Narrative:

On arrival into mexico city; we were on the AVSAR4A arrival. After being cleared direct to mavek; the plane began a slight banking to the left and right while continuing direct to mavek. Within a few seconds of crossing mavek; approach queried us on our position as they showed us almost 2 miles off course. They issued us a vector of 140 to join the 164 radial off smo. At this point the aircraft was still in navigation mode and showed it was turning to join the assigned course. However approach quickly queried us a second time to inform us we were now 3 miles off course and below the assigned minimum altitude for the area. Approach assigned a further left turn and told us to maintain VFR and to join the final approach course on this heading. At this point I; as pilot monitoring switched to 'green needles' and saw we were too far to the right and full scale deflection on the 164 radial from smo. Right about this point the captain disconnected the autopilot to join the final approach course as was shown on the FMS. The FMS showed us to be very high on a short final to runway 5R which did not add up to what our position and distance off the smo radial showed. At this point I noticed a 'FMS dr' flag on our mfd. Upon seeing this I immediately called for a go around. The captain immediately added power and began a climb. As we were climbing we reported a navigation failure to the approach controller. He instructed us to turn to a heading of 050 and to climb and maintain 12;000. Shortly after; he gave us a further clearance to turn direct to the slm VOR and hold as published. At this point we once again had navigation troubles as the FMS was directing us in one direction while the approach controller was directing us in another. After tuning the slm VOR we proceeded directly to the VOR and held without further navigational problems. At this point I ran the QRH for a position unknown which was the correct QRH to run for a FMS dr flag. This instructed us to inhibit GPWS terrain. While in holding; the FMS navigational performance degraded further and displayed 'FMS dr 5.0NM.' according to the mfd; our position was displayed nearly thirty miles from the slm VOR where we were holding.the root cause was the failure of the GPS as we crossed the IAF of the RNAV/ILS 5R approach. By the time we had been alerted to the problem; we were already several miles off course.a contributing factor was mavek depicted as a flyover waypoint. Because of this we expected to cross over mavek with some overshoot as is natural of a flyover waypoint.I have experienced the aircraft bank to the left and right repeatedly only one other time. This was when the military was jamming GPS. In that instance we merely deselected the GPS and allowed the aircraft to continue strictly on RNAV equipment. In the future; if I begin to feel the aircraft waver on a course again I will make sure to immediately check the GPS equipment and rnp and to ensure we have adequate performance. Additionally I will cross check using ground based navigation aids.

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Original NASA ASRS Text

Title: Air carrier First Officer reported a GPS failure went undetected on the MMMX AVSAR4A Arrival until ATC issued heading and altitude vectors. A go-around was begun when the FMS DR alerted.

Narrative: On arrival into Mexico City; we were on the AVSAR4A arrival. After being cleared direct to MAVEK; the plane began a slight banking to the left and right while continuing direct to MAVEK. Within a few seconds of crossing MAVEK; approach queried us on our position as they showed us almost 2 miles off course. They issued us a vector of 140 to join the 164 radial off SMO. At this point the aircraft was still in NAV mode and showed it was turning to join the assigned course. However approach quickly queried us a second time to inform us we were now 3 miles off course and below the assigned minimum altitude for the area. Approach assigned a further left turn and told us to maintain VFR and to join the final approach course on this heading. At this point I; as pilot monitoring switched to 'green needles' and saw we were too far to the right and full scale deflection on the 164 radial from SMO. Right about this point the Captain disconnected the autopilot to join the final approach course as was shown on the FMS. The FMS showed us to be very high on a short final to runway 5R which did not add up to what our position and distance off the SMO radial showed. At this point I noticed a 'FMS DR' flag on our MFD. Upon seeing this I immediately called for a go around. The Captain immediately added power and began a climb. As we were climbing we reported a navigation failure to the approach controller. He instructed us to turn to a heading of 050 and to climb and maintain 12;000. Shortly after; he gave us a further clearance to turn direct to the SLM VOR and hold as published. At this point we once again had navigation troubles as the FMS was directing us in one direction while the approach controller was directing us in another. After tuning the SLM VOR we proceeded directly to the VOR and held without further navigational problems. At this point I ran the QRH for a Position Unknown which was the correct QRH to run for a FMS DR flag. This instructed us to inhibit GPWS terrain. While in holding; the FMS navigational performance degraded further and displayed 'FMS DR 5.0NM.' According to the MFD; our position was displayed nearly thirty miles from the SLM VOR where we were holding.The root cause was the failure of the GPS as we crossed the IAF of the RNAV/ILS 5R approach. By the time we had been alerted to the problem; we were already several miles off course.A contributing factor was MAVEK depicted as a flyover waypoint. Because of this we expected to cross over MAVEK with some overshoot as is natural of a flyover waypoint.I have experienced the aircraft bank to the left and right repeatedly only one other time. This was when the military was jamming GPS. In that instance we merely deselected the GPS and allowed the aircraft to continue strictly on RNAV equipment. In the future; if I begin to feel the aircraft waver on a course again I will make sure to immediately check the GPS equipment and RNP and to ensure we have adequate performance. Additionally I will cross check using ground based navigation aids.

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.