GIII IS DETERMINED BY RADAR TO BE 35 MI S OF COURSE EVEN THOUGH THE ACFT NAV GEAR SHOWED THE ACFT TO BE ON COURSE.

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ON A TRANS-ATLANTIC FLT FROM EGGW TO BGR; A GIII WAS CLRED VIA THE FLT PLAN ALONG THE NATS. NXXX HAS 3 LONG RANGE NAV (LRN) SYS CERTIFIED TO USE THE NATS: AN FMS AND 2 INS'S. AT OUR FINAL LATITUDE/LONGITUDE COORDINATE (N52 W050); WE CONTACTED FIRST GANDER RADIO AND THEN GANDER RADAR ON VHF RADIO. AFTER WE GAVE OUR POS RPT: 'NXXX 52050 AT XA20; FL430; ESTIMATE DOTTY AT XA50; TOPPS NEXT.' WE WERE ASSIGNED A XPONDER CODE AND CLRED DIRECT TO BANGOR. WE SELECTED 'DTO' ON THE FMS AND PROCEEDED DIRECT TO BGR. SHORTLY THEREAFTER; GANDER RADAR SAID WE WERE 40 MI S OF 52050 AND THERE APPEARED TO BE A GROSS NAV ERROR. WE IMMEDIATELY RAN CHKS. I ASKED GANDER RADAR FOR A POS FIX AND ALL 3 LRN'S AGREED WITH THE COORDINATES THEY GAVE US (N5050; W5205). WE CHKED THE FMS DATA PAGE 3/4 FOR THE ACCURACY OF THE FMS SENSOR INPUTS. A SUBSEQUENT CHK OF THE FMS DATA PAGE 2/4 SHOWED AN FMS QUALITY FACTOR OF 'Q01.' UPON LNDG A BGR WE CHKED THE RAMP COORDINATES WITH THE 3 LRN'S. A SUBSEQUENT PHONE CALL TO THE GANDER RADAR SHIFT MGR CONFIRMED THEIR RADAR TRACK SHOWED NXXX WAS 35 NM S OF 52050 AT N51 12.59 W50 34.52 AND THEY WERE FILING A GROSS NAV ERROR RPT TO THE NATS COORDINATING COMMITTEE AND THE CANADIAN MOT.