B747-400 RECEIVES FALSE GPWS WARNING ON POMOK SID FROM ZSPD.

Narrative

ON POM22D SID PASSING FL190; ON COURSE ENRTE FROM PK TO POMOK WITH THE R AUTOPLT IN COMMAND; THE GPWS 'WHOOP; WHOOP; PULL UP TERRAIN' AURAL WARNING SOUNDED; ACCOMPANIED BY ITS ASSOCIATED VISUAL WARNINGS ON THE PFD'S AND NAV DISPLAYS. THE MSA FOR THIS RTE SEGMENT IS 3000 FT. WE WERE IN CLR AIR IN DAYLIGHT BTWN CLOUD LAYERS. WE VERIFIED THERE WAS NO TERRAIN HAZARD AND THAT ACFT PERFORMANCE (PITCH; THRUST; AIRSPD AND VERT SPD) WAS NORMAL. WHILE THE AURAL WARNING WAS AUDIBLE AND THE NAV DISPLAYS WERE COMPLETELY OBSCURED BY THE VISUAL WARNING SO THE MAGENTA LINE WAS NOT VISIBLE; WE BELIEVE THE FO NAV DISPLAY MAP SHIFTED TO THE R ABOUT 3 NM BECAUSE THE FMC MESSAGE 'VERIFY POS' APPEARED ON THE CAPT'S AND FO'S FMS CDU'S. WITH THE R AUTOPLT STILL IN LNAV; THE ACFT SIMULTANEOUSLY INITIATED A R TURN. TO SILENCE THE AURAL WARNING; AND TO CLR THE COLORED VISUAL WARNINGS OFF THE NAV DISPLAYS AND PFD'S; WE SELECTED 'TERRAIN OVERRIDE.' WE IMMEDIATELY NOTED THE FO'S NAV DISPLAY SHOWED WE WERE L OF TRACK. ALSO SIMULTANEOUSLY; ATC ASKED IF WE WERE PROCEEDING TO POMOK. THE FO SELECTED HDG SELECT AND THE CAPT REQUESTED A VECTOR TO POMOK TO VERIFY NAV ACCURACY. ATC ASSIGNED AN INITIAL HDG TO POMOK AND THEN ASSIGNED A HDG FOR TFC SEPARATION TO PIKAS. THE CAPT EXPLAINED WE HAD EXPERIENCED A MOMENTARY NAV SYS MALFUNCTION. ATC SAID ROGER. APCHING PIKAS; ATC CLRED US DIRECT TO UNTAN. AFTER EXECUTING THE DIRECT; WE VERIFIED WITH ATC THAT OUR RADAR TRACK WAS CORRECT. WHILE THE CREW WAS DISCUSSING WHAT HAPPENED AND WE WERE ESTABLISHED ON TRACK; THE ACFT STARTED ANOTHER R TURN TO CAPTURE THE TRACK. THE FO IMMEDIATELY WENT TO HDG SELECT. THE R NAV DISPLAY TRACK HAD SHIFTED ABOUT 3 MI R AND THE L NAV DISPLAY SHOWED ON TRACK. THIS CONFIRMED WHAT WE BELIEVED TO HAVE HAPPENED JUST MINS EARLIER. WE ASKED ATC TO VERIFY OUR RADAR POS AGAIN. THEY VERIFIED THAT WE WERE ON TRACK. THE CAPT'S NAV DISPLAY WAS CORRECT. WE DISCONNECTED THE R AUTOPLT AND SELECTED THE L AUTOPLT COMMAND AND MONITORED THE TRACK. WE WENT TO THE POS REF PAGE 3/3. THE GPS-R POS DISPLAY WAS INTERMITTENT. WHEN IT WAS VISIBLE; GPS-R WAS NOT RELIABLE. WHEN COMPARED WITH THE 3 IRS AND GPS-L; GPS-R WAS ABOUT 40 MINS LATITUDE S; 2 DEGS LONGITUDE E AND 12 KTS SLOWER THAN THE OTHER POS REFS. ALTHOUGH NO GPS EICAS MESSAGE APPEARED; THE RELIEF OFFICER LOCATED THE 'GPS L; R' CHKLIST AND SUGGESTED WE TRY THE PROC. THE CREW DISCUSSED IT AND WE ALL AGREED. USING THE CHKLIST; WE INHIBITED GPS NAV INPUT TO THE FMS AND PURGED. FOR THE REMAINDER OF THE FLT; WE MONITORED THE POS REF 3/3 AND 2/3 PAGES; SPECIFICALLY THE RELIABILITY OF GPS-R AND THE REQUIRED NAV PERFORMANCE. ABOUT 2 HRS 43 MINS INTO THE FLT; THE EICAS 'UNABLE REQUIRED NAV PERFORMANCE' APPEARED. WE VERIFIED OUR NAV ACCURACY WITH KMG RADAR WHO CONFIRMED WE WERE ON THE AIRWAY. WE DISCUSSED THE SIT AND DECIDED TO RE-ACCOMPLISH THE 'GPS L; R' CHKLIST EVEN THOUGH WE DID NOT HAVE THAT EICAS MESSAGE. AFTER THIS PROC; GPS-R WAS IN REASONABLE AGREEMENT WITH THE OTHER POS REF. WE ENABLED THE GPS INPUT TO THE FMS. WE CONTINUED TO MONITOR ALL POS REF FOR ACCURACY AND REASONABLENESS FOR THE DURATION OF THE FLT TO DXB. THE REMAINDER OF THE FLT WAS NORMAL. ON ARR AT DXB; I WROTE UP THE FAULTY GPWS 'PULL UP' WARNING; THE FO MAP SHIFT; THE GPS-R AND THE 'UNABLE REQUIRED NAV PERFORMANCE.' I DEBRIEFED MAINT.