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Which navigation aid may be allocated the frequencies of 108.4 MHz and which may be allocated 109.5 MHz?
  • A
    108.4 MHz may be allocated to a VOR and 109.5 MHz to an ILS.
  • B
    109.5 MHz may be allocated to a VOR and 108.4 MHz to an ILS.
  • C
    Either of the two frequencies may be allocated to a VOR.
  • D
    Either of the two frequencies may be allocated to an ILS.

Learning Objective State that the frequency band allocated to VOR according to ICAO Annex 10 is VHF, and the frequencies used are 108.0-117.975 MHz.
Learning Objective State that frequencies within the allocated VOR range 108.0-111.975 MHz, which have an odd number in the first decimal place, are used by instrument landing system (ILS).

There are 2 different nav-aids which operate in the VHF-NAV band, which spans from 108 MHz to 117.95 MHz. VORs and ILSs (specifically the ILS localisers).

The VHF-NAV frequency band is split into frequencies with 50 kHz spacing, so 108.000 MHz, 108.050 MHz, 108.100 MHz, etc.

ILS localiser frequencies are less common, and have smaller range, so less chance of 2 nearby frequencies interfering. They therefore require less available frequencies, and only use the band from 108 - 112 MHz. Within this range, they only use the odd numbered decimals (and odd +50kHz), so ILSs would be allocated onto frequencies such as:

  • 108.500 MHz, 108.55 MHz, 111.900 MHz, 111.950 MHz, etc.

but VORs would be allocated the even decimals, and the evens plus 50 kHz, such as:

  • 108.600 MHz, 108.65 MHz, 110.200MHz, and;
  • all frequencies between 112 MHz and 117.95 MHz, so 112.200 MHz, 113.250 MHz, 115.300 MHz, etc.

This means that the frequency of 108.4 MHz would be allocated to a VOR (as it is an even decimal between 108 and 112 MHz)

The frequency of 109.5 MHz is between 108 and 112 MHz but has an odd decimal, so is an ILS localiser frequency.

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