Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 20; Flight Crew Total 3500; Flight Crew Type 300
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Airspace Violation All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural FAR
Flight Crew Overcame Equipment Problem; Flight Crew Exited Penetrated Airspace
ATC Equipment / Nav Facility / Buildings; Aircraft
While enroute to OSH; were using VFR flight following until past Chicago Center where this was terminated; encountered limited GPS signal; and changed to VOR guidance for enroute navigation. While approaching RFD VOR from the south nearing Class C airspace at 5500 feet; DME came active indicating 15 miles south of the VOR and immediately turned west to leave airspace in the event this was entered. Continued west then north to make sure clear and continued to OSH.  Better preflight planning for back up VOR guidance in the event of loss of GPS signal would have helped route planning to correct situation and prevent future occurrence.
Piper 34 pilot reported loss of GPS signal when approaching RFD led to a Class C airspace penetration.