AN ACR DISPATCHER RPTS THAT ONE OF HIS ACFT LOST ITS MAJOR NAV SYS. THE FLC WAS ABLE TO MAINTAIN VISUAL CONTACT ON AN ACFT IN FRONT OF THEM UNTIL THEY PARTIALLY RECOVERED THEIR NAV CAPABILITY. THE DISPATCHER WAS UNABLE TO MAKE CONTACT WITH THE FLC THROUGH SATCOM DUE LIMITATIONS IN THE COCKPIT'S SATCOM ALERTING SYS.

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THE FLT HAD A TOTAL LOSS OF THE LONG RANGE NAV SYS REQUIRED TO NAV ACCURATELY THE REMAINDER OF THE OCEAN XING. ACFT WAS ALREADY E OF 4940. THE CREW DID HAVE A VISUAL OF TFC ON THE SAME ACFT. ATC ADVISED THE CAPT TO DSND TO FL290 BELOW REDUCED VERT SEPARATION MINIMA SPACE. THE CAPT REFUSED SAYING; BECAUSE OF CLOUD COVER; HE WOULD LOSE THE VISUAL CONTACT AND WOULD; IF NECESSARY; USE HIS EMER AUTH TO REMAIN AT FL310. SHORTLY AFTER THE FLT WAS ABLE TO RESTORE PARTIAL NAV FUNCTION. DURING THIS TIME THE DISPATCHER HAD CONTACT WITH MAINT TO SET UP A PHONE PATCH VIA SATCOM TO ASSIST THE CREW TO RESTORE THE NAV FUNCTION. I HAD ALSO CALLED SHANWICK CTR AND ADVISED THEM OF OUR PROB AND DEMANDED WE BE ABLE TO REMAIN AT FL310 ABOVE THE CLOUD DECK. SHANWICK AGREED AND WAS TO CONTACT GANDER WITH THIS INFO. I TRIED SEVERAL SATCOM ATTEMPTS AND WAS UNABLE TO REACH THE FLT. THE LINE JUST RANG. IT IS MY OPINION THAT BECAUSE IT WAS DAYLIGHT THE CREW; AS IS USUALLY THE CASE; WAS UNABLE TO SEE THE DIMLY ILLUMINATED LIGHT ADVISING THEM OF A SATCOM CALL. THEREFORE; A VALUABLE CREW RESOURCE WAS UNAVAILABLE IN TIME OF AN EMER. THIS IS NOT THE FIRST TIME A DAYLIGHT SATCOM CALL HAS BEEN UNANSWERED BY A FLC AND NEEDS TO BE ADDRESSED. THEY THOUGHT IT WAS THE FLT ATTENDANTS CALLING. IT'S THE SAME CHIME AND THEY CAN'T SEE THE LITTLE BLUE LIGHT IN THE DAYLIGHT.