Time System
For every command, telemetry or event generated in the system, a time-stamp must be included, intended to be sufficient to rendezvous information associated with a particular image or state of the observatory. As specified in LSE-30, all time tagged events reported both internally and externally by the LSST system shall be done with an accuracy of 0.010 seconds, and all internal events within the LSST system shall be recorded with a precision relative to the master clock of 0.001 seconds. The time reporting standard shall be International Atomic Time (TAI).
Time-stamps must be provided by every subsystem utilizing the SAL package, where time-stamp fields will be included in the topic definitions. The SAL package will verify the time-stamp to be present in the message content.
A GPS disciplined quartz oscillator forms the basis of the time system and the timing signal is distributed using the PTP Protocol, that does not require an independent time distribution network and is transmitted over the normal Ethernet cables. PTP is widely used to synchronize a computer to Internet time servers or other sources, such as a radio or satellite receiver or telephone modem service. It can also be used as a server for dependent clients. It provides accuracies typically less than a millisecond on LANs and up to a few milliseconds on WANs. Typical PTP configurations utilize multiple redundant servers and diverse network paths in order to achieve high accuracy and reliability.