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

magnetic pole. This event triggered pre-programmed safemodes,which in turn resulted in the automatic switch-over. We haveprovided a more complete description of this anomalous event and theensuing recovery efforts in the Mission News Section below.The possibility of a B-side switch during the science phase of themission was anticipated, and the team had recently rehearsed theprocedures for dealing with such an event. As a result, much of therecovery process—which otherwise might have taken two weeks orlonger—was accomplished last weekend. This event has caused us tolose about a week’s worth of science data. Our data collection rate hasbeen running above 99% up to this point in the science phase of themission, and the loss of a week’s data will only reduce that level byabout 2%, which is still well within the mission requirement of 90%.Assuming that this event did not place any non-relativistic torques(forces) on the gyros, the loss of this small amount of data will nothave any significant effect on the outcome of the experiment. Analysisto ensure that this is the case is underway.On Monday evening, 14 March 2005 at 11:38PM PST, the spacecraft'ssafemode system triggered another computer reboot—the secondflight computer reboot within the past two weeks. Telemetry indicatedthat this most recent safemode response was activated when a memorycheckout procedure detected at least three multi-bit errors (MBE’s)within a 0.2 second interval in the B-side computer.Since the switchover from the A-Side (main) computer two weeks ago,the spacecraft has remained in control of the B-Side (backup) flightcomputer. In this configuration, if a new series of MBEs occurs, thespacecraft does not automatically switch back to the A-side computer.Rather, the B-side computer reboots itself, and the mission operationsteam follows a pre-defined set of procedures to restore the normalsystems configuration for science data collection-as was the case thispast Tuesday and Wednesday.This week, the team is still in the process of fine-tuning the backupcontrol systems in order to provide the same level of response andquality of science data that we had been obtaining from the main (Aside)systems prior to this event. These fine-tuning efforts areproceeding well, and we have already resumed science data collection.18 MARCH 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 331 days (47 weeks/11.00 months) as of3/17/05Science Data Collection: 202 days (29 weeks/6.75 months) as of3/17/05Current Orbit #: 4,883 as of 2:00 PM PST on 3/17/05Spacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.80 kelvin, rising slowlyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 3 as of 3/17/05, Single-bit errors(SBE): 7 (daily average) as of 3/17/05In response to the reboot, the mission operations team uploaded andran a set of pre-approved recovery commands on the B-side computer.The spacecraft's roll rate, which had automatically decreased duringthe reboot, was quickly restored to the science value of 0.7742 rpm.The team also sent commands to reboot the SQUID ReadoutElectronics (SRE) as part of the initial recovery response, prior to reacquisitionof the guide star and return to drag-free flight. Thisaccelerated the overall recovery process by a full day. We returned todrag-free operation on Gyro #3 at 5:34PM PST on Tuesday, less than18 hours after the B-side reboot event. After several guide star searchmaneuvers Tuesday night, the guide star was successfully acquired at4:41AM PST on Wednesday, 29 hours after the reboot event.498 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission