Air carrier pilot crew reported GPS jamming departing Tel Aviv and continuing for hours. The crew used an alternate method to regain GPS guidance and continued the flight.
Synopsis
Air carrier pilot crew reported GPS jamming departing Tel Aviv and continuing for hours. The crew used an alternate method to regain GPS guidance and continued the flight.
Narrative
Aircraft X from Tel Aviv. Encountered GPS jamming from Takeoff and had subsequent loss of GPS signals Left and/or Right. Followed the company location manual and selected RAD NAV INHIBIT OFF. This drove the FMC to display the guidance it was using to INERTIAL. We continued to experience loss of one or both GPS systems continually as we flew near the border of Ukrainian. Then I went on my rest break while we were flying over Eastern Europe. Navigation updated well while land based sources like VOR's were available.The Relief Pilot called me about two hours later to return early from break when we were approaching the coast of Iceland. We were in radar contact with Reykjavik. The Relief Pilot had reset the RAD NAV INHIBIT to OFF to recapture the use of the single GPS signal the aircraft was receiving as the source of navigation. This did not work. The NAV system was using INERTIAL and the ANP (Actual Nav Performance) was steadily increasing past 3.00 toward 4.00; which we believed would preclude us from entering Oceanic Airspace since the limit is 4.00. Upon seeing this I quickly called company Maintenance on the Satellite phone. The technician indicated he was unsure how to proceed but told us to use the CDU on the FMC and select the INIT REF key; INDEX; POS; Page 2. On Page 2 of the POS REF display at the 1R key position select UPDATE ARM and then select GPS at 3R. Just as we were working through this the ANP (Actual NAV Performance) increased to 4.02 and received FMC checklist message NAV UNABLE RNP. Within seconds we worked through the above steps described by Maintenance and were able to force the FMC to use GPS as its navigation source. The aircraft immediately made a turn to the left and the ANP value decreased to 0.05. This was a perfect result: on course and within navigation accuracy limitations.A aircraft malfunction was not the cause of this eventual navigation problem. I believe it was the continual GPS jamming in the vicinity of Israel; Turkey and Ukraine. In speaking to my three First Officers we agree we never trained in how to force the Navigation system to return to using GPS as its navigation source. Once we called the maintenance technician he also voiced his uncertainty how to proceed. Thankfully he quickly came up with a successful set of keystrokes via the CDU to fix our problem.After we were confidently back navigating properly we dug into our flight manual. We found instruction on selecting GPS vice INERTIAL as the navigation source. We found it in our flight manual.Receiving an Oceanic Clearance by voice from Iceland Radio at the same time our navigation accuracy was nearing unacceptable limits made this a very difficult situation. The additional guidance I mentioned above from the flight manual placed in the Company location manual would really have helped.Cause: Sustained GPS jamming for hours after leaving Tel Aviv all the way by Ukrainian boarder. Nav System did not automatically return to highly accurate GPS even after following the procedure in our Tel Aviv Briefing Guide. Suggestions: Add steps to the Company location manual on what to do to recover GPS as the navigation source once the GPS is available if it hasn't done so automatically.
Second reporter narrative
Aircraft X from LLBG to ZZZ. Departed LLBG uneventfully. Encounter GPS jamming from shortly after takeoff until well past the Ukrainian border area. Followed the procedures from the manual (Rad/Nav inhibit off.) Upon returning from crew rest break; approaching Iceland; and under Reykjavik radar control we were unable to get nav systems to use GPS for navigation even though we now had Rad/Nav inhibit on and we had at least one good GPS. The Nav system was still using inertial; and since we were now coast out; the ANP was increasing fairly quickly towards RNP of 4.0. Captain immediately called Maintenance via Satcom. Maintenance advised crew to select: init ref/index/pos/page2. Now select update/arm and then select GPS. This forced FMC to use the good GPS signal. At this very moment ANP exceeded RNP and received message : NAV UNABLE RNP. ANP was in excess of RNP on the oceanic portion of our routing although we were still under Reykjavik radar control. Upon forcing the FMC to use GPS the aircraft made an immediate left turn to intercept what was now the correct track. No further issues with Nav systems the rest of flight.Suggestion: Increased training and awareness in Nav systems in recurrent/initial would have greatly helped. Previously in my experience when Rad/Nav has been switched back to on; FMC will now switch back to GPS for position. Have never been trained or seen a situation where crew had to force FMC to accept a good GPS position. Company location manual does not address this. Additional info included in Company location manual.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.