B737 flight crew experiences loss of GPS signal at FL350 in ZLC airspace due to notamed GPS jamming ...

Narrative:

Center later amended our routing to MVA dta ekr then as filed. We entered that routing into the FMC; then cross-checked and executed the change. While in ZLC airspace; we began observing intermittent GPS-left and GPS-right invalid FMC scratchpad messages. We referred to the FMC position page 2 and observed that both GPS position lines were blank. We determined that radio position updating was still valid and that our anp (actual navigation performance) was .12; which was satisfactory for our phase of operation. We did not observe any anomalous navigation performance; map shifts; or uncommanded aircraft turns.although the GPS fail light on the overhead panel was not illuminated; we reviewed the GPS inoperative non-normal procedure. We then discussed our approach limitations at denver given the apparently inoperative GPS equipment. Denver was IMC at the time of our discussion but the airport was using ILS approaches so we were comfortable continuing to destination.we then sent a message to maintenance control informing them of our equipment problem and asked if they could advise us on possible system resets. At that point we began hearing other aircraft on the frequency discussing GPS problems. ATC was discussing a possible navigation deviation with another aircraft on frequency. ATC stated that the military was conducting GPS jamming in the area and that there was an enroute NOTAM to that effect. We informed ATC and dispatch that we too were experiencing intermittent GPS issues. We briefly discussed the issue with ATC; but were never informed of a possible navigation deviation.as we continued toward denver; both of our GPS systems recovered operation and functioned normally for the remainder of the flight. We informed maintenance control that our equipment had recovered without having to be reset. We conducted a normal ILS approach into denver and landed uneventfully.we were later informed by the duty manager that ZLC had charged us (and several other aircraft) with an 'undirected turn' while in their airspace. This came as a surprise to both of us; since we had neither observed nor been informed by ATC of any possible navigation deviations.

Original NASA ASRS Text

Title: B737 flight crew experiences loss of GPS signal at FL350 in ZLC airspace due to NOTAMed GPS jamming by the military. Several other crews report GPS interruptions to ATC and ATC detects some track deviations.

Narrative: Center later amended our routing to MVA DTA EKR then as filed. We entered that routing into the FMC; then cross-checked and executed the change. While in ZLC airspace; we began observing intermittent GPS-L and GPS-R INVALID FMC scratchpad messages. We referred to the FMC POS page 2 and observed that both GPS position lines were blank. We determined that radio position updating was still valid and that our ANP (Actual Navigation Performance) was .12; which was satisfactory for our phase of operation. We did not observe any anomalous navigation performance; map shifts; or uncommanded aircraft turns.Although the GPS Fail light on the overhead panel was not illuminated; we reviewed the GPS Inoperative Non-Normal procedure. We then discussed our approach limitations at Denver given the apparently inoperative GPS equipment. Denver was IMC at the time of our discussion but the airport was using ILS approaches so we were comfortable continuing to destination.We then sent a message to Maintenance Control informing them of our equipment problem and asked if they could advise us on possible system resets. At that point we began hearing other aircraft on the frequency discussing GPS problems. ATC was discussing a possible navigation deviation with another aircraft on frequency. ATC stated that the military was conducting GPS jamming in the area and that there was an enroute NOTAM to that effect. We informed ATC and Dispatch that we too were experiencing intermittent GPS issues. We briefly discussed the issue with ATC; but were never informed of a possible navigation deviation.As we continued toward Denver; both of our GPS systems recovered operation and functioned normally for the remainder of the flight. We informed Maintenance Control that our equipment had recovered without having to be reset. We conducted a normal ILS approach into Denver and landed uneventfully.We were later informed by the Duty Manager that ZLC had charged us (and several other aircraft) with an 'undirected turn' while in their airspace. This came as a surprise to both of us; since we had neither observed nor been informed by ATC of any possible navigation deviations.

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	1261505
Time
Date	201505
Place
Locale Reference	ZLC.ARTCC
State Reference	UT
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 220 Flight Crew Total 6200 Flight Crew Type 750
Person 2
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Track / Heading All Types

B737 flight crew experiences loss of GPS signal at FL350 in ZLC airspace due to notamed GPS jamming by the military. Several other crews report GPS interruptions to ATC and ATC detects some track deviations.