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You are established on an ILS approach at 4 NM on the DME and get some signal interference. What could be the reason for this?
  • A

    You get interference because of being outside the published coverage area.

  • B
    Multipath interference from reflections off objects within the published coverage area.
  • C
    Reverse-sensing side lobes.
  • D
    Picking up the "false glide slope"
Learning Objective 062.02.05.04.08: Explain that multipath interference is caused by reflections from objects within the ILS coverage area.

We can be sure that the aircraft is within the coverage area in this question, as it is fully established on the ILS approach (at 4 NM it is most likely to be descending on the glide path as well as on the localiser). This leaves only one answer available, as none of the other options happens when within the correct coverage area of the ILS.

Reflections of the ILS signals off objects can hit the aircraft at certain stages of the approach, altering the difference in depth of modulation between the 90 Hz and the 150 Hz lobes, and can cause a false movement of the ILS needles, which in turn can affect the autopilot if one is engaged.

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