FLT CREW OF MD80 DEVIATE FROM CLRED RTE WHEN ENGROSSED IN RESOLVING GPS ANOMALY.

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ENRTE ON CLRED DIRECT ILM CLRNC; THE #1 GPS BECAME INCREASINGLY INACCURATE. THE #2 GPS IS PLACARDED 'NOT TO BE USED AS PRIMARY NAV; USE AS REF ONLY.' IT WAS A SUBTLE INACCURACY NOT IMMEDIATELY NOTICED BY THE CREW OR ATC AS WELL SINCE WE WERE NOT ON A SPECIFIC AIRWAY. AS WE GOT NEARER TO ILM AND WERE ABLE TO IDENT THE MORSE CODE OF THE VOR AND ASCERTAINED THAT BOTH #1 AND #2 RMI'S WERE SHOWING THE DIRECT COURSE TO ILM AS APPROX 345 DEGS VERSUS 014 DEGS AND THAT THE #2 GPS WAS IN AGREEMENT WITH VOR'S #1 AND #2; MADE CORRECTIVE ACTION WITH A L TURN TO FLY DIRECTLY OVER ILM. ATC FREQ WAS EXTREMELY BUSY AND ALTHOUGH WE CALLED ONCE PREVIOUSLY WITH NO REPLY FROM THE CTLR; HE QUESTIONED OUR 30 DEG L TURN AND WE REPLIED THAT WE HAD A #1 GPS FAILURE AND WERE TURNING DIRECT ILM. MADE LOGBOOK WRITE-UP ON THIS MATTER; AFTER WE HAD FIGURED OUT WHAT WENT WRONG AND ASCERTAINED #1 GPS FAILURE BY VERIFYING ON MESSAGE ANNUNCIATOR; '#1 GPS FAIL SYS TIMEOUT.' TURNED OFF #1 GPS TO TRY TO REINSTATE THE SYS; WHILE THIS WAS OCCURRING AND IN THE MIDDLE OF REVIEWING GPS BOOK REGARDING SYS ERRORS; RECEIVED ATC REROUTE 'AFTER FAK DIRECT LDN-LDN 031 DEG RADIAL TO JOIN V-377 HAR V-162 DUMMR INTXN DIRECT ABE.' AFTER XING FAK; INADVERTENTLY TURNED DIRECT SCAPE INTXN WHICH IS THE EXTENSION OF THE LDN 031 DEG RADIAL WHERE V-377 INTERCEPTS TO HAR; THINKING WE HAD JUST CROSSED LDN INSTEAD OF WHERE WE ACTUALLY WERE WHICH WAS FAK. NOTICED THIS APPROX 10 NM N OF FAK AND MADE CORRECTION TO NAV DIRECT LDN AND WE WERE IN THE TURN ATC QUERIED US TO VERIFY ROUTING WHICH WE DID. IN ADDITION; ATC ONLY ADMITTED THIS FACT ON ZJX FREQ AFTER BEING 'GRILLED BY ACR Y' ZJX OVERLOADED AND OVERWORKED. I HAVE JUST BEEN INFORMED BY ATC THAT THE MIL HAS BEEN JAMMING THE GPS SIGNALS IN THE WARNING AREAS ADJACENT TO OUR RTE OF FLT. WE RECEIVED NO NOTAMS ON THIS; THIS MORNING. THIS WOULD EXPLAIN THE WHOLE SIT BUT IT NEEDS MORE INVESTIGATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STRESSED HIS CONCERNS ABOUT 'JAMMING' OF THE GPS.' THIS IS INCONSISTENT; HOWEVER; WITH HIS COINCIDENT STATEMENT THAT BOTH OF HIS GPS RECEIVERS AT ONE TIME INDICATED NO SATELLITES AVAILABLE. HE DISCUSSED AT SOME LENGTH THE POOR QUALITY OF HIS RNAV TOOLS ON THIS ACFT AND THE FACT THAT HIS ACR HAD A VARIETY OF DIFFERENT TYPES OF RNAV SYS ON BOARD ACFT HE FLEW IN ADDITION TO SOME WITH NO RNAV CAPABILITY. FROM HIS DESCRIPTION OF THE SYS ON BOARD THE ACFT IN QUESTION; IT APPEARS THAT CONVENTIONAL VOR/DME NAV WAS THE PRIMARY NAV TOOL AND THE DUAL GPS SYS WERE AN ADD-ON THAT PROVIDED LIMITED INTERFACE WITH THE ACFT'S AUTOPLT SYS. IN OTHER WORDS; IT IS ONE OF AN INCREDIBLE ARRAY OF COMPONENT RNAV SYS OF BROADLY DIFFERING CAPABILITIES CURRENTLY CERTIFIED TO CONDUCT RNAV NAV. HE FURTHER STATED THAT THE ACR MAINT PERSONNEL CHKED THE #1 GPS WHICH HE HAD WRITTEN UP AND FOUND NO FAULTS; ALTHOUGH THEY DID REPLACE THE DATABASE.