VHF Omnidirectional Range (VOR)

The VHF Omnidirectional Range (VOR) is a ground‑based radio navigation aid that transmits a signal from which an aircraft can determine its bearing (radial) relative to the station. The system operates in the VHF band (108.0 MHz to 117.975 MHz) and provides 360 distinct radials (each 1° apart) that extend outward from the station like spokes on a wheel. By tuning to a VOR frequency and interpreting the received signal, a pilot can determine which radial they are on, and by combining this with distance information (from DME), can fix their position. VOR is the cornerstone of the airway system, used for en‑route navigation, terminal area procedures, and non‑precision approaches.

The VOR's purpose is to give every aircraft within range a continuous, real-time bearing to a known fixed point, and it is that continuously available radial structure, rather than any single station, that forms the skeleton of the airway system pilots have navigated by for decades. Because the 360 radials radiate outward like spokes, a network of VOR stations lets routes be defined as sequences of radials and intersections rather than requiring point-to-point dead reckoning, which is what made structured, repeatable airway navigation possible before satellite navigation became widespread. Combined with DME for distance, a single VOR/DME station gives a pilot a full position fix, which is why the two are so often co-located rather than deployed as separate services.

VOR is used across all phases of flight and is installed at thousands of locations worldwide:

• En‑route Navigation: VORs define the airway structure (Victor Airways in the US, Blue and Red Airways elsewhere). Aircraft navigate from one VOR to the next, following designated radials.
• Terminal Area: VORs are used for arrival and departure procedures, providing bearing information to airports and waypoints.
• Non‑precision Approaches: VOR approaches provide lateral guidance to runways where ILS is not available (VOR/DME and VOR approaches).
• Holding Patterns: VORs are used as holding fixes for traffic sequencing.
• Military Operations: Tactical navigation and training.

The VOR ground station sits on its own counterpoise (a circular ground-plane structure) at a site chosen for a clear, obstruction-free horizon in all directions, since buildings, terrain, or even large aircraft parked nearby can distort the radiated pattern and cause course errors. The antenna array and equipment shelter are set inside a fenced, restricted-access compound with its own earthing and lightning protection, positioned clear of the runway/taxiway obstacle limitation surfaces despite typically sitting well away from the movement area itself. Power and monitoring cabling runs underground from the compound back to the airport electrical supply and to the ATC facility for remote status monitoring; site selection is verified by a flight-check survey before the station is commissioned, and re-checked if nearby construction changes the surrounding terrain.

The VOR system operates on the principle of phase‑comparison between two signals:

• Reference Signal: A 30 Hz signal frequency‑modulated onto a 9960 Hz subcarrier (which in turn amplitude‑modulates the main carrier) is transmitted omnidirectionally. This signal is constant in phase regardless of bearing and serves as the phase reference.
• Variable Signal: A second, 30 Hz amplitude‑modulated signal is produced by a rotating or electronically scanned antenna pattern. This signal's phase varies with the bearing relative to the station. At the moment the antenna pattern points north, the variable signal is in phase with the reference; at other bearings, the phase difference equals the bearing angle.
• Bearing Determination: The aircraft's VOR receiver compares the phase of the variable (AM) signal with the reference (FM subcarrier) signal. The phase difference (in degrees) directly indicates the radial from the station.
• Identification: The station also transmits a Morse‑coded identifier (three‑letter code) at regular intervals, allowing the pilot to confirm correct station selection.
• Coverage: VOR signals are line‑of‑sight, with published service volumes ranging from about 25 nautical miles (Terminal) up to roughly 40–130 nautical miles depending on altitude, for Low and High Altitude station classes.

The VOR ground station sits outside the AGL lighting control hierarchy entirely — it is a navigation aid, not a lighting circuit, and has no connection to the ALCMS. Its own architecture centres on the transmitter and its continuous self-monitoring: the reference and variable phase signals are generated and radiated from the antenna array, while a co-located monitor system independently receives the broadcast and checks the phase relationship, modulation, and radiated power against tolerance. If the monitor detects an out-of-tolerance condition, it does not simply raise an alarm — it can automatically switch the transmitter to a standby unit or shut the station down and flag it as unusable, because a VOR broadcasting bad navigation data is more dangerous than one that is off the air. Station status (operating, standby active, or off-air) is reported to ATC and maintenance over a separate remote-control/monitoring link, distinct from any airfield lighting network.

The station's radiating element is an Alford loop or similar antenna array mounted atop a ground-plane counterpoise, typically a raised metal disc or mesh structure a few metres in diameter that shapes the radiation pattern needed to produce accurate radials. The antenna and counterpoise assembly sits on a low, obstruction-free mound at the station centre, chosen so surrounding terrain and structures do not distort the radiated signal, with the transmitter, monitor, and standby equipment housed in an equipment shelter beside or beneath the antenna structure. The shelter is a weatherproof, temperature-controlled building housing the dual (hot-standby) transmitters, monitoring receivers, and battery/UPS backup, connected to the antenna via a short, low-loss coaxial feed to minimise transmission loss between the equipment and the radiating element. Both the antenna structure and the equipment shelter are rated for continuous outdoor exposure (IP65/IP67 on external components), with lightning protection and earthing designed into the counterpoise and shelter grounding system given the station's isolated, elevated siting.