An air carrier crew found a hole in the GPS coverage nearing the east coast of korea.

Attributes
ACN	285903
Time
Date	199410
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : kpo
State Reference	FO
Altitude	msl bound lower : 29000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : rjtg artcc : rktt
Operator	common carrier : air carrier
Make Model Name	DC-8 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 150 flight time total : 8500
ASRS Report	285903
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : instrument pilot : atp pilot : commercial
Experience	flight time last 90 days : 160 flight time total : 3900
ASRS Report	286289
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During a flight from rkso (osan AFB, rok) to panc on oct/mon/94, we were cleared from mule intersection on A582 to sapra intersection on G585. Sapra is the korean/tokyo fir boundary. Northwest of kpo, both trimble 7900 units went into standby mode. No navigation information is available in this mode. At the same time, taegu center advised 'radar service terminated, contact tokyo radio on 118.9.' I stated we had lost ability to navigation to sapra intersection. Taegu controller 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 controller. We advised several times that we were unable to navigation and requested radar vectors. The tokyo controller assigned us a transponder code, but never acknowledged our problem or advised radar contact. We continued on our pre-standby mode heading and intercepted G585 to jec and continued on course. We were able to regain navigation capability within approximately 15 mins. It was impossible to determine exactly how far off course we were until intercepting jec. A crew information letter issued by my company had advised of VLF-omega-GPS difficulty in that area. Further investigation might be helpful.

Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.