The VHF Omnidirectional Range navigation system, VOR, was probably the most significant aviation invention other than the jet engine. With it, a pilot can simply, accurately, and without ambiguity navigate from Point A to Point B. The widespread introduction of VORs began in the early 1950s and 50 years later it remains the primary navigation system in the overwhelming majority of aircraft. VHF omnidirectional radio range (VOR), is a type of short-range radio navigation system for aircraft, enabling aircraft to determine their position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons with a receiver unit. It uses radio frequencies in the very high frequency (VHF) band from 108 to 117.95 MHz. Developed in the US beginning in 1937 and deployed by 1946, VOR is the standard air navigational system in the world, used by both commercial and general aviation. There are about 3000 VOR stations around the world. It is practically free from static and night effect therefore is a reliable navigational aid by day and night.
VOR Ground Station (antenna)
The prefix “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. TWO CATEGORIES OF VOR
1. Normal VOR Beacon for en-route navigation- has radio frequency carrier output of about 200 Watts to provide a service range of up to 200 nautical miles. (Category A) 2. Terminal VOR- has a lower output of about 50 Watts to provide the limited coverage(25 nautical miles) required for approach and let down to an airport. (Category B) BASIC PRINCIPLE OF OPERATION
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. VOR stations broadcast a VHF radio composite signal including the station's identifier, voice (if equipped), and navigation signal. The identifier is typically a two- or three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts. The navigation signal allows the airborne receiving equipment to determine a magnetic bearing from the station to the aircraft (direction from the VOR station in relation to the Earth's magnetic North at the time of installation). VOR stations in areas of magnetic compass unreliability are oriented with respect to True North. A VOR ground station sends out a master signal, and a highly directional second signal that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle (bearing) to the station can be determined. This bearing is then displayed in the cockpit of the aircraft, and can be used to take a fix as in earlier radio...