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Please advise if the glideslope flag validates that the received frequency is being modulated at nominal 80% or 40% for a localizer.

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I wanted to post this as a comment, but it's a bit long.

From ICAO Chicago Convention annex 10, vol 1:

2.3 Alarm conditions for ILS airborne equipment

2.3.1 Ideally, a receiver alarm system such as a visual flag should warn a pilot of any unacceptable malfunctioning conditions which might arise within either the ground or airborne equipments. The extent to which such an ideal may be satisfied is specified below.

...

2.3.3 The alarm system should indicate to the pilot and to any other airborne system which may be utilizing the localizer and glide path data, the existence of any of the following conditions:

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b) the percentage modulation of either the 90 Hz or 150 Hz signal reduction to zero with the other maintained at its normal 20 per cent and 40 per cent modulation respectively for the localizer and glide path;

Note.— It is expected that the localizer alarm occur when either the 90 Hz or 150 Hz modulation is reduced to 10 per cent with the other maintained at its normal 20 per cent. It is expected that the glide path alarm occur when either the 90 Hz or 150 Hz modulation is reduced to 20 per cent with the other maintained at its normal 40 per cent.

Glide path receiving equipment operational standards are detailed in RTCA DO-192 and EUROCAE ED-47B

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