Flight Crew Air Transport Pilot (ATP)
Troubleshooting
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification
ATC Equipment / Nav Facility / Buildings; Aircraft
Approaching RAP VOR we were advised a re-route for active military airspace. Then [received] transponder L EICAS message.  We switched to R transponder and also got an associated R transponder EICAS message. We asked ATC if they still had us on radar and they said yes they did. ATC advised it was most likely military jamming. Next we got a FMC verify position. Then unable RNP. I noticed both GPS L and R were way off and causing FMC position updating to be off RNP 2 ANP was 20. We advised ATC of navigation issues due to GPS jamming and asked for vectors. ATC advised this was not a problem and had been happening in the area. We decided to inhibit the GPS NAV and see if it would correct itself by radio NAVAID. This did seem to be working as the Actual Navigation Performance (ANP) was getting better but as we were getting close to [top of descent] and ready for an RNAV arrival and RNAV approach we decided to just ask for vectors to an ILS with a slight tailwind due to ANP would not be acceptable for an RNAV approach. If we had had more time before descent to advise Maintenance and possibly reset breaker GPS L might have fixed the issue.
B747 Captain reported possible GPS jamming near active military airspace.