Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Aircraft
I was acting as the pilot flying. While enroute cruise Mexico and about half way into this 3 hour flight we were given a shortcut to CJS VOR. I was navigating with the GPS in NAV mode and for at least 30-40 minutes the aircraft was right on course. I was monitoring the airplane but also somewhat distracted because I was finishing up some trip paperwork. About 45 miles outside of CJS; ATC Center suggested us to turn about 10 degrees right for a more direct course to CJS. At this time I noticed that the aircraft had drifted about 8 miles south of course. The NAV selector was still in NAV mode and linked to the GPS. The First Officer recalled that he experienced a loss of RAIM (unreliable GPS) in this exact location before; though ATC said they had no reports of this. We turned back on course and re-engaged the NAV mode to the GPS and for the remainder of the flight it seemed to work fine.Our automation is often not very reliable and requires constant monitoring. I; like many captains; can get distracted by doing trip paperwork while in cruise (especially on longer legs). Drifting off course is very easy to do and can be very un-noticeable (the autopilot can make very slight banking turns). The First Officer (the pilot in the right seat) has a harder time noticing course deviations because while in NAV mode linked to the GPS; the deviation information is only in front of the left seat and not the right seat (only indicates on the Captain HSI).
While enroute to Mexico at FL340 over Central Texas we were given a shortcut to go direct CJS VOR. At this point it was not possible to navigate solely with the VOR due to normal range limitation (CJS VOR is a Terminal VOR which is limited to 25 NM laterally). We used GPS direct to CJS. The aircraft was tracking the course successfully until a few miles prior to the point at 45 NM east from CJS VOR on the 090 degree radial; heading westbound. The aircraft; while on autopilot in NAV mode linked to GPS mode; slowly drifted south of course by about 8 miles. Albuquerque Center identified our deviation and gave us a recommended heading of another 10 degrees to the right to head direct to CJS again. I; as the pilot not flying; remembered prior RAIM losses in the past at this very location while enroute to El Paso so I inquired with the Center Controller about this. I also suggested we may have lost RAIM. The Controller said no GPS jamming operations were taking place at the time. The autopilot was re-engaged to track the new course and for the rest of the flights during that trip it had no issues successfully tracking.It's difficult for the First Officer to notice course deviations when the GPS course is only displayed on the Captain's CDI/HSI; especially when we are beyond or on the edge of VOR range limitations. We add the ability of the First Officer's CDI to link up to the GPS course. We should as pilots also be extra careful when navigating 'direct to' with no official airways to back ourselves up with and be more vigilant of course deviations; especially since the nearest course deviation indicator while operating 'GPS direct' for the First Officer is a tiny LCD indicator on the Trimble GPS.
An Air Carrier was advised by ATC of a track deviation near CJS when the GPS was engaged and indicated on track. The flight crew suspected GPS DOS but ATC had no indication of interference.