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

Furthermore, as a result of the transient behavior from the initial conditions while turning on the filter after theSAA, during parts of the year, the notch filter must remain off even after the SAA. This affects data on one ortwo orbits out of every day, or one or two out of every fifteen orbits. On orbit data has been used to map out thestrongest section of the SAA, where more than 50% of the attitude error data points are lost due to protonactivity, as shown in Figure 7-22 below.Figure 7-22. South Atlantic Anomaly Region produced from telescope proton hit data7.5.6.2 Solar FlaresSolar flares have increased the proton activity that the satellite has observed in much of its sensitive equipment.The ATC system has been affected most by solar activity in the telescope detectors. Specifically, the January 20,2005 solar flare charged up the north pole strongly enough to corrupt 90-100% of the telescope data. The dataloss lasted long enough for the satellite to drift far enough to lose the guide star. Several hours of science datawere lost while the team reacquired the guide star. Most other solar flare activity has been weak enough so thecontrol system performance has been unaffected. The telescope data corruption and subsequent spacecraftattitude error is shown in Figure 7-23 below.Gravity Probe B — Post Flight Analysis • Final Report March 2007 197