Sometimes on ILS approach glide slope deviation shows 'above G/S' constantly on approach. I could follow it to the ground and still be 'above G/S' on approach. Problem seems to be random. I took off from Heathrow (EGLL), flew to Manston (EGMH) with GPS DirectTo, activated GPS vectors to final on 28 and did ILS approach with autopilot. Glide slope was showing proper reading and autopilot followed ILS perfectly. Go around and repeat with hand flying and same result - glide slope showing proper reading. Then flew back to Heathrow with GPS DirectTo, activated GPS vectors to final 27L. This time glide slope was constantly showing 'above G/S', even though I was below or at glide slope.

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i1198.photobucket.com/albums/aa457/depenthene/egllgs2.jpg

Checked Heathrow ILS with slew and got correct glide slope.

I had same problem in various different airports too (PHNL, KORD, PANC, EFHK), but problem is not probably the airports since never had problem with other aircraft on glide slope. I can't seem to recreate problem or circumstances leading to it.

Anyone else experience anything like this?