Performance of the Pierre Auger Observatory Surface Array
  The surface detector of the Pierre Auger Observatory is a 1600 water Cherenkov tank array on a triangular 1.5 km grid. The signals from each tank are read out using three 9'' photomultipliers and processed at a sampling frequency of 40 MHz, from which a local digital trigger efficiently selects shower candidates. GPS signals are used for time synchronization and a wireless communication system connects all tanks to the central data acquisition system. Power is provided by a stand-alone solar panel system. With large ambient temperature variations, that can reach over 20 degrees in 24 hours, high salinity, dusty air, high humidity inside the tank, and remoteness of access, the performance and reliability of the array is a challenge. Several key parameters are constantly monitored to ensure consistent operation. The Surface Array has currently over 750 detectors and has been in reliable operation since January 2004. Good uniformity in the response of different detectors and good long term stability is observed. 
The Pierre Auger Collaboration