An air carrier was advised by ATC of a track deviation near cjs when the GPS was engaged and indicat...

Narrative:

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 navigation 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 navigation selector was still in navigation 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 navigation 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 navigation 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).

Original NASA ASRS Text

Title: 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.

Narrative: 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).

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1075440
Time
Date	201303
Local Time Of Day	1201-1800
Place
Locale Reference	ZAB.ARTCC
State Reference	NM
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Medium Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Commercial
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Track / Heading All Types

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.