Air carrier flight crew reported GPS jamming caused an inability for the flight crew to safely accept a visual approach without terrain awareness at night. ATC was able to temporarily stop the GPS jamming to allow the flight crew to safely land with GPS per RNAV approach procedures.
Synopsis
Air carrier flight crew reported GPS jamming caused an inability for the flight crew to safely accept a visual approach without terrain awareness at night. ATC was able to temporarily stop the GPS jamming to allow the flight crew to safely land with GPS per RNAV approach procedures.
Narrative
While enroute and upon initial check-in with Albuquerque Center; our aircraft was cleared for the initial descent into ABQ. We were set up for the LZZRD FOUR RNAV Arrival to the RNP Z Approach for Runway 21. Prior to descent; we were aware via ATC communications that GPS jamming activity was ongoing in the area. Several aircraft were being offered vectors due to potential GPS and navigation degradation.As we entered the descent phase into the terrain environment; we queried ATC regarding the expected duration of the GPS jamming; noting that we were executing an RNAV arrival and approach. At that point; our FMS navigation status indicated a complete loss of GPS signal. We had already inhibited the DME VOR IRS (Inertial Reference System) updating and had selected RNP 0.3 in the FMS.Although no navigational flags were triggered at that time - since we were still in the enroute phase - we closely monitored lateral navigation integrity via the progress page and overlaid the aircraft position on the navigation display to detect any potential cross-track errors. GPS - L invalid" followed by "GPS - R invalid" appeared in the FMS scratch pad. Afterwards; the status page GPS signals showed ( ); indicating the absence of 2 GPS signals. We also had TERR POS (Terrain Position) and a lack of all display terrain. As of yet there was no evidence of navigational degradation or error with the progress page and position indicators.We advised ATC that we were effectively losing terrain awareness and clearance due to the GPS outage combined with the RNAV arrival and approach procedures. This presented a significant concern; as the destination airport runway in use only supports RNAV approaches; and there were no available ground-based navigation alternatives to ensure vertical or lateral terrain clearance. With no GPS; no ground-based NAVAIDs; and without a published conventional approach; our terrain avoidance tools were compromised and our lost communications procedures in a terrain environment were rudimentary at best. As such; we were left relying solely on Grid MORA (Minimum Off-Route Altitude); MSA altitudes; and the like for terrain clearance during arrival. Although weather conditions were VMC; it was nighttime with no significant moonlight. As a result; terrain features were not visually distinguishable; and the environment appeared as a black void throughout the arrival and approach phases. We were not comfortable with accepting visual approach procedures as all of the terrain was not in sight; we were unfamiliar with the airport; and did not have the time or capacity to reference VFR charts.Given the operational limitations imposed by the GPS jamming and lack of terrain awareness in a high terrain environment at night; we turned on the DME and VOR updating to the IRS; began working with Dispatch to coordinate a diversion; requested vectors; and communicated our concerns to ATC stating we would be unable to except the approach and arrival procedures without GPS. We asked if the government was able to turn the GPS jamming off to return the GPS signals back to normal. ATC was very helpful in coordinating this and was able to reestablish GPS navigation by coordinating with the GPS jamming unit. I am estimating from the time of request - to the time the GPS signal returned - was approximately five to seven minutes.We then accepted vectors north of the airport to join the RNAV Y Runway 21 approach to an uneventful landing. ATC was again extremely helpful and reestablishing GPS and providing adequate vectors and altitudes to keep us clear of terrain. Had we had gone lost comms; our VFR procedure was going to be "turn to the southwest away from all terrain and climb well past the GRID MORA." This was essentially mimicking the engine out procedures for the airport; which we were gratefully familiar with from the previous safety briefing events and training."
Second reporter narrative
My First Officer and I were cruising at flight level 380 just southeast of Roswell; New Mexico. We were direct LOKKE which is a fix on the LZZRD4 RNAV Arrival into ABQ with LNAV/VNAV and Command A engaged. At that point; we had set up and briefed to fly the LZZRD4 down to the PILLA transition for the RNAV (RNP) Z 21 approach. Setting up for the RNP we had made sure that we have 2 GPS signals; DME/VOR updating is turned off; RNP value of 0.30 as per the minimums; and finally we made sure the fixes and altitude were matching as well as a present glide path of 3.00. Center Controller advised us that White Sands was conducting GPS jamming. He then advised us that it may affect us and to advise if we need vectors. At this point everything looked normal so out of precaution we checked that the IRSs (Inertial Reference System) are operational. Both me and the FO turned on position to show the IRS stars" on our ND (Navigation Display). Shortly after that we had an FMC scratchpad message that said "GPS-L invalid." We cleared the message only to immediately get a "GPS-R invalid."At this time we were approximately 10 - 15 NM southeast of LOKKE the FMAs (Flight Mode Annunciator) still showed a green engaged LNAV and VNAV. Our ND still showed our route; with my data engaged it showed all the altitudes and time of arrival at each fix. We then went into the FMC to check out Navigation Status page. This time instead of 'GPS (2)' indicating that we have two good GPS signals; it showed 'GPS ( )' showing we have no good GPS signal. At this point we noticed our XPNDR (Transponder) FAIL light illuminated amber on the ATC/TCAS panel. We also lost our terrain awareness; which was indicated by a TERR POS (Terrain Position) amber flag on the left-hand side of the ND and the absence of terrain mapping on the ND. Out of precaution we turned DME/VOR updating back on. Given that it was night; mountainous terrain; and we had no terrain awareness or GPS which was the only available approach at the field; we decided to let ATC know that we will be unable to continue. At that time we sent Dispatch a message via ACARS alerting them of the situation and that we might need an alternate with a FOB of 13.0 lb. After advising ATC that we are unable to continue my FO suggested that we ask them to unjam the GPS. I thought it was a great idea so he let Center know that if they can unjam the signal we can continue. ATC then gave us a descent down to FL200 and a heading of approximately 350 initially then 340 while they coordinated a possible unjamming of the GPS.At that time I put the airplane in heading mode and descended at 1000 - 1500 FPM in vertical speed and slowed the airplane down to 270 as to give us time and room to work the issue out. In the meantime we referenced our Jepp charts for obstacles and grid MORAs (Minimum Off-Route Altitude) to make sure we are well clear of terrain. ATC then vectored us around for a couple of minutes. They then came back and informed us that White Sands will be unjamming the signal and to let them know when we have everything back. After approximately 1 - 2 minutes we got the L GPS back associated with that was the return of terrain display on our NDs; then another minute and we got the R GPS back. We then went over all the RNP approach procedures confirming that we had 2 good signals and we turned off the DME/VOR updating. We then informed ATC that we are okay to continue. They vectored us in to approach the airport from the east. About 20 miles from the airport ATC informed us that there is a helicopter that would block our path for the RNAV RNP Z 21 and offered us a visual with vectors or a vector to the north side approach which was the RNP Y. We informed Approach that we would like the Y approach. They then vectored us west as we set up for the new approach and briefed it. We then preformed the RNP Y 21 with accordance to the FM (Flight Manual). The plane preformed normal all systems were functioning properly to includethe progress page showing our deviations to be minimal and within limits. We continued to a normal landing and to a normal taxi with no further incidents. Overall from losing both GPS to getting them both back we estimate approximately 10 minutes."
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.