CloudSat Science Data are time stamped with UTC time by matching CPR profile times with the spacecraft time stamp located in the SSOH data stream.
The spacecraft contains a clock that latches a Vehicle Time Code Word (VTCW) to the engineering data stream. VTCW is referenced to an epoch time that begins when the VTCW clock is reset at some time shortly after launch. All instrument data is associated with a VTCW time.
Concurrently, each CPR profile is given a time stamp by the CPR instrument based on a separate clock on the instrument an knowledge of the VTCW time. To produce the CPR time stamp, the VTCW is latched to the CPR instrument by the spacecraft once per second. In a separate operation, the CPR clock increments a millisecond counter. (note: If the second counter is not reset by the latching of a VTCW time, once per second, the CPR clock will update the second counter when the millisecond counter reaches one second of elapsed time. Because the CPR instrument samples every 160 milliseconds, the second counter should be reset between CPR sample times). The millisecond counter is also reset when the VTCW is latched once per second. Thus, the instrument will attach a time to each CPR profile that includes the second counter (under normal operation this will be the VTCW time provided by the spacecraft) plus the value of the millisecond counter. The VTCW time of the CPR profile will then be the value of the second counter plus the millisecond counter. A separate and simultaneous data stream (state of health) produced by the spacecraft includes the VTCW time and the GPS time that have been latched together. The GPS time is used, during ground data processing at the RSC, to convert the VTCW time to UTC time. Both the VTCW and UTC time are included in the SSOH data stream. The final step, then, is to convert the VTCW time of the CPR profile to a UTC time by interpolation of the 1-second interval SSOH VTCW and UTC times.