pjdj, have to agreee the RSC (Reduced Shakespeare Company) rule. They are the dons!
Gaunty, maybe I'm wrong, but I thought that planes don't use GPS as far as I know. People use GPSs, planes (at least, the big ones) use INS (Inertial Navigation System). It's a bit like an electronic gyroscope that measures acceleration in all axes (I'm sure you know what it is, but I'm explaining it to Halibut as it was explained to me by a Maersk captain).
If it [the INS] knows your start point on the ramp (which one of the pilots enters into the FMC: it's written on the aerodrome info. pages of the Aerads under 'Ramp INS positions' for each stand), it can calculate your position later on based on what your starting point was.
Indeed 'tis a cunning device. I believe it's more reliable than GPS at lower altitudes.