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What is GP?

Yes 2.5 is only the scale deflection, not (DOC) direct operational coverage when you would get “tune & flag”, also does +/-35deg apply to GP or it has a different lateral coverage than the LOC?

Last Edited by Ibra at 06 May 13:31
Paris/Essex, France/UK, United Kingdom

GP is within 8 degrees to 10 NM, although it can be certified for a higher range.

In practice this is irrelevant because you should never descend unless established (i.e., within half-scale deflection of the localizer).

What is more important is the vertical coverage, to avoid capturing a false glideslope.

Biggin Hill

Ibra wrote:

LOC: two directional signals from 2 antennas and you measure phase difference along the runway axis, you can only receive LOC in direct coverage area +/-2.5deg range (and reverse runway in back-course BC LOC as well, unless it’s Germany where they explicitly “hide” the back-course LOC signal to make sure you don’t fly it consciously or unconsciously)

It’s not the phase difference that is measured by the ILS receiver but the relative strength of the two signals. The glidepath works the same way. It would not even make sense to measure a phase difference as the two signals have different frequencies – 90 and 150 Hz. The localiser and glidepath carriers are modulated with these two frequencies.

In case someone doesn’t know, what is referred to as the “ILS frequency” is actually only the localiser frequency. The glidepath (GP) transmitters use an entirely different frequency (see e.g. LeSving’s AIP excerpt above) but the LOC and GP frequencies are paired so it is sufficient to tune the localiser frequency.

ESKC (Uppsala/Sundbro), Sweden

At least in FAA-land the terminology GP (glide path) is used for GPS approaches and GS (glide slope) for ILS. A LOC approach doesn’t have vertical guidance.

172driver wrote:

At least in FAA-land the terminology GP (glide path) is used for GPS approaches and GS (glide slope) for ILS..

ICAO terminology is GP (glide path) for both.

ESKC (Uppsala/Sundbro), Sweden
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