Citation Excel (C560XL)
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 77; Flight Crew Total 6423; Flight Crew Type 121
Confusion; Distraction; Troubleshooting
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Environment - Non Weather Related
Our initial takeoff clearance was to turn left to heading 055; and climb to 1;500 feet. After departure; we started our turn and initiated level off at 1;500 feet as we were handed off to Departure Control. Departure assigned a climb to 2;000 feet. We initiated a climb and leveled off at 2;000 feet. We received FMS messages 'GNSS 1/2 NOT NAV' and 'POSITION UNCERTAIN' as we were simultaneously queried by ATC regarding our altitude. The controller stated that she showed us at 2;500 feet; while our primary instruments read level at 2;000 feet. We advised that we would attempt to recycle our transponder; which we did. Immediately following; the controller issued a clearance to climb to 3;000 feet and to turn left heading 270. We initiated a climb and left turn as instructed. The controller then stated that she needed us to start the turn immediately; and we responded that we were already in the turn. After recycling the transponder; all altitude information appeared to be accurate. The aircraft was flown to the Service Center where the FMS messages were written up as a discrepancy to be evaluated for functionality. There were no TCAS alerts activated and the controller did not note any further issues after recycling the transponder. I am unaware if this was reported as a potential pilot deviation. I am aware of the fact that degraded GPS information can lead to erroneous ADS-B readouts; which is why the aircraft was written up at [the service center]. Our instruments did not agree with what the controller was reporting during the 2;000 feet level off.
A pilot reported a GPS anomaly.