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

Figure 7-8. Correlation of pointing roll rate component with telescope/sun angle7.3.1.4 SAA BehaviorThe configuration of the ATC has been slightly adjusted to allow for continued pointing toward the Guide Starduring periods of South Atlantic Anomaly (SAA). This was done at the request of the Science Team, and whileexcessive noise to the telescope detectors is noticed during these periods, much of the noise is filtered out byATC filters and limits. The remaining noise which does slightly effect high-precision pointing during SAA isfound to be a manageable risk as compared to the large benefit of greater mission duration in GSV (anadditional 70 minutes, approximately., per day of GSV). Further improvement of the pointing accuracy throughthe SAA has been achieved by setting the TRE gains and clamp settings to avert the effects of a compilation errorin the exception handling of the ATC slopes. This is discussed in more detail in a section below.7.3.1.5 Guide Star Capture TimesFigure 7-9 and Figure 7-10 below show yaw errors immediately at the beginning of a GSV period, taken fromtwo different dates (07 July 2004 and 27 Aug. 2004, respectively). First, one may note that the overall “capture”duration, the time required to damp out gyro hold errors accumulated during the previous GSI period, has beensubstantially reduced from approximately 13 minutes (07 July 2004) to about 3 minutes. This has beenaccomplished through the judicious selection of ATC controller gains and limits, and by removing the 7th orderpolynomial fit for ATC slope in favor of a truncated linear fit. The immediate benefit of this reduced captureduration (10 minutes shorter, approximately) is an additional 10 minutes of science data during a typical 55minute GSV period. This represents nearly a 20% gain in usable science data per GSV period.In addition to the gain and limit changes that have been made, we have also expanded the definition of thelinear range of the telescope to more heavily weight the proportional term even outside the linear range. Part ofthe reason the guide star captures were so long, was the rate capture mode was actually driving the telescopeaway from the guide star before the integrator caught up to the error in the rate gyros due to the motion of theARP.Gravity Probe B — Post Flight Analysis • Final Report March 2007 181