$File TIME_CLOCKS.DOC Nov 26, 1991 KES -------------------------------------------------------------------------- The Galileo spacecraft clocks are based on the imaging instrument (SSI) readout time, called a Major Frame (MjF). It is the time to read the 800 lines of the SSI image into the Command and Data Subsystem (CDS) computer for construction of the downlink telemetry block. This spacecraft clock is called the RIM clock (for Real Time Image Counter) and is 60.333 seconds. There are 91 minor frames (mf) in a Major Frame. Minor frames are numbered 0 to 90, where each minor frame is 0.666 seconds. The Real Time Interrupt (RTI) clock is the next finer time. The RTIs are numbered from 0 to 9 and are therefore 0.0666 seconds. RTIs are divided into eight divisions (called the MOD8 clock) and represent 8-2/3 milliseconds. Major Frame = 1 RIM = 60 2/3 seconds = 91 minor frames minor frame = 2/3 second = 10 RTIs RTI = 66-2/3 millisecond = 8 MOD8s MOD8 = 8-2/3 millisecond Telemetered data is time tagged, when it is received at the ground station, with the Earth Receipt Time (ERT). The Spacecraft Event Time (SCE) is calculated from the ERT and one way light time (OWLT) by the JPL Data Management System (DMS) and applied to the time header supplied with the data. Therefore, three time values are supplied with the data set. The Spacecraft Clock (SCLK) is the only value actually transmitted with the data. ERT and SCE are given in Universal Time Coordinated (UTC).