VHF Omnidirectional Range (VOR)

How does it work?

• it depends upon a phase difference between two signals transmitted simultaneously from a ground station
• position sensitive, NOT azimuth (i.e. no matter what direction you are facing, your instruments will give you the same VOR information about your position)
• line of sight: reception distance = 1.23 x sqrt(altitude above station)

Components:

• Omni Bearing Selector (OBS) >> select radials "FROM top, TO bottom"
• Course Deviation Indicator (CDI needle) >> centre to full scale is 10 degrees
• TO / FROM indicator >> directional ambiguity when crossing a radial 90 degrees from what is set on OBS

Tolerances:

• airway radials maintained by TC within +/- 3 degrees
• VOR checkpoint +/- 4 degrees
• dual VOR check +/- 4 degrees on ground or in air
• visual check over landmark +/- 6 degrees

To calculate distance (NM) from station:

• (groundspeed x time in minutes) / degrees of bearing change

Radio Theory

Radio... what FGU calls the 'modern magic genie' :)

Some basic definitions:

• wavelength: linear measurement of a wave in meters
• cycle: peak to peak vibration period measured in seconds
• frequency: number of cycles per second measured in hertz

Wavelength remains constant, but the amplitude (strength) decreases with distance from the transmitting station.

Frequency Bands:

• Low and Medium Frequency (LF / MF): NDBs
• High Frequency (HF): long range communication
• Very High Frequency (VHF): communication, VOR, ILS
• Ultra High Frequency (UHF): DME, glideslope of ILS

Characteristics:

• LF/MF/HF
• ground waves: follow the surface of the earth; can bend and be attenuated (higher frequencies = more attenuation = less reception distance)
• sky waves: travel up into the atmosphere and are reflected back to earth from the ionosphere
• skip distance: the distance between the transmitting antenna and the point where the skywave first returns to the surface of the earth
• skip zone: the space between the end of ground wave coverage and the point at which the first sky wave is returned to earth
• VHF
• line of sight

Comm and nav systems will get their own separate posts soon - stay tuned!