GROSS NAV ERROR.

Narrative

DEPARTED BERMUDA ENRTE TO EWR. AT RPTING POINT FLANN (FIX) NYC RPTS OUR POS AS BEING 63 MI E OF THE FLANN INTXN. AUTOPLT COUPLED TO #1 OMEGA #2 VOR WAS CTRED ON JFK RADIAL DEPARTING FLANN INTXN AND CONCURRING WITH DME READOUT. NYC DIRECTED FLT TO TURN L TO HDG 270 DEGS IN WHICH WE DID. AT THIS POINT; WE VERIFIED PROPER WAYPOINT COORDINATES AND INSERTION INTO OMEGA. CAPT MADE LOGBOOK ENTRY FOR MAINT TO INSPECT BOTH OMEGA SYS. SUPPLEMENTAL INFO FROM ACN 208497: NY THEN ASKED FOR OUR POS COORDINATES AND WHAT TYPE OF NAV WE WERE EQUIPPED WITH. UPON RPTING OUR INDICATED COORDINATES. CTR STATED THEIR COORDINATES DIFFERED WITH OURS. CTR STATED A RPT WOULD BE FILED AND THAT OUR MAINT SHOULD INSPECT THE OMEGAS. SUPPLEMENTAL INFO FROM ACN 208394: WAYPOINT PROGRAMMING WAS VERIFIED. ALL OMEGA AND AUTOPLT ACTIONS WERE BACK- TRACED BY FLC TO VERIFY; AND TO IDENT POSSIBLE ERRORS. NEITHER I NOR EITHER OF THE OTHER 2 FLC MEMBERS COULD DETERMINE THE REASON THAT WE SHOWED OFF-COURSE BY RADAR UPON CONTACT OVER FLANN. I WROTE UP BOTH OMEGA UNITS FOR MAINT INSPECTION AS HAVING BEEN RPTED 63 MI OFF-COURSE BY ATC.