This is possibly one of the biggest current traps on the Radio Navigation exam.

The first thing to emphasise is that depth of modulation and difference in depth of modulation are not the same thing.

  • Depth of modulation refers to one of the ILSILS —Instrument Landing System lobes on its own.
  • Difference in depth of modulation refers to a comparison between the two lobes.

A rough analogy could be that the first is the height of any mountain, but the second is a comparison between the relative heights of two mountains

The depth of modulation is greatest at the centre of a lobe (not at the ILSILS —Instrument Landing System centreline) and decreases as you go away from the centre of the lobe.

The difference in depth of modulation is zero at the ILSILS —Instrument Landing System centreline and increases with angular distance away from the centreline.

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Allow multiple correct answers

What happens to the ILSILS —Instrument Landing System signal as you get further away from the centreline or glidepath?