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Concerning the variable, or directional, signal of a conventional VOR:
  • A
    The transmitter changes the frequency of the variable signal by 30 Hz either side of the allocated frequency each time it rotates.
  • B
    The rotation of the variable signal at a rate of 30 times per second gives it the characteristics of a 30 Hz amplitude modulation.
  • C
    The receiver adds 30 Hz to the variable signal before combining it with the reference signal.
  • D
    The transmitter varies the amplitude of the variable signal by 30 Hz each time it rotates.

The VOR shall radiate a radic frequency carrier' with two associated 30 Hz modulations. The phase of one of these modulations shall be independent of the azimuth of the point of observation (reference phase). The other modulation (variable phase) shall differ from that of the reference phase by an angle equal to the magnetic bearing of the point of observation with respect to the VOR. Therefore, we can say that a VOR bearing is obtained by phase comparison - The directional information is derived by comparing the phase between a 30 Hz reference signal and a 30 Hz variable phase signal.

  • For the conventional VOR, the phase of the 30 Hz component of the FM subcarrier is fixed without respect to azimuth. It is termed the “reference phase”.
  • The other signal component shall be 30Hz amplitude modulation. For the conventional VOR, this component results from a rotating field pattern (rate of 30 times per second), the phase of which varies with azimuth. It is termed the "variable phase" and is of constant amplitude.

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