INS, almost invariably. Some large jets still do not have GPS, for the rest there are still many areas of the world where it is illegal to use GPS as the primary means of navigation, notably the UK lest some of the private pilots forget they are still legally obliged to fix their position by visual reference under VFR or ground-based radio aids under IFR!
The INS position over the Atlantic can be out by a few miles (depends on the age and design of the kit to degree) but before landfall FMS (Flight Management System) position is updated by ground-based nav aids, reducing the errors to something very small. The error then depends on the radio aids used and the Kalman filter used to combine all the data being input to the FMS. The INS itself cannot be updated when the aircraft is moving, the last point of update is either on the stand or when take-off power is selected on the runway, depending on aircraft type.
Send Clowns
General Nav instructor,
BCFT