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Is a 3-degree glideslope based from the runway surface slope at the point where the antenna is installed or is it referenced to a zero-degree plane?

If you know of a design document or FAR/AIM reference, please include that as well.

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  • $\begingroup$ The runway slope doesn't matter, it's 3 degrees below horizontal. Don't have a reference for you, but that's what you see in the aircraft - reference is the horizon. $\endgroup$
    – Ralph J ♦
    May 21, 2018 at 19:06
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    $\begingroup$ I'll be surprised if the answer is that it is based in any way on the runway surface slope. Allowing the surface slope where the antenna is installed to determine the glideslope angle would be introducing varying descent rates for different ILSes, and pilots expect the same general descent rate on glideslopes for the the aircraft landing configuration, including power settings. A 2 degree upslope would mean a 1 degree glideslope, lots of power. A 2 degree downslope, a 5 degree glideslope, extremely hard to follow. $\endgroup$
    – Terry
    May 21, 2018 at 19:08

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It is referenced to the zero-degree plane, and depending on obstacle clearance it may be a little steeper (3.5° or 4°). There is no reason to refer it to the runway slope as it defines a sink rate to speed ratio during final approach.

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