times per second (30Hz).

VOR measurement of radial angle 


When the signal is received in the aircraft, the two 30 Hz signals are detected and then compared to determine the phase angle between them. The phase angle by which the AM signal lags the FM subcarrier signal is equal to the direction from the station to the aircraft, in degrees from local magnetic north, and is called the radial angle.   
 

Site Errors in VOR

The VOR is vulnerable to multi-path errors caused by poor terrain that result into course roughness and bends, and course scalloping leading to measurement errors in the aircraft receiver.

Polarization errors also arise due to delicate balance between the predominant horizontal polarization and the unwanted vertical polarization that may arise due to electronic rotation of the antenna system. Any slight imbalance in wave polarization is likely to impact negatively on the accuracy of measuring instruments. This is one why this study chose to investigate effects of roofing materials on wave polarization.