Not all the BA 747-400s have got GPS. It was only approved quite recently as a modification for the whole fleet, based on a number of reasons that flight ops presented to the management/finance. As the 57 aircraft come in for routine major maintenance, it is installed and tested by us. Consists of two crown mounted antennas (left and right systems) and associated coaxial cable, complex wiring integration with other systems in the main equipment centre and some updated FMC software. No new LRUs, same Multi mode receivers. Not a great deal really, can be fully installed in a few days across shifts.