HDG TRACK POS DEV DURING AN OVERWATER OP.

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RPTED N54 W20 AT ATA AM26Z; FL410; ETA N53 W15 AT AM49Z; BREAK JUST PRIOR TO N53 W15 SHANNON CALLED US ON UHF 135.6 AND INFORMED US WE WERE OFF COURSE BY 38 MI N. WE RECEIVED HIS COORDINATES AND COMPARED THEM TO OUR OMEGA'S. WE SHOWED WE WERE NO MORE THAN 5-6 MI N OF COURSE. ON CHKING OTHER NAV INPUTS (#1 IRS INPUTS AND #2 IRS) ON THE #1 SIDE SHOWED US 35 MI L. #1 IRS INPUTS AND #2 IRS ON THE R SIDE SHOWED US NO MORE THAN 2 MI L. WE TOOK A HDG FROM SHANNON AND LANDED WITH NO FURTHER COMMENTS. PLOT AT N54 W20 WAS N54.06 - W20.07 #1 SIDE; N54.05 - W20.03 #2 SIDE. THIS WAS NOT SIGNIFICANT OFF COURSE FOR CONCERN. 3 NAVAIDS WERE SHOWING PLUS OR MINUS 5 MI OFF COURSE AND INBOUND TO N53 W15. WE WERE SURPRISED THAT JUST PRIOR TO N53 W15 WE WERE INFORMED WE WERE SO FAR OFF COURSE. AT NO TIME PRIOR TO N53 W15 INCLUDING OUR RECORDED POS AT N54 W20 INDICATED WE HAD TO CORRECT SIGNIFICANT COURSE DEV. NO CORRECTIVE ACTION RECOMMENDED. MAINT IS CHKING NAVAIDS FOR PROBS. WE WILL FLT CHK NAVAIDS FOR DEVS. ALL PLTS WILL BE INFORMED AND UPDATED ON CURRENT LONG RANGE NAV.