B777 Undifferentiated or Other Model
Confusion
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented
Environment - Non Weather Related; Aircraft
Descending on SAS 11E STAR past SASAN with all four pilots in cockpit. Shanghai app control called us and stated that we were 2 miles right of course and requested that we correct back to course. Navigational Display (ND) showed us on course for the selected STAR. Captain (CA) Pilot not Flying (PNF) confirmed with approach the assigned STAR and was again told that we were off course. We had no initial indications that we were off course. CA requested radar vectors and app control did not understand. CA stated that we apparently [were] having navigation problems and requested a heading. App assigned a heading to fly and cleared us direct to EKIMU. As CA again was explaining that we could not determine if we were flying direct to EKIMU if we were having navigation issues. I noticed that ND was now NOT showing GPS but was showing INERTIAL. We had still NOT received any EICAS message (that I could see from the 2nd observers seat). While CA was verifying position we received a UNABLE RNP EICAS message. CA then found that FMC was showing up to a 20 mile difference between GPS and INERTIAL navigation positions and found that on the GPS page the L GPS data was now blank. We were given vectors to join the ILS 34R approach and landing was uneventful. Sometime after landing rollout I noticed that GPS was again showing on the ND.
Upon approach into ZSPD leaving approximately 13;000 feet the approach controller asked us to get back on course. LNAV was engaged showing us correctly on the course with no cross track error. Correct STAR was also selected and active. Approach informed us that we were 2 miles off course and asked us to rejoin the arrival. We showed exactly on the magenta line. Several seconds later UNABLE RNP EICAS message displayed. We informed approach controller that we were having navigation problems and ran UNABLE RNP checklist. Approach controller advised us to turn immediately for traffic avoidance. No TCAS events notes. Controller apparently did not understand our situation due to language barrier. Though we continued to ask for radar vectoring he gave us fixes. At some point he then began to give us vectors. L GPS showed 20 miles off. No map shift was observed at any time. After landing GPS appeared to rectify itself.We informed maintenance and the outbound crew of the situation. This event is very disconcerting. What if this had happened in IMC and mountainous terrain? The aircraft literally showed itself on the course while being at least 2 miles off of the airway. Our only alert was UNABLE RNP shortly after ATC had advised us of the issue. There was no way to correct because the airplane believed itself to be on course. The aircraft should automatically disregard the information given by a malfunctioning GPS. It did not.
B777 flight crew arriving ZSPD is informed they are off course by ATC; but no deviation can be detected in the cockpit. An EICAS message 'NAV unable RNP' is then displayed and the crew requests vectors to the ILS; which are eventually provided. Once on the ground the left GPS returns to normal operation.