DURING A B757-200 PREFLT; THE CAPT ENTERED INCORRECT FMS COORDINATES. AFTER DEP; THE FLT RETURNED TO LAND BECAUSE OF EXTREME NAV INACCURACIES.

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PRIOR TO GATE DEP; CAPT ENTERED GATE COORDINATES INTO FMS. RTE CHK AND LEG CHKS WERE ACCOMPLISHED WITH TOTAL RTE DISTANCE WITHIN 6 NM. ON TAXI OUT; CAPT WAS USING WX RADAR THAT OBSCURED THE RWY SYMBOL DISPLAY. AFTER TKOF; CREW RECEIVED A GPWS TERRAIN WARNING; A CHK OF SYS NAV REVEALED A POS ERROR. THE ACFT MANEUVERED AROUND WX; RETURNED FOR LNDG AT ZZZZ. POST-LNDG; CREW OBSERVED A 500 NM ERROR. CAPT HAD ZZZZ GATE COORDINATES IN HIS FLT BOOK WITH ZZZZ1 PAGES UNDERNEATH. IT IS VERY LIKELY WRONG COORDINATES WERE ENTERED. CAPT REMEMBERS ENTERING NXXX55 AND HAVING TO RE-ENTER AS NXXX5.5. FLASHING ALIGN LIGHT WAS POSSIBLY MISSED BY HEAD DOWN ACTION. AT GATE AFTER RETURN; MAINT WAS PRESENT AS CREW ENTERED CORRECT COORDINATES AND IRU ALIGN LIGHTS FLASHED. ON SECOND ENTRY; COORDINATES WERE ACCEPTED. CREWS HAVE BEEN TAUGHT 3 ATTEMPTS WERE REQUIRED TO ENTER WRONG DATA. A NUMBER OF PROCS WERE MISSED. GATE COORDINATE VERIFICATION BY CREW; AND ACFT ALIGNMENT WITH RWY SYMBOL WERE MISSED. ATTN TO DETAIL A MUST AND A VALUABLE LESSON LEARNED.