B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft; Human Factors
At or near ZZZ VOR; I noticed the aircraft started to turn when on a direct clearance to fix ZZZ1. Followed quickly by an Unable RNP. I quickly went to heading mode and requested and was given a heading from ATC. Aircraft was about a mile left of course. Trouble shooting looked to be a GPS issue. Turned off GPS navigation and used IRS function and aircraft tracked courses normally. We changed flight plan with ATC to no GPS and continued on with a normal flight and approach and landing.
B767-300 Captain reported having a GPS issue resulting in a track heading deviation.