scope:

Description of Driving Event:

The Space Shuttle Main Engine (SSME) telemetry system does not show absolute Greenwich Mean Time (GMT), but rather, the number of controller cycles since change in mode. Approximate timing had been established (during the STS 51-L launch process) by aligning the first guidance cycle after engine start with the engine start command time determined by analysis of the General Purpose Computer (GPC) event times. This aligned the SSME event times to the GPC event times to within a few tens of milliseconds, sufficient for normal flight analysis but insufficient for determining event sequencing for a system failure, which involves an interaction between the SSME and the rest of the vehicle. Without precise correlation between multiple data systems, there may be a delay in subsequent anomaly investigations due to timing uncertainties or a need to correct data timing.