DC10-30 SUSTAINS TRACK AND HDG DEV AT CDG ON DEP.

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WE WERE ON THE AMOGA-G SID AT THE PARIS CDG ARPT IN FRANCE. WE TOOK OFF IN VMC ON RWY 9 AND WERE TO FLY THE SID; WHICH REQUIRED A STRAIGHT OUT CLB ON RADIAL 88 OFF THE CDG VOR UNTIL AN NDB/DME FIX AT WHICH WE HAD TO TURN L TO DEPART THE TERMINAL AREA. ORIGINALLY; OUR FLT PLAN CALLED FOR AN AMOGA-H DEP OFF RWY 8L; FOR WHICH OUR GPS COMPUTERS WERE PROGRAMMED. AS WE RECEIVED CLRNC FROM PARIS ON THE GND TO EXPECT RWY 9 FOR DEP; I REPROGRAMMED THE DEP SID IN THE GPS COMPUTERS. I HAND FLEW THE DEP WITH REF TO MY GPS/FLT DIRECTOR; WHILE THE CAPT HAD HIS FLT DIRECTOR SET TO VOR WITH THE CDG VOR COURSE OF 88 DEGS SELECTED. FOR SOME REASON; MY FLT DIRECTOR COMMANDED ME TO FLY A R TURN TO INTERCEPT A DIFFERENT RADIAL THAN REQUIRED BY THE SID FOR THE RWY WE USED (RWY 9). I INTERPED THIS COMMAND AS A SLIGHT COURSE CORRECTION TO STAY ON THE 88 DEG RADIAL/HDG. I INITIATED THIS SLIGHT TURN; REALIZED SOON; HOWEVER; THAT MY FLT DIRECTOR/GPS SHOWED A DESIRED TRACK OF 109 DEGS AND CALLED FOR A CONTINUING COURSE CORRECTION/TURN. I LOOKED AT THE CAPT'S COURSE BAR WHICH USED THE VOR NAV MODE AND REALIZED THAT IT COMMANDED A TURN TO THE L INSTEAD. THAT IS WHEN I RECOGNIZED MY GPS COURSE TO BE IN ERROR AND I CORRECTED THE TURN BY BANKING TO THE L TO RETURN TO THE ORIGINAL TRACK OF 88 DEGS. THAT IS WHEN THE DEP CTLR CALLED AND ASKED FOR AN IMMEDIATE TURN TO 360 DEGS BECAUSE WE WERE ABOUT TO ENTER THE DEP CORRIDOR FOR RWYS 8L&R. I TURNED THE ACFT TO THE NEW HDG WHILE CONTINUING OUR CLB. THERE WERE NO OTHER ACFT IN OUR VICINITY WHICH WOULD HAVE CAUSED A TFC CONFLICT; AND OUR TCASII SYS NEVER INDICATED ANY TFC CLOSE TO OUR POS. THE CTLR SAID THAT; DUE TO THE UNAUTH COURSE CORRECTION TO THE L; HE WAS REQUIRED TO FILE A TFC RPT OF THIS INCIDENT. WHEN WE EVALUATED THE SIT; WE CAME TO THE CONCLUSION THAT; EVEN THOUGH I HAD REPROGRAMMED THE GPS COMPUTERS TO THE NEW DEP SID; MY GPS; FOR SOME UNKNOWN REASON; STILL DIRECTED ME TO INTERCEPT THE INITIAL OUTBOUND COURSE FOR RWY 8L. WE DID NOT COME UP WITH ANY EXPLANATION AS TO WHY THE GPS HAD NOT ACCEPTED MY PROGRAM CHANGE. EVEN THOUGH I CORRECTED MY OUTBOUND COURSE AFTER REALIZING THE ERROR; I SHOULD HAVE CAUGHT THIS INITIAL TURN COMMAND AS BEING INCORRECT SOONER AND CONTINUE ON RWY HDG; USING RAW VOR DATA FROM THE CAPT'S NAV SYS; UNTIL THE REASON FOR MY GPS ERROR COULD BE DETERMINED. LUCKILY; NO TFC CONFLICT HAD BEEN CREATED AND THE SIT DID NOT BECOME DANGEROUS.