VHF Omni Directional Radio Range (VOR)

Wing Commander Rajagopal

Direction Measurement using Very High Frequency Omni-Directional Radio Range (VOR)

VOR (VHF Omnidirectional Range) is a radio navigation system that provides aircraft with accurate directional guidance. It helps pilots determine their bearing (radial) from a VOR ground station. VOR is widely used for en-route navigation and instrument flight procedures.

Introduction to VOR

  • Very High Frequency Omni-Directional Radio Range (VOR) is a short-range radio navigation aid approved by ICAO.
  • VOR is largely free from static interference and night effect, making it suitable for both day and night operations.
  • VOR determines aircraft bearing using the principle of phase comparison.
  • The station simultaneously transmits a Reference Signal and a Variable Signal.
  • The phase difference between these signals determines the aircraft’s radial.
  • Aircraft equipment consists of a VHF dipole antenna and a VOR receiver.
  • VOR information is displayed on the Radio Magnetic Indicator (RMI) and Course Deviation Indicator (CDI).

VOR Transmission

  • VOR transmits magnetic bearings known as radials.
  • There are 360 radials, each separated by 1°.
  • VOR transmits horizontally polarised VHF radio waves.
  • Operating frequencies range from 108.00 MHz to 117.95 MHz.
  • VOR frequencies use even first decimal digits (e.g., 108.60 MHz).
  • Each station broadcasts a unique three-letter Morse code identification or station name.
  • The identification is transmitted at least seven times per minute.
  • ATIS or a continuous tone may replace the Morse identification.

Emission Designator

  • The emission designator for VOR is A9W.
  • A – Double-sideband amplitude modulation.
  • 9 – Composite modulation system.
  • W – Telemetry or transmission of measured information.

Principle of VOR

  • VOR determines bearing by comparing the phase of the Reference and Variable signals.
  • The combined radiation pattern forms a Limacon.
  • The Reference Signal is a constant-phase, omnidirectional 30 Hz frequency-modulated signal.
  • The Variable Signal is a 30 Hz amplitude-modulated signal whose phase changes as the Limacon rotates.
  • The airborne receiver measures the phase difference between the two signals.
  • At Magnetic North, the Reference and Variable signals are in phase.
  • The measured phase difference corresponds to the aircraft’s magnetic bearing (radial).

Doppler VOR (DVOR)

  • Doppler VOR minimises site errors caused by uneven terrain and ground reflections.
  • A DVOR uses a circular array approximately 13.4 m in diameter with 48 transmitting elements.
  • The Variable Signal is frequency modulated to create the Doppler effect.
  • This method produces less interference than conventional amplitude modulation.
  • An electronically rotating antenna produces the required phase shift.
  • The antenna rotates electronically at approximately 30 revolutions per second.
  • The airborne receiver measures phase differences to determine the selected radial.

Range and Accuracy of VOR

Designated Operational Coverage (DOC)

  • The maximum theoretical range is determined by the line-of-sight principle.
  • Actual range also depends on transmitter power.
  • The inverse square law governs transmission power requirements.
  • Designated Operational Coverage (DOC) is the protected service area around a VOR.
  • DOC prevents interference from nearby VOR stations operating on the same frequency.
  • DOC is expressed as Distance/Altitude.
  • DOC 50/25 indicates service up to 50 NM and 2,500 ft.
  • Different DOC values exist for Terminal, Low Altitude, and High Altitude VORs.

Accuracy of VOR

  • Overall VOR accuracy depends on both ground station and airborne equipment.
  • Proper site selection limits beacon error to less than 1°.
  • Doppler VOR significantly reduces site errors.
  • Airborne receiver error is normally limited to less than 3°.
  • Total system error is generally limited to less than 5°.

VOR Monitoring Equipment

  • The station identification is automatically removed whenever system errors exceed permissible limits.
  • Conditions that trigger shutdown include:
    • Radial transmission error greater than 1°.
    • Transmitter power loss greater than 15%.
    • Failure of the ground monitoring equipment.
  • The aircraft indicator displays a red or yellow warning flag whenever VOR information becomes unreliable.

Cone of Ambiguity (Cone of Confusion)

  • The airspace directly above a VOR station is known as the Cone of Ambiguity or Cone of Confusion.
  • Within this region, VOR indications fluctuate and become unreliable.
  • ICAO limits the cone to approximately 40° elevation.
  • In practice, unreliable indications rarely extend beyond about 80° elevation.

Airway Navigation Using VOR

  • Typical VOR system accuracy is approximately ±5°.
  • Standard airway width is 10 NM (5 NM on either side of the centreline).
  • According to the 1-in-60 rule, a 5° error corresponds to approximately 5 NM at 60 NM.
  • Airways normally have VOR stations spaced approximately 120 NM apart.
  • This ensures aircraft remain within acceptable navigation accuracy.

Types of VOR

  • CVOR – Conventional VOR used along airways.
  • BVOR – Broadcast VOR providing weather and airfield information.
  • DVOR – Doppler VOR designed to minimize site errors.
  • TVOR – Terminal VOR with reduced power for airport operations.
  • VOT – Test VOR used to verify airborne VOR equipment.
  • During a VOT test, airborne equipment should indicate 360° ±4°.
  • VORTAC – A co-located VOR and TACAN station.
  • DBVORTAC – Combination of DVOR, BVOR, and VORTAC facilities.

Cockpit Indications of VOR on RMI, CDI and HSI

Radio Magnetic Indicator (RMI)

  • The Radio Magnetic Indicator (RMI) displays magnetic bearings from both NDB and VOR.
  • The head of the pointer indicates QDM (bearing to the station).
  • The tail of the pointer indicates QDR (bearing from the station).
  • The ADF pointer converts relative bearings into magnetic bearings.
  • The VOR pointer displays magnetic radials directly.

Course Deviation Indicator (CDI)

  • The Course Deviation Indicator (CDI), also called the Omni Bearing Indicator (OBI), displays deviation from a selected radial.
  • The desired course is selected using the Omni Bearing Selector (OBS).
  • The CDI is used to intercept and maintain a selected VOR radial.
  • The deviation bar always points toward the selected course.
  • The TO/FROM indicator shows whether the selected course leads toward or away from the station.
  • In VOR mode, the CDI provides approximately 10° of full-scale deflection on each side.
  • On a five-dot indicator, each dot represents approximately 2°.
  • The CDI provides left/right course guidance without considering aircraft heading.

Horizontal Situation Indicator (HSI)

  • The Horizontal Situation Indicator (HSI) combines heading and navigation information into a single display.
  • It displays the selected magnetic track or radial.
  • It indicates deviation from the selected VOR course.
  • It simultaneously displays aircraft heading and required heading.
  • The HSI provides intuitive left/right steering commands that account for aircraft heading.

Electronic Horizontal Situation Indicator (EHSI)

  • The Electronic Horizontal Situation Indicator (EHSI) provides additional navigation information.
  • Displays terrain and weather information from the Airborne Weather Radar.
  • Displays terrain proximity warnings from the Terrain Collision Avoidance System (TCAS/TAWS, depending on installation).
  • Displays distance information from Distance Measuring Equipment (DME).