RPTR WAS NAVING BY VLF OMEGA FROM HNL TO SAC AND WAS 40 MI OFF COURSE APCHING THE CALIFORNIA COAST. ZOA CONFIRMED OFF COURSE BY RADAR IDENT PROCS NEAR CLUKK INTXN.

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THE FLT DEPARTED HNL; HI; UNEVENTFULLY AT XX24 AM ON AUG/XX/96. PREFLT AND IRU INITIALIZATION WAS NORMAL AND THE APPROPRIATE VLF STATIONS HAD BEEN DESELECTED. DEP WAS UNEVENTFUL AND ALL SYS SEEMED TO BE TRACKING TOGETHER AND WELL WITHIN TOLERANCES. SHORTLY AFTER REACHING CLURE RPTING POINT ON R465; HNL SWITCHED THE ACFT TO ENRTE FREQS WITH OAK FIR. WE WERE UNABLE TO MAKE HF CONTACT WITH THEM AND OUR POS WERE PASSED TO THEM BY TANGO GULF POLAR 80. CONTINUED SEARCH OF ALL AVAILABLE HF FREQS WAS IN VAIN. THE HF SYS HAD WORKED PROPERLY ON THE FLT TO MIDWAY ISLAND AND BACK ON JUL/YY/96. THE ENRTE PORTION OF THE FLT WAS UNEVENTFUL UNTIL REACHING CLUKK INTXN WHERE ZOA NOTIFIED US THAT THE ACFT WAS 40 NM N OF THE INTXN. DURING THE ENRTE PORTION OF THE FLT PRIOR TO CLUKK; I HAD TUNED IN ALL AVAILABLE NAVAIDS TO ENSURE THAT WE WERE ON TRACK. NO VOR/TAC WERE REACHABLE AT THAT POINT. THE NDB FARALLON ISLAND 314 (F) WAS ALSO TUNED IN; BUT IT DID NOT COME IN UNTIL WE WERE RELATIVE CLOSE IN TO THE STATION AND WAS OF NO NAV USE TO US AT THAT POINT. I HAD TUNED IN AM STATION KFRC 610 ON THE NDB AS A REF ONLY; AND IT HAD IN FACT POINTED TO SAN FRANCISCO FROM SOME 600 NM OUT AND PLACED THE ACFT IN RELATIVE GOOD POS ON R465 INDICATING ITS POS JUST TO THE N OF THE R465 TRACK. ALL INDICATIONS ON THE NAV EQUIP AT THIS TIME APPEARED NORMAL. ALL HDGS AND TIMES APPEARED IN AGREEMENT WITH THE FLT PLAN THAT WE HAD SO WE FELT RATHER CONFIDENT THAT WE WERE WELL ON TRACK. FREQUENT COMPARISON WITH THE GPS POS AT THE FITS PANEL CONFIRMED THAT ALL WAS APPARENTLY WELL. THE GPS WAS GIVING VERY LOW POS VALUES. UPON REACHING ABOUT 250 NM FROM THE COAST; THE VLF AND IRS STARTED TO DRIFT OFF GETTING TO 49.2 ON BOTH. AT THIS TIME; WE WENT TO HDG AND MAINTAINED FLT PLAN HDGS UNTIL WE WERE CALLED ON RADAR BY OAKLAND AND TOLD WE WERE 40 NM N OF THE INTXN OF CLUKK. SHORTLY AFTER THE CALL SAUSALITO AND OAKLAND VORS CAME UP AND SHOWED US TO BE N OF TRACK. AS I RECALL OAKLAND VOR SHOWED US ABOUT ON THE 226 DEG RADIAL. ZOA THEN CLRED US DIRECT TO SACRAMENTO AND SAID THEY WOULD HAVE TO FILE AN ONER RPT. NOTHING ELSE WAS SAID. THE REST OF THE FLT WAS UNEVENTFUL. ON AUG/XX/96 ON THE FLT BTWN SACRAMENTO AND OAK THE FOLLOWING OCCURRED. ABOUT AN HR INTO THE FLT; THE VLF SHOW A DRIFT OF 18.9; THE IRS SHOWED A DRIFT ERROR OF 4.9; AND BOTH VPU'S SHOWED 3.8 ERRORS. AT THIS TIME I DESELECTED VLF ENTIRELY. WITHIN 3 MINS; THE IRS WAS AT 0.7 AND THE VPU'S INDICATED 0.1 AND 0.1. WITHIN 5 MINS THE IRS WAS INDICATING 0.2 ERROR AND THE VPU'S BOTH 0 ERROR.