Thanks for the replies re GPS altitude. I had a very strong belief already that the GPS altitude isn't worth a cracker after calling it up often on the ACMS and seeing what it read compared with our F/L. Even allowing for baro variations, it's still way out - in the order of 1500 - 2000 ft or so. So much of the inherent inaccuracy comes from the poor grazing angles of the SVs used for nav solutions. As it has been pointed out, nav solutions prefer SVs with a diferent sight line than altitude and that's what the GPS is optimised to do.