Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert
Procedure; ATC Equipment / Nav Facility / Buildings
We were taking off from 26L out of Atlanta. At 600 feet CA called for the Auto Pilot on. It was engaged and we started flying the departure. We made the turn towards MPASS. Tower called and asked if we were direct MPASS and from all of our indications in the flight deck; we were. Later Departure asked us the same thing; if we were direct MPASS. We told him yes we were; every indication from our flight instruments indicated we were direct MPASS. Departure came back and told us we were a little north of course. We responded with we were on the departure and going direct to MPASS; we had no indications saying otherwise. Departure told us later that they were doing some GPS testing and that might be the cause of the northerly course. Captain had put the flight plan into the FMS. We did a clearance verification procedure that verified our flight plan. Everything looked normal and correct. I personally went through the legs page and verified each leg from the departure and afterwards to our destination. Everything was correct. Taxing down to the runway we did our Before Takeoff checklist which we verified for the second time that we were doing the correct departure with the first fix MPASS. Three times we verified our route and first fix. I honestly think it had something to do with GPS testing that Departure later told us they were doing.
Air carrier First Officer reported that a course disparity between ATC and cockpit indications while departing ATL was apparently due to GPS testing.