What are those strange white cones in fields and what do they have to do with aviation? Although they’ve been around since the 1930s and widely in use since the 1970s, VOR (pronounced “VEE OH ARE”) stations are uncommon enough to be foreign and mysterious to those who are unfamiliar with them. VOR navigation includes a ground-based element (those white cones, called stations) and a receiver component installed in an aircraft.
Long before GPS was available for aircraft navigation, VOR stations guided aviators around the world. Although this technology is aging and many VORs are being decommissioned, VORs still play an important role in aviation. The VOR infrastructure itself is currently being repurposed as a backup navigation service that can be used during GPS outages.
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This backup infrastructure is known as the VOR Minimum Operational Network (VOR MON). According to the FAA’s Aeronautical Information Manual (AIM), “the key concept of the MON is to ensure that an aircraft will always be within 100 nm of an airport with an instrument approach that is not dependent on GPS.”
FAA knowledge tests and private pilot practical tests will include some discussion of VOR navigation. When going over the elements of a sectional chart, an examiner may point out a blue circle with radial marks and ask you what it is. Private pilot and instrument rating candidates may be asked to explain the difference between a VOR, a VOR-DME, and a VORTAC.
This article will discuss what a VOR is and how VOR navigation works.
VOR Definition
VOR stands for very high frequency omni-directional range and is a navigation aid (navaid). At the most simple level, a VOR is a type of navigation system for aircraft, using very high frequency radio signals emitted by radio beacons.
VOR stations broadcast the three letter identifier in Morse code. Because VOR signals have a range of about 200 miles, it is possible for an aircraft to receive multiple VOR signals. Therefore, it is necessary that pilots identify a VOR before navigating to it to ensure the proper navigation aid is selected. In some cases a voice signal is broadcast with the station name with recorded advisories.
There are about 3,000 VOR stations worldwide, however, the number of VORs in the U.S. is declining. In the last decade the FAA has identified underutilized and redundant VOR stations which no longer provide adequate benefit to justify their cost.
In an update published on April 27, 2021, the FAA stated it would discontinue approximately 34 percent (307) of VORs in the contiguous U.S. by 2030. When a VOR is decommissioned, it is replaced with a GPS based intersection. For the majority of pilots, the effect of decommissioning is minimal.
VOR Components and Features
VOR components include the ground component and the aircraft component. The ground component transmits signals to the aircraft component.
Antenna and VHF
Aircraft equipment includes a VOR antenna, a VOR frequency selector such as the Bendix/King KNS 80 or the Garmin GNC 255, and a cockpit instrument to display the course information—either a Course Deviation Indicator (CDI) or Horizontal Situation Indicator (HSI). Handheld VOR transceivers may also be used. Some general aviation and all transport aircraft have multiple independent VOR systems. VOR navigation is also incorporated into many integrated flight decks along with GPS nav equipment, offering pilots the flexibility of both systems.
Distance Measuring Equipment (DME)
It is common for distance measuring equipment (DME) to be co-located with a VOR station. DME equipment uses a UHF (L-band) transmitter/receiver (interrogator) in the aircraft and a UHF (L-band) receiver/transmitter (transponder) on the ground to measure distance from a station, giving pilots an indication of the aircraft’s distance from the VOR station.
DME is a “slant distance”— a straight line from the station to the aircraft receiver. An aircraft 1 nm laterally from the station would receive the same DME indication as an aircraft 1 nm, about 6,000 feet, directly above the station. A VOR station with a collocated DME is called a VOR-DME.
As VORs are becoming decommissioned, there are some instances of standalone DMEs popping up on sectional charts, such as the Goodsprings DME (GOG), which can be utilized by advanced navigational systems in airliners and some corporate aircraft.
How Does a VOR Work?
VOR navigation technology uses a ground-based antenna at a station to send a directional signal that rotates clockwise 30 times a second, or 360 degrees in azimuth. A reference signal is also emitted timed to be in phase with the directional signal as the directional signal passes magnetic north.
The “phase difference” between the reference signal and the directional signal is the bearing from the station to the aircraft position. This bearing or line of position is called the VOR radial and can be used to fix the position of the aircraft towards the station.
Advantages of VOR Navigation
GPS is far more reliable than VOR navigation with global coverage and no ground station location or maintenance required. Going forward, the advantage of VORs will be realized as a back-up system in the event of GPS outages or failures.
Disadvantages of VOR Navigation
VOR navigation is limited by distance (range) and terrain and requires a ground station with its associated power and maintenance requirements. As a short to medium range system, VOR navigation is more limited than space-based GPS navigation, which can be established solely on waypoints.
VOR signals may also suffer interference from thunderstorms and the lower accuracy requires instrument approaches based on VOR signals to have higher weather requirements and minimum descent altitudes.
VOR vs VORTAC
A common practical test question is what is the difference between a VOR and a VORTAC? A VORTAC is simply a VOR with a co-located tactical air navigation system (TACAN), which is a navigation system used by military aircraft.
Both VOR and VORTAC navaids can be used by general aviation pilots, though civilian pilots should be alert for notices to air missions (NOTAMs) specific to the VOR side of the facility when combined with a TACAN, as the systems run independently despite sharing an identifier.
Stay Up to Date With the Latest in VOR and GPS Navigation
Navigational aids are necessary to provide pilots with guidance on the location and direction of flight. Understanding VORs and GPS is critical for pilots flying in busy airspaces, whether local or across the country. As the airspace system and navigational aids change, pilots must stay up to date.
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FAQ
The three types of VOR are VOR, VORTAC (VOR with TACAN), and VOR-DME (VOR with DME).
VOR navigation is still in use and will continue to be part of the VOR Minimum Operational Network (MON) for the foreseeable future.
In a 2012 publication of SatNav News, the FAA reported that the cost of operating and maintaining VORs was $110 million per year.
