GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Figure 9-20. Representative drag-free performance together with gravity gradientacceleration on the space vehicle9.3 GSS SoftwareThe GSS software (GSW) residing in the RAD6000-based processors in each chain performs the controlcalculations and telemetry processing required to operate the suspension for each gyroscope. The code itself waswritten in the C language, rather than ADA used on the CCCA and the SRE systems, for two reasons: 1)compatibility with prototype code developed at Stanford prior to the design of the GSS, and 2) efficiency inexecution to allow the flight system to suspend a gyroscope on the ground. To this end, the VxWorks operatingsystem used on the CCCA and SRE subsystems was replaced with a single thread, non-preemptive real timeoperating system with a fixed scheduler. Since the GSS processor needed to execute a real-time control loop athigh speed without hiccups, the software team could control explicitly the priority of the control calculationswith other less-critical telemetry or housekeeping functions. This level of explicit control also made the codingof the transitions between spin-up and science configurations much more tractable.The code image is relatively small at 480 kB; with data storage and scratchpad space, the code occupied about37% of the 3 MB static RAM aboard the RAD6000 processor.274 March 2007 Chapter 9 — Gyro Suspension Subsystem (GSS) Analysis