Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew FLC complied w / Automation / Advisory
Aircraft
About an hour into our flight near ZZZZZ Intersection we received a Degraded message on our Honeywell FMS.  This was followed by a DR or dead reckoning annunciation.  The Honeywell FMS manual defines DR mode as follows: 'DR mode is defined as the loss of radio updating and all other position sensors (IRS and GPS).'  When we looked at our GPS sensor page which normally shows the coordinates; the GPS sensor coordinates were blank.  We then asked ATC if they showed us on course and they informed us that we were off by 5 NM.  We then got vectors until we could pick up the ZZZ VOR with short range needles.  The rest of the flight was flown using short range NAV.  After approximately 30 minutes after the FMS issue we powered the FMS down and powered it back up but still had the same problem for 20 minutes or; so then the previous annunciations went away.  We monitored the FMS and at times it appeared to be off course when comparing it to our VOR course.  We landed uneventfully and wrote up the FMS.Suggestions: A dual FMS configuration would have given us a backup other than short range nav.
CE-560XL Captain reported a loss of all GPS/IRS input to the aircraft's FMS during cruise.