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What is the difference between glidepath and glideslope? How would you define each, and the key differences?

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Glideslope: describes the systems that generate, receive, and indicate the ground facility radiation pattern.

Glidepath: is the straight, sloped line the aircraft should fly in its descent from where the glide slope intersects the altitude used for approaching the FAF, to the runway touchdown zone.

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A glideslope is an inclined plane - the glidepath is the line created by the intersection of the glideslope and the plane of the localizer.

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  • $\begingroup$ So the glideslope is a line in 2D (height vs distance), while the glidepath is a line in 2D (additionally including the heading)? $\endgroup$
    – Dakkaron
    Commented Oct 11, 2022 at 21:07
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    $\begingroup$ glideslope is a plane in 3d and a line in 2d (most prominent in the localizer plane), the glidepath is a line in both 3d and 2d $\endgroup$
    – tsg
    Commented Oct 12, 2022 at 6:01
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They serve the same purpose for the pilot, the difference is buried deep in the implementation concepts and administration of the systems. I would almost call it a philosophical difference. They have different points of failure, failures and inspections being distributed vs centralized, ease of confirming correctness, that sort of thing.

A Glidepath is a 3D path calculated by RNAV equipment, most commonly a GPSS receiver. It is based on extrapolating the most recent series of coordinates to a predicted position then comparing that to a mathematical model of the intended path. If the receiver has the wrong map-model or math, then the path of that plane will be wrong even if the calibration of the satellite signals are correct.

A Glideslope is an inclined radio signal, it is not used for calculation, it is a physical signal that can be followed. The equipment indicates where the plane actually is at that moment relative to the signal without extrapolating any past positions. The receiver is not following a map-model, if the signal is not aligned then the slope that all planes follow will not be aligned even if the receiving instruments are correct.

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  • $\begingroup$ You should add that this definition is only used by some manufacturers (e.g. Boeing). Airbus uses the term F-G/S (FMS glideslope) instead of G/P (glidepath), see e.g. this answer. The other answers refer specifically to ILS. $\endgroup$
    – Bianfable
    Commented Oct 12, 2022 at 6:06
  • $\begingroup$ @Bianfable my answer is independent of manufacturer software UI quirks. I am using the FAA concepts of glideslope(a physical signal on an inclined plane indicating a particular slope; either visual or radio) and glidepath(a charted/calculated or actual decent angle to be taken by an aircraft), as used internally in the agency for terminal procedure development and inspection. Though I may revise my answer tomorrow for clarity, I'm a bit fatigued. $\endgroup$
    – Max Power
    Commented Oct 12, 2022 at 7:06

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