The Very High Frequency Omnidirectional Range (VOR) system is a type of air navigation system. Though older than GPS, VORs are very commonly used and have been a very reliable source of navigation information since the 1960s.
VOR System Components:
A VOR system is made up of a ground component and aircraft receiver component.
The VOR ground stations are located both on and off airports to provide guidance information to pilots both en route and during arrival and departure. The VOR system is expansive, and pilots can use VORs to navigate throughout the country.
Aircraft equipment includes a VOR antenna, a VOR frequency selector, and a cockpit instrument. The instrument type varies, but will consist of one of the following: an Omni-Bearing Indicator (OBI), Horizontal Situation Indicator (HSI) or a Radio Magnetic Indicator (RMI), or a combination of two different kinds.
Distance Measuring Equipment (DME) is often times collocated with a VOR to give pilots a precise indication of distance from the VOR station.
VORs typically have AM voice broadcast ability. Each individual VOR has its own Morse code identifier that it broadcasts to pilots. This ensures that pilots are navigating from the correct VOR station, as there are often many VOR facilities within range of a single aircraft.
How it Works:
The VOR ground station is aligned with magnetic north, and it emits two signals -- a 360-degree sweeping variable signal and an Omni-directional reference signal. The signals are compared by the aircraft's receiver, and a phase difference between them is measured, giving a precise radial position of the aircraft and displaying it on the OBI, HSI or RMI.
VORs come with different service volumes and dimensions: High, Low and Terminal. High-altitude VORs can be used up to 60,000 feet and 130 nautical miles wide. Low-altitude VORs service aircraft up to 18,000 feet and up to 40 nautical miles wide. Terminal VORs go up to 12,000 feet and 25 nautical miles. The network of VORs typically provides thorough coverage along published VFR and IFR routes.
As with any system, the VOR comes with some faults. While it's more accurate and usable than the old NDB system, it's still a line-of-sight instrument. Pilots flying in low or mountainous terrain may find it difficult to identify a VOR facility.
In addition, there exists a "cone of confusion." For a brief period when an aircraft flies near or over the top of a VOR station, the aircraft instrument will give erroneous readings.
Finally, VOR ground systems require constant maintenance, and they are commonly out of order for short periods of time while maintenance is performed.
Practical Applications of the VOR Navigation System:
After tuning in a VOR facility's frequency and identifying that the Morse code is correct, the pilot will be able to determine which radial to or from the VOR station the aircraft is located. The OBI, HSI or RMI indicator in the cockpit looks like a compass or a heading indicator, with a superimposed Course Deviation Indicator (CDI) needle on it. The CDI will align itself with the radial that the aircraft is on. Paired with DME, a pilot can determine a precise location from the station.
In addition, the use of two VOR stations makes determining a precise location even more accurate by using cross-radials, even without DME.
Pilots fly certain radials to or from VORs as a primary way of navigating. Airways are often designed to and from VOR facilities for ease of use.
In its more basic form, a VOR facility can be used to go directly to an airport. A large number of VOR facilities are located on airport property, allowing even student pilots to fly directly to a VOR to find the airport easily.
The VOR system is at risk of being decommissioned by the FAA due to the popularity of new technology such as GPS, WAAS and ADS-B. For the time being, pilots will continue to use VORs as a primary navigational aid, but in the distant future, as more and more aircraft are equipped with GPS receivers, VORs will most likely be retired from use.