CAM-REQ-0111

1.1.1  Timestamp Accuracy and Precision

Specification:  Computer clocks used to produce timestamps shall be synchronized with an observatory master clock to a precision of timestampPrecision and an accuracy of timestampAccuracy, as given in the table below. This requirement shall apply separately to each computer clock.

Discussion: The purpose of time synchronization is to ensure that timestamps recorded in the database are meaningful regardless of which computer generated the timestamp. To achieve this, an observatory master clock is distributed to all computer hosts that generate timestamps recorded in LSST telemetry. Current protocols (PTP and NTPv4) allow system clocks to be synchronized well within the requirement. Timestamps are used to record both internal and external events as observatory telemetry. The relationship between the timestamp and the actual physical event, expressed as latency/jitter, depends on both the computer and the hardware (mechanical, electrical, etc.). It is the responsibility of individual hardware design teams to determine the relevance of latency/jitter. When cross subsystem dependencies on timestamps exist, additional requirements can be documented in ICDs. The term "precision" is to be interpreted as a one-sigma statistical measure, and the term "accuracy" as a statistically determined mean measurement.