A counter on a spacecraft that records the passing of time during a mission. It regulates nearly all activities, and many commands uplinked to the spacecraft are set for execution on counts of the spacecraft clock. In telemetry, the SCLK (pronounced ‘sklock’) helps in the processing, storage, retrieval, distribution, and analysis of data. SCLK is the best on-board estimate of spacecraft event time (SCET), but is not as constant as SCET.