B777 captain reported both the left and right ads-B out failed near fir north korea. The GPS was nor...

Narrative:

We received an EICAS message; ads-B left out. A few minutes later we got an ads-B left out. I wrote them both up and we then started discussing if this was a GPS jamming event since we were just north of north korea. The first officer and I referenced the B777 GPS jamming update and our situation was the first example listed. We notified dispatch as required. A few minutes later we lost the GPS indication at the bottom of the nd [navigational display]. It indicated inertial; but would momentarily go back to GPS. We again notified dispatch. I then gave dispatch a quick call on satcom because our controller had tried to call us repeatedly and she received a message that we were unavailable. I got through on my first attempt. Chinese ATC then asked us if we were 5 miles right of course; and we became concerned there was a navigational error. After determining we were still on course; I believe they were asking us if we were offset 5 miles right of course. We were not and this was a communication issue probably. GPS was back up by the time we started our descent. Things got busy as we approached top of descent and we signed off with dispatch. Rksi control asked us if we needed assistance and to fly our filed plan as it seemed chinese ATC had forwarded our GPS event to them.

Original NASA ASRS Text

Title: B777 Captain reported both the Left and Right ADS-B out failed near FIR North Korea. The GPS was normal at the top of descent after some troubleshooting was performed.

Narrative: We received an EICAS message; ADS-B L out. A few minutes later we got an ADS-B L out. I wrote them both up and we then started discussing if this was a GPS jamming event since we were just north of North Korea. The FO and I referenced the B777 GPS Jamming update and our situation was the first example listed. We notified Dispatch as required. A few minutes later we lost the GPS indication at the bottom of the ND [Navigational Display]. It indicated inertial; but would momentarily go back to GPS. We again notified Dispatch. I then gave Dispatch a quick call on SATCOM because our controller had tried to call us repeatedly and she received a message that we were unavailable. I got through on my first attempt. Chinese ATC then asked us if we were 5 miles right of course; and we became concerned there was a navigational error. After determining we were still on course; I believe they were asking us if we were offset 5 miles right of course. We were not and this was a communication issue probably. GPS was back up by the time we started our descent. Things got busy as we approached top of descent and we signed off with Dispatch. RKSI control asked us if we needed assistance and to fly our filed plan as it seemed Chinese ATC had forwarded our GPS event to them.

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.

Attributes
ACN	1538338
Time
Date	201804
Local Time Of Day	0001-0600
Place
Locale Reference	ZYSH.ARTCC
State Reference	FO
Aircraft 1
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Flight Dynamics Navigation and Safety
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Inflight Event / Encounter Other / Unknown

B777 captain reported both the left and right ads-B out failed near fir north korea. The GPS was normal at the top of descent after some troubleshooting was performed.