Air carrier flight crew reported GPS service problem due to ATC receiving conflicting position information resulted in crew returning to correct routing.
Synopsis
Air carrier flight crew reported GPS service problem due to ATC receiving conflicting position information resulted in crew returning to correct routing.
Narrative
While operating Aircraft X we had an issue after takeoff and continuing enroute. The Captain was resting while myself and the relief pilot were operating the flight. The relief pilot was pilot flying while I was pilot monitoring. After takeoff the aircraft was having intermittent ACARS connection issues which was a previous writeup. The system was still operating mostly and we communicated the connection issues with PANC Center. We had further issues around the time I replaced the closest ETOPS alternate after passing the ETP on route 2. Shortly afterwards; the aircraft encountered a navigation issue around NRKEY at FL380. The center seemed unsure of our position and requested resetting the ADSB/C connections and ACARS. PANC further requested ACARS CPDLC test messages and position reports via ACARS then had us contact San Francisco radio on HF to verify our position verbally. While talking to San Francisco; we received an ACARS message to contact RJJJ on HF. We completed the conversation with San Francisco and then contacted Tokyo on HF as requested. RJJJ sent an ACARS indicating we were flying east. We verified with the magnetic compass and all cockpit indications that we were indeed flying west on the correct heading of around 245. We verified that LNAV was engaged and green and we were still in VNAV Path. The Captain then zoomed in the max range on the ND (Navigation Display) and noticed we were roughly .48nm left of course. The ND indicated the correct values of 4nm RNP/.05nm ANP and the FMA's (Flight Mode Annunciators) were also correct SPD/LNAV/VNAV Path. We verified LNAV was engaged and green and the Captain engaged HDG mode on an intercept course. He then rearmed LNAV; however; after crossing the course LNAV did not engage. We verified RTE 1 was the active route and we were on the course via the ND. I then recommended we go direct the current fix to see if the FMS would recalculate the route. After doing so; the box engaged LNAV and resumed tracking our course correctly. We encountered no further issues on the flight and the navigation error was active for approximately five minutes at most. Cause: I'm really not sure if this was an aircraft issue or maybe I hit an additional button without my knowledge. There was another aircraft in our area having similar issues at the same time. I don't know if this was just a weird issue or some sort of spoofing.
Second reporter narrative
Received ATC message to give NUZAN report and NIPPI estimate. ADS-C indicates you are flying east. I selected 10 on the range rings for the Nav display. It showed us left of course or southeast of it. The distance indicator for the nav display located at the bottom of the triangle representing the aircraft; showed 0.48L. (Approximately half a mile left of course)I confirmed position on high IFR chart on iPad and EFB and both confirmed left by the same distance. All of this time the FMA's (Flight Mode Annunciators) showed Green SPD LNAV VNAV/PATH. ANP was .04 and GPS was green. LNAV was clearly not engaged.At approximately XA:29 I told the FO (First Officer) what I was going to do. I selected heading and turned us right 10 degrees. Armed LNAV while on the intercept heading. We passed the magenta line and it didn't intercept. We went right of course .25 miles.Still in heading I brought the aircraft back to course. The FO came up with the idea of re-sequencing while we were on the magenta line. Less than 50 miles from NUZAN when we crossed the magenta line we selected direct NUZAN and LNAV engaged. No more issues for the rest of the flight.Cause: Still not sure. CPDLC was intermittent. Satellite comms were intermittent as well. Before all of this happening I recall a Check ADSC on the EICAS. We were also at FL380 near the Kamchatka peninsula.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.