AN ACR CREW FOUND A HOLE IN THE GPS COVERAGE NEARING THE EAST COAST OF KOREA.

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DURING A FLT FROM RKSO (OSAN AFB; ROK) TO PANC ON OCT/MON/94; WE WERE CLRED FROM MULE INTXN ON A582 TO SAPRA INTXN ON G585. SAPRA IS THE KOREAN/TOKYO FIR BOUNDARY. NW OF KPO; BOTH TRIMBLE 7900 UNITS WENT INTO STANDBY MODE. NO NAV INFO IS AVAILABLE IN THIS MODE. AT THE SAME TIME; TAEGU CTR ADVISED 'RADAR SVC TERMINATED; CONTACT TOKYO RADIO ON 118.9.' I STATED WE HAD LOST ABILITY TO NAV TO SAPRA INTXN. TAEGU CTLR DIDN'T APPEAR TO UNDERSTAND BUT REPLIED; 'ROGER; CONTACT TOKYO 118.9.' AFTER CONTACTING TOKYO RADIO; IT WAS VERY DIFFICULT TO COMMUNICATE OUR DIFFICULTY TO THE CTLR. WE ADVISED SEVERAL TIMES THAT WE WERE UNABLE TO NAV AND REQUESTED RADAR VECTORS. THE TOKYO CTLR ASSIGNED US A XPONDER CODE; BUT NEVER ACKNOWLEDGED OUR PROB OR ADVISED RADAR CONTACT. WE CONTINUED ON OUR PRE-STANDBY MODE HDG AND INTERCEPTED G585 TO JEC AND CONTINUED ON COURSE. WE WERE ABLE TO REGAIN NAV CAPABILITY WITHIN APPROX 15 MINS. IT WAS IMPOSSIBLE TO DETERMINE EXACTLY HOW FAR OFF COURSE WE WERE UNTIL INTERCEPTING JEC. A CREW INFO LETTER ISSUED BY MY COMPANY HAD ADVISED OF VLF-OMEGA-GPS DIFFICULTY IN THAT AREA. FURTHER INVESTIGATION MIGHT BE HELPFUL.