Air carrier flight crew reported GPS interference during cruise in vicinity of the fix LINSO.
Synopsis
Air carrier flight crew reported GPS interference during cruise in vicinity of the fix LINSO.
Narrative
Approaching LINSO I contacted Kunming ATC and was instructed to offset 5NM right of course. I input the 5NM offset; but it didnt show any offset on the ND (Navigation Display) Map. Attempted this multiple times with the same result. I elected to execute the offset as we needed to comply with ATC hoping it would work after EXEC was pressed; but the offset didnt take and the aircraft remained on route centerline. After sequencing LINSO the majority of the route disappeared from the FMC/ND MAP so I requested a direct to GMA and advised ATC we experienced GPS jamming (occurred over Yangon) and were unable to offset. Further clarification with ATC I told them GPS operational; but unable to offset. ATC cleared us direct SGM flight plan route. After rebuilding our route and verifying it in the FMC; we investigated the route offset problem further. FMC would take 1NM right offset and display properly; but any offset greater than 1NM it would display an approximately 45 degree turn off the route to infinity. There were no EICAS or FMC messages. We consulted the AOM and decided to delete the STAR and Approach from the FMC at ZSPD. Attempted the route offset again; and it displayed properly. Informed ATC we were able to offset the route and they cleared us to offset 5NM right of course. We complied with the clearance and did not have any other offset issues on the remainder of the flight.
Second reporter narrative
Approaching fix LINSO we were advised to offset 5 miles right of track. The PM tried multiple times to put in the offset on the route page. For some reason it was not showing the offset on the map. We elected to execute the offset to comply with atc instructions. Once executed plane stayed on the route centerline. Upon reaching LINSO the majority of the route disappeared from the FMC and ND map. We requested direct the next fix on our flight plan and rebuilt the route into the fmc. After getting the route built we tried the offset again without success. We tried putting different offsets into the route page but the plane show a 45 degree off route for infinite distance. The only offset that looked correct was a 1nm offset. We advised ATC we received gps jamming earlier and were unable to accept the offset at this time. The PM and I keep researching the problem and found in the AOM with offset problems that were linked to approach and arrivals. I decided to delete and reload the arrival and approach into PVG then tried to add the offset. Everything showed perfect and we advised atc we could accept an offset at this time. We did not have any other issues on the remainder of the flight.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.