On the B777 the FMC determines present position from these navigation systems; ADIRS, GPS and navigation radios. When receiving reliable GPS data, the primary mode of navigation is from a GPS updated FMC position. If GPS data is not available, cannot be validated, or is inhibited, the FMC position is updated using navigation radios. When navigation radios are available or no reliable, the FMC position comes from the ADIRU.
FMC position may be manually updated to any of the navigation system positions and this is accomplished on POS REF page 2.