Narrative:

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 left 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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.