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

Figure 7-23. Effects on telescope pointing from the solar flare on January 20, 20057.5.6.3 ATC slope exception handlingThere appears to be an error in the proton filter in the telescope detectors that remained unexplained for almosthalf of the science mission. The error is now believed to be a compile-time error where the programmedalgorithm is correct and the compiled code is incorrect. The problem lies in the size of the largest numberallowed in the exception handling. Basically, numbers twice as big as the intended threshold pass through theexception handler without causing an exception. However, the ATC slope algorithm still runs on theassumption that the numbers can be no larger than the threshold.The resulting behavior is a “wrap-around” effect where negative numbers of larger magnitude than thethreshold wraps around and are interpreted as a very large positive number. Due to this error, proton activity isespecially potent in the SAA. The large data points getting through to the control system pushed it just about toits limits and resulted in large attitude error oscillations coming out of the SAA.This could have been fixed by changing the onboard code, but a much simpler solution was implemented. Thevehicle is now set up so that only half of the range of the telescope detectors is used. Since, we only use half of thedetector range, we are guaranteed that the largest negative number will still have a smaller magnitude than the“wrap-around” threshold.198 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis