ZDC ALLEGED THAT THE RPTR WAS OFF COURSE IN THE VICINITY OF CSN.

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APPROX 40 NM NE OF CSN; ZDC TOLD US TO TURN R TO 260 DEGS TO INTERCEPT J48. I ADVISED CTR THAT A 260 DEG HDG WOULD TAKE US OFF THE AIRWAY. THEY SAID WE WERE S OF COURSE. I LOOKED AT OUR MAP; WHICH SHOWED US ON COURSE. WE DID HAVE 'DIR' ON OUR MAP; SO WE DID TURN TO 260 DEG; UPDATED OUR POS OFF THE CSN VOR; AND THEN WENT TO NAV AGAIN. CTR THEN CALLED TO SAY WE WERE S OF COURSE AGAIN. THIS TIME WE CORRECTED AND FLEW THE REMAINDER OF THE FLT USING VOR/AIRWAY RADIALS AND USED THE 'MAP' MODE AS A BACKUP. NO FURTHER INCIDENT OCCURRED. I WAS NOT OVERLY CONCERNED; SINCE ALL OF THE NAV PROBS COULD HAVE BEEN JUST ON OUR ACFT. HOWEVER; ON WED/AUG/94 I FLEW THE SAME FLT AGAIN; AND ALTHOUGH I DID NOT EXPERIENCE A NAV PROB I HEARD ZDC TALK WITH ANOTHER ACFT THAT WAS 'OFF COURSE.' THIS OCCURRED NEAR CSN VOR. VERY STRANGE. ZDC TOLD THIS OTHER ACFT; 'WE HAVE HAD A LOT OF TROUBLE IN THIS AREA.' MY QUESTION IS; IS ANYBODY DOING ANYTHING ABOUT THIS NAV PROB? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT WAS FLYING LGA-TYS OVER CSN WHEN THIS ANOMALY OCCURRED. HE THOUGHT NOTHING OF IT UNTIL HE HEARD A CTLR SAY THAT IT HAPPENS ALL OF THE TIME. THE RPTR WAS FLYING THE MD-88 WITH AN AHARS INSTRUMENTATION SYS. THIS IS NOT AN INS OR IRS ACFT. THE RPTR HAS NOT TALKED TO OTHER PLTS FLYING THROUGH THIS AREA TO SEE IF INS OR IRS ACFT HAVE THE SAME SORT OF NAV PROBS; ACCORDING TO THE CTLRS.