VHF Omni-directional Range (VOR)

Figure 8.1 A combined VOR/DME

The VHF Omni-directional Range (VOR) was adopted as the standard short range navigation aid in 1960 by ICAO. It produces bearing information usually aligned with magnetic north at the VOR location. It is practically free from static interference and is not affected by sky waves, which enables it to be used day and night. When the VOR frequency is paired with a co-located Distance Measuring Equipment (DME) an instantaneous range and bearing (Rho-Theta) fix is obtained. The equipment operates within the frequency range of 108 - 117.95 MHz.

VOR has the following uses:

•Marking the beginning, the end and centre line of airways or sections of airways.

•As a let-down aid at airfields using published procedures.

•As a holding point for aircraft.

•As a source of en route navigational position lines.

Figure 8.2 A VOR Polar Diagram

VHF Omni-directional Range (VOR)

(VOR) Range directional-Omni VHF 8

The Principle of Operation

VOR bearing is obtained by phase comparison:

•An aircraft’s VOR receiver measures the phase difference (angular difference) between two signals from the VOR transmitter:

◦◦ a 30 Hz frequency modulated omni-directional, reference signal which produces constant phase regardless of a receiver's bearing from the VOR, and

◦◦ a 30 Hz amplitude modulated variable phase (directional) signal created by the rotating transmission pattern (limaçon).

•The 30 Hz FM reference signal is synchronized with the 30 revs/sec rotating directional AM signal (limaçon) such that:

◦◦ the two 30 Hz modulations are in phase to an aircraft’s VOR receiver when it is due magnetic north of the VOR beacon, and

◦◦ the phase difference measured at any other point will equate to the aircraft’s magnetic bearing from the VOR.

The two 30 Hz signals are modulated differently to prevent interaction and merging at the aircraft’s receiver. The rotating limaçon polar diagram, which provides the directional information, is created by combining the polar diagrams of the rotating loop and reference signal. In early VORs the loop rotation was mechanical; modern VORs use electronic circuitry.

Phase Difference 000°

N

Phase Difference 270°

W + - E

S

Phase Difference 090°