A320
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 70; Flight Crew Total 15000; Flight Crew Type 7000
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Environment - Non Weather Related
With autopilot 2 engaged; the aircraft initiated a turn to the left to correct back to what it perceived as its FMGC generated course. The turn was fairly aggressive; 30 - 40 degree intercept; and was questioned by the pilots. A message then appeared on MCDU screen FMGC 1/2 disagree. We noted that; although both boxes indicated that they were both in high accuracy with estimated position deviations of only 0.08 mile; they did; in fact; disagree by almost four miles. We did not know which; if either was correct and were about to query ATC when they contacted us; asking why we were deviating to the north. Aircraft was already correcting to course when we received the call. Turns out; FMGC 2's course was possibly corrupt and that was what the aircraft was following. Within approximately one minute; the two FMGC's updated each other; generating a single accurate course. We informed ATC of the transient downgrade. In addition; GPS was being jammed across the Utah area; with GPS lost message on CRT; but had recovered to GPS primary at time of event.
While in cruise with autopilot 2 on; aircraft made an unexpected 20-30 degree turn left. Analysis showed FMGC 2 showing 3 nm Right of course; but FMCG 1 showing 1.3 nm left. We had been in an area of GPS jamming and the GPS primary had come and gone several times. Data page showed FMCG 1 most closely in occurrence with GPS position; but GPS wasn't reliable. Discussion ensued about which FMGC was most correct and why radio updating would show 2 different positions between the FMGCs to determine correct navigation course. Then FMGC 2 position seemed to update and move toward FMGC 1. Both positions moved to agree and airplane nav moved to correct back on that course. Salt Lake Center queried about turn and we informed of our navigation uncertainty but that problem seemed to be fixed and we showed on course.
A320 flight crew experiences GPS jamming in ZLC airspace resulting in a momentary track deviation. The jamming was NOTAMed but the aircraft's reaction to the jamming was unexpected.