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

Distance Measuring Equipment (DME)

Operating Principle: the measurement of the time between the transmission of an aircraft interrogating signal and the receipt of a matched pulse reply signal from a ground station (the 2 signals having different frequencies).

DME operates in the UHF range, but can be collocated with a VOR or ILS, or occasionally NDB. Distance information can also be obtained from TACAN stations by selecting the paired VOR frequency.

DME measures slant range, not distance along the ground. Therefore, the greatest error occurs when flying high and close to the station.

Tolerance at a VOR checkpoint: +/- 0.5 NM