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

One of the effects of geomagnetic storming is a significant increase inhigh-energy proton bombardment in any part of the Earth facing theSun, and especially in the region known as the South AtlanticAnomaly, which normally experiences heightened levels of solarparticle bombardment. Thus, it was not surprising that at 4:58 PMPST on 10 November 2004, as the GP-B spacecraft was entering theSouth Atlantic Anomaly region, a proton hit a critical memorylocation in the SQUID Readout Electronics (SRE). This eventtriggered a chain reaction of safemodes that, among other things,halted the GP-B mission time line, caused the SRE computer to reboot,and transitioned all four gyros to analog backup suspension mode.Over the past few weeks, the SRE electronics had sustained two protonhits to non-critical memory locations, and we were already in theprocess of planning the necessary steps to manually reboot it, but as ithappens, nature apparently took care of this issue for us. Fortunately,the effects of this event were limited to the SRE electronics, and thespacecraft’s main (A-side) computer was not affected. Our teamworked through the night this past Wednesday and most of Thursday,sending commands to the spacecraft to reset affected systems andrestore all four gyros to digital suspension modes. As of this afternoon,the spacecraft has returned to normal operation. The effect of thisevent on the experimental data is not yet known. A few data pointsfrom this period may have to be omitted, but such an omission willhave no significant effect on the overall experimental results.19 NOVEMBER 2004 – GRAVITY PROBE B MISSIONUPDATE: DAY 213The GP-B spacecraft remains in good health and is continuing toperform well. We have been collecting science data for 83 days, anddata acquisition process is proceeding as expected. The spacecraft ismaintaining a constant roll rate of 0.7742 rpm (77.5 seconds perrevolution), and it is flying drag-free around gyro #3. The dewar’sinternal temperature remains steady at just under 1.82 Kelvin.At this time of year, the spacecraft is entering a 6-week period where itremains in full sunlight—that is, light from the Sun shines broadsideon the spacecraft. As a result, the dewar’s outer shell has been warmingup, and some of this heat is transferred to the liquid helium inside.Because the temperature and pressure inside the dewar are maintainedat constant levels, this expected seasonal warming of the dewar causesthe helium inside to boil off at a faster rate. To compensate for thisincreased helium flow and maintain the correct pointing position, thespacecraft’s Attitude and Translation Control system (ATC) has been“null dumping” (uniformly venting) the excess helium out through themicro thruster system.Last week, we reported that on 10 November 2004, the SQUIDReadout (SRE) computer failed a fundamental “health check,” whichresulted in its rebooting itself and triggering six protective safemodesthat, among other things, halted the spacecraft’s on-board timeline,unlocked the telescope from the guide star, closed the telescopeshutter, and transitioned all four gyros into analog backup suspension.On first analysis, it appeared that the root cause of this anomalousevent was that a high-energy proton had struck a critical memorylocation in the SRE computer during the severe geomagnetic stormthat was raging all last week.However, further analysis now suggests that the spacecraft did notsuffer a new proton hit after all. Rather, the SRE computer hadpreviously suffered high-energy proton hits resulting in multi-bit (notself-correctable) errors in two of its memory locations. These memorylocations were thought to be non-critical, but it now appears that oneof these affected memory locations was accessed during a routinemaintenance procedure.This caused a health check of the SRE computer to fail, which in turn,automatically triggered the safemodes and re-booted the computer.For precautionary purposes, our team had already prepared aprocedure to manually reboot the SRE computer and clear out anymulti-bit errors, but this will no longer be necessary, since thecomputer’s self-reboot last week cleared and reset all of the computer’smemory locations. The fact that this event happened while thespacecraft was entering the South Atlantic Anomaly region of theEarth (where protection from proton bombardment is significantlyreduced) during a severe geomagnetic storm, was apparently acoincidence.Events such as the one that occurred last week always prompt anumber of inquiries about whether such glitches cause any loss ofscientific data or detriment to the experimental results. The shortanswer to this question is “no.” Such events typically do not—and havenot—had any significant effect on the GP-B experimental data or theresults.492 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission