THE VLT TIME REFERENCE SYSTEM: A MICROSECOND-ACCURATE TIME/SYNCHRONIZATION BUS FOR DISTRIBUTED CONTROL SYSTEMS
F. Biancat Marchet, B. Gustaffson, Garching B. Muenchen · 1999
The demanding timing and synchronisation requirements of the VLT (Very Large Telescope) array of telescopes operating in interferometric mode, have led to the development of a dedicated fibre-optic bus, the Time Bus, distributing accurate timing to more than 100 Local Control Unit computers (LCU) scattered on a wide area. The UTC (Universal Time Coordinated ) is received from the Globalstar GPS (Global Positioning System) satellites, processed and distributed through the Time Bus to the Local Control Computers with accuracy in the range of few microseconds. Means for processing, backing up and relay the time information as well as for interfacing the Local Control Computers with the Time Bus, have been developed and integrated. Software drivers have also been developed providing means to synchronise processes running on different local computers to the absolute time from better than 10 microsecond accuracy at interrupt level to scheduling processes, or sending messages with millisecond accuracy. Tests have been performed to evaluate the performances both at ESO (European Southern Observatory) headquarters and at the VLT observatory on Cerro Paranal, Chile, where the system is now in regular operation. The compliance with leap seconds, Y2K and GPS week rollover has been investigated.