A B777 CREW CONTINUES ON A PACIFIC RTE WHEN THEIR EICAS RPTS 'NAV UNABLE REQUIRED NAV PERFORMANCE.' AFTER CONFERRING WITH THE CARRIER'S MAINT DEPT GPS UPDATING OF THE FMS WAS DISABLED. NO APPARENT DEVS FROM TRACK WERE NOTED BY THE CREW OR ATC.

Narrative

EICAS MESSAGE 'NAV UNABLE REQUIRED NAV PERFORMANCE.' INVESTIGATION SHOWED L GPS TO BE 57 MI OFF COURSE. NO OTHER EICAS OR STATUS MESSAGES FROM ACFT. REQUIRED NAV PERFORMANCE VALVE WAS 2.0. ACTUAL NAV PERFORMANCE WAS 4.14. CONSULTED WITH AIRLINE MAINT CTL CTR AND B777 FLEET MGR BY SATCOM. SWITCHED OFF GPS (BOTH) SYS AND CONTINUED IN 'INERTIAL' NAV. 3 HRS LATER; EICAS MESSAGE 'NAV UNABLE REQUIRED NAV PERFORMANCE' REAPPEARED. SATCOM TO COMPANY AND GPS TURNED BACK ON. REQUIRED NAV PERFORMANCE WAS 4.0; ACTUAL NAV PERFORMANCE WAS 4.17. THE L GPS SYS SLOWLY CORRECTED TO THE IDENTICAL POS ON THE R GPS. AT SHUTDOWN AT THE GATE; THE NAV SYS 'DRIFT' WAS 0.1. FLEET MGR IS INVESTIGATING THE PROB. AT NO TIME DURING THE FLT DID THE ACTUAL ACFT POS APPEAR TO BE 'OFF COURSE.' (ZZZ CTR FOUND US ON COURSE AT FIRST RADAR CONTACT.) CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINT RPTED AFTER INVESTIGATION THAT THE FAILURE IN THE SYS WAS THE GPS ANTENNA. IT WAS FOUND TO HAVE MOISTURE IN THE ANTENNA AREA. IT WAS FOUND THAT A SINGLE FAILURE OF THE SYS IS NOT ALERTED TO THE CREW BY ANY MESSAGE AND A DUAL FAILURE IS REQUIRED BEFORE A MESSAGE IS DISPLAYED. FURTHER COMPANY INVESTIGATION REVEALED THAT THE SYS OPERATED AS IT WAS DESIGNED WHEN ERRONEOUS INFO WAS BEING RECEIVED. THE BAD INFO WAS BIASED OUT AND BACKUP INFO FROM IRS AND RADIO NAV UPDATING RESULTING IN AN ACCURATE TRACK BEING FLOWN.