Thank you for answers. The GPS receiver is on the second floor of the house and the antenna --Lucent 26db gain conical -- is just above in the attic, the cable passing through the ceiling : I guess that this system introduces a little delay-frequency shift following the way through the tiles. The roof was recently thermaly insulated with a thick layer of ? covered with a tiny aluminium foil. GPS out, no satellite seen... The GPS of my labtop receives, in less than 1 minute on the first floor, a dozen of satellites. I have observed on the field that receiving a dozen of satellites gives a more precise position than receiving 4 ...8.