What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO SEA. PASSING THROUGH 8K, FIRST OFFICER CHECKED HYDRAULIC SYNOPTIC/LOWER ECAM AND SAW GREEN SYSTEM HYDRAULIC HAD DROPPED FROM FULL LEVEL TO 50-60% LEVEL- NO ECAM DISPLAYED. THE AIRCRAFT LANDED SAFELY AT SEA WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE GREEN SYSTEM QUANITY GUAGE TRANSMITTER AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK.