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B747 captain reported possible GPS jamming near active military airspace.
| Attributes | |
| ACN | 1586978 |
| Time | |
| Date | 201810 |
| Local Time Of Day | 1801-2400 |
| Place | |
| Locale Reference | ZDV.ARTCC |
| State Reference | CO |
| Aircraft 1 | |
| Make Model Name | B747 Undifferentiated or Other Model |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | GPS & Other Satellite Navigation |
| Person 1 | |
| Function | Captain |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Inflight Event / Encounter Other / Unknown |
Narrative:
Approaching rap VOR we were advised a re-route for active military airspace. Then [received] transponder left EICAS message. We switched to right transponder and also got an associated right 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 left and right 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 navigation 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 left might have fixed the issue.
Original NASA ASRS Text
Title: B747 Captain reported possible GPS jamming near active military airspace.
Narrative: 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.
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.