As mentioned in Chapter 2, the spacecraft clock (SCLK, pronounced "sklock") is typically a counter maintained by CDS. It meters the passing of time during the life of the spacecraft. Nearly all activity within the spacecraft systems is regulated by the SCLK (an exception is realtime commands). The spacecraft clock may be very simple, incrementing every second and bumping its value up by one, or it may be more complex, with several main and subordinate fields that can track and control activity at multiple granularities. The SCLK on the Ulysses spacecraft, for instance, was designed to increment its single field by one count every two seconds. The Galileo and Magellan clocks, on the other hand, were designed as four fields of increasing resolution. Many types of commands uplinked to the spacecraft are set to begin execution at specific SCLK counts. In telemetry, SCLK counts that indicate data creation time mark engineering and science data whether it goes to the onboard storage device, or directly onto the downlink. The presence of SCLK in telemetry facilitates processing, storage, retrieval, distribution, and analysis.