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

visitors on a tour of the GP-B facilities, including our MissionOperations Center (MOC), the Anomaly Resolution Board Room(ARB), and the display of GP-B technology in the lobby of ourbuilding.15 JULY 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 451 days (64 weeks/14.79 months)Science Data Collection: 322 days (46 weeks/10.56 months)Current Orbit #: 6,655 as of 4:30 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 0, Single-bit errors (SBE): 8 (dailyaverage)As of Mission Day 451, the Gravity Probe B vehicle and payload are ingood health. All four gyros are digitally suspended in science mode.The spacecraft is flying drag-free around Gyro #1.The Poker Flats ground station in Alaska has been experiencinghardware problems, and for this reason, we have had to re-schedulesome of our data telemetry sessions at other NASA TDRSS groundstation facilities. We have recently run tests at the McMurdo station inAntarctica, and we successfully completed a data capture session fromMcMurdo this past Monday, 11 July 2005, after some last-minutescrambling to get the connection properly set up. We have also beenusing the Wallops ground station in Virginia.On Wednesday, 12 July 2005, we completed a paper simulation of thecalibration procedures we will be performing towards the very end ofthe mission, just before the helium runs out, to move our telescopefrom our guide star, IM Pegasi, to a nearby star and back to IM Pegasi.As reported last week, gyro #3 transitioned into analog backupsuspension mode during the first phase of a calibration test that beganon Thursday, 7 July 2005, that involves electrically “nudging” the gyrorotor to various pre-defined positions within its housing. We restoredgyro #3 to digital suspension last Thursday evening and continuedphase 2 of the test last Friday. We suspected that the root cause of thetransition to analog mode was likely due to a known “race” condition,which occurs when the gyro rotor reaches a low threshold, set by thehardware. For this reason, we suspected that gyro #3 would transitionto analog mode again during the second phase of the calibration test,and this was the case last Friday. We again returned gyro #3 to digitalsuspension and completed the test successfully.Yesterday, 13 July 2005, marks the one-year anniversary of our fullspeedgyro spin-up in space (gyro #4 was spun up to 105 Hz/6,300rpm). That was a very tense and exciting time in the MissionOperations Center (MOC) here at GP-B. Also yesterday, we usedultraviolet light to reduce the electrostatic charge on all four gyros.508 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission