The 2 ILS localiser lobes and the 2 glideslope lobes can cover a wide area, especially when further from the airfield. This is known as the ILS coverage area.
Any obstacles within this area can reflect the ILS signals, potentially sending a different amount of one frequency to your ILS receiver than it should. This is called multipath interference and can cause an ILS display to show untrue fluctuations, in both the horizontal and the vertical.
This can be especially dangerous when connected to autopilot, which could pitch or roll excessively due to the ILS fluctuations.
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