Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Distraction; Workload; Troubleshooting; Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented
ATC Equipment / Nav Facility / Buildings; Aircraft; Human Factors
While at cruise at 35;000 feet in Cairo control airspace; we began to receive a series of Engine Indicating and Crew Alerting messages- ATC Fault; Unable RNP and Terrain Position.  We referenced the Quick Reference Handbook for each message but were unable to clear the faults. We asked ATC if they saw any issue with our transponder and they replied that they saw no problems. Shortly after crossing from Amman control to Jeddah on airway Y415; we received the FMC message 'Verify position'. We saw that we were off the airway to the left and ATC asked if we were on the airway. The First Officer and I put our range scales to 5 miles and noted that the left side showed us approximately 3 to 5 miles left of course; First Officer side showed the same distance to the right of course.We checked the FMC position reference pages and saw that GPS position was blank and substantial errors on the other positions - FMC; IRS and Radio. We advised ATC of the GPS failure and could not determine which source was accurate. ATC said we were showing right of course. I selected GPS off then on; selected HDG (heading) select to manually steer to course. Reselected LNAV. Nothing seemed to be working. I asked for direct to a fix further down the airway. No VORs were on this airway to use for navigation. I repeated switching heading to LNAV but the aircraft symbol did not move closer to the course line as if it was frozen. We were continually advising ATC and monitoring position reference as GPS was now intermittent and RNP-ANP errors were constantly changing. Eventually GPS began to work and we were able to get back on course.  This entire time we never received any EICAS or FMA indications of any faults such as 'GPS'; 'IRS Nav Only'; 'LNAV'.  ATC passed the information to subsequent ATC and they offered vectors if needed. After position MOGON; navigation functions all appeared normal for the remainder for the flight.
An air carrier Captain reported GPS interference caused a course deviation.