Transponder altimetry
I am sure this has been discussed before, I have searched but have been unsuccesful!
Question:- If transiting VFR beneath hanging airspace on a given QNH, a pilot and all around him (or her) will be using the same datum for height reference - their altimeter.
Controllers however will be using data transmitted from the aircraft's XPNDR, (mode S) which we know to be more accurate and will almost certainly be transmitting a figure that is unlikely to be the same as the altimeter. So there will be two definitions of the vertical dimension. the altimeter and the XPNDR. (Pilot's and controllers if you will)
The controlller decides that there's been a vertical incursion, but the pilot maintains that his altimeter was correctly set and accurately flown - Discuss.
This is a hypothetical question I'm not asking for a mate! :-)