VHF Omni direction Range (VOR)

VHF Omni direction Range (VOR)

1. Short Range

   • Line of sight

2. Very high frequency

   • 108.00 - 117.95 MHz (50 kHz spacing
     • 1st 4 MHz is shared with ILS, 108-111.95 MHz
       • Odd - ILS (108.10, 108.15,108.30,108.35,...)
       • Even: VOR (108.00, 108.05, 108.20, 108.25,...)

3. Replaced by GPS

   • Many small general aviation aircraft are not equipped with a GPS certified for primary navigation or approaches

4. Operation

   • Send omnidirectional master signal
   • Send highly directional second signal by phase antenna array
   • Rotates clockwise 30 times a second
   • Signal is timed
     • Compared to master varies as secondary signal rotates
       • bearing to aircraft from the station can be determined
     • Phase difference is same as angular direction of spinning signal
       • Signal send 90 degrees clockwise, 90 degrees out of phase with master

5. Intersection of 2 radial (line of position) from 2 VOR to fix position

6. Line of sight propagation

   • 200 miles range

7. VHF radio composite signal

   • Navigation Signal
   • Station Identifier
     • 3 letter string morse code
   • Voice Signal
     • Station name, in-flight recorded advisories or live service broadcast
     • Continuous broadcast recorded broadcast of Hazardous Inflight Weather Advisory Service (HIWAS)

8. Developed from Visual Aural Radio Range (VAR)

   • Short Range
   • 100 miles range
   • 4 courses of navigation
     • 2 visual instrument signal
       • Similar to modern localizer & backbeam in Instrumental Landing System
     • 2 audio signal
       • Similar to Low-Frequency Radio Range

9. More reliable than NDB

   • Provides bearing from station to aircraft that does not vary with wind and orientation of aircraft
   • Less vulnerable to diffraction (course bending around terrain and coastline
   • Phase encoding suffer less interference from thunderstorm

10. 4 common type of indicators

    • Omni bearing indicator(OBI) & course deviation indicator
    • Radio Magnetic Indicator
    • Horizontal Situation Indicator
    • Area Navigation

11. Collated

    • Distance Measuring Equipment (DME)
      • VOR-DME
        • 1 station position fix
    • Tactical Air Navigation (TACAN)
      • VORTAC
        • VOR & TACAN provide azimuth
        • Civil DME is built to match military DME specification
    • Standardardized scheme of VOR frequency to TACAN/DME pairing
      • Automatically selected

12. Same antenna, receiving equipment & indicator as ILS

    • VOR station selected
      • OBS is functional
    • Localizer frequency selected
      • OBS not functional
      • Indicator is driven by localizer converter

13. Service Volume

    • Volume of airspace
    • 3 standard service volumes (SSV)
      • Terminal
        • 1000 feet AGL to 12,000 (including) feet AGL
        • 25 NM radial distance
      • Low Altitude
        • 1000 feet AGL to 18,000 (including) feet AGL
        • 40 NM radial distance
      • High Altitude
        • 1000 feet AGL to 14,500 (including) feet AGL
          • 40 NM radial distance
        • 1000 feet AGL to 18,000 (including) feet AGL
          • 100 NM radial distance
        • 18,000 feet AGL to 45,000 (including) feet AGL
          • 130 NM radial distance
        • 45,000 feet AGL to 60,000 feet (including) feet AGL
          • 100 NM radial distance

14. Most aircraft IFR have 2 VOR receiver

    • Backup primary receiver
    • Confirm aircraft exact position (cross position

15. Future

    • Global Navigation Satellite System (GNSS)
      • Lower transmitter cost per customer
      • Provide distance & altitude data
      • Future GNSS eventually provide accuracy same or greater than VOR
      • Subject to sabotage & interferences
        • VOR as backup
          • VOR signal static mapping to local terrain

16. Accuracy & Reliability

    • Predicted accuracy of ±1.4°
    • Actual data 99.94% < ±0.35° of error
    • Internal monitoring
      • Shut down or change to standby station if exceeds limit
      • Doppler VOR
        • 1.0° limit boundary
      • > 1 receiving antennas located away from beacon
        • Monitoring parameters
          • Bearing accuracy
          • Reference
          • Variable signal modulation indices
          • Signal level
          • Presence of notches
            • Caused by individual antenna failures
        • Signals received are different for aircraft
          • Due to proximity effect
            • Free space path loss from nearby sideband difference from farside
              • 1.5 dB different (113 MHz, 80m)
            • Doppler shift effect
            • No measurable difference for distant aircraft
          • Peak rate of phase change from tangential antenna
            • Aircraft tangential antenna will be parallel but not tangential antenna
      • CVOR
        • 4° limit
    • No signal is better than poor signal

17. Doppler VOR (DVOR)

    • Elevated groundplane (counterpoise)
      • Creates strong lope at elevation 30°
        • Complements 0° lobe of the antennas
      • Rarely does it correctly
        • Edge of counterpoise can absorb & re-radiate signals
    • More accurate than conventional VOR
      • Less affected by hills & buildings
    • 30 Hz FM variable signal
      • Signal bounces off
        • 2 signal of unequal strength & phases
          • Instantaneous deviation of the two signals will be same
            • Phase lock loop will be confused
          • Signal drift apart
            • Phase lock loop follow the strongest strength (LOS signal)
      • CVOR - 30Hz AM signal
        • Signal bounces off
          • Aircraft see phase that appears to be at phase centre of main signal & reflected signal

18. Intercepting VOR radials

    • T.I.T.P.I.T
      • Tune
        • Tune to desired frequency
      • Identity
        • Identify the correct VOR by morse code
      • Twist
        • Twist OBS to desired radial course until CDI is centered
      • Parallel
        • Bank the aircraft until heading indicator indicate radial or course set in VOR
      • Intercept
        • Fly towards the needle
      • Track
        • VOR needle centred, turn the heading of the aircraft back to the radial or course to track down the radial or course flown.
          • If there is wind, a wind correction angle will be necessary to maintain the VOR needle centred

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