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

DateSeverity68 16-Jun-04 Medium Decline in GPS lock GPS lock time ratiodecreased from a nominal98% to 35% over a few dayperiod, roughly 60 days intomission.69 19-Jun-04 Obs T GSS Cal14 Sequence Gyros 1-4 did not re-centerat conclusion of CalSequence70 24-Jun-04 Obs T Thruster 12 bottomingout during roll-up to 0.6rpm71 27-Jun-04 Obs T Loss of Star Sensor IDs athigher roll rates72 28-Jun-04 Obs F GPS First Failure TestActivated73 29-Jun-04 Obs O SSR not in playbackmode during playbackpass74 29-Jun-04 Obs T Roll down is less thanexpectedSubtypeTitle Description Root cause Corrective ActionThruster 12 commandedcompletely closed duringroll-up to 0.6 rpm. Mayreduce ATC dynamic range.At 0.6 and 0.9 rpm, initial staridentifications decreasedresulting in attitude errorsGPS First Failure TestActivatedSSR not in playback modeduring playback passSV roll down only to 0.7 rpm,0.5 rpm expected.Likely cause: SEU or longperiodsoftwareerror/overrun.Template scheduling error.Thruster pressure sensorbiasMost probable causes are:1) Gyro Bias errors, 2) sensorgate strategy, 3) star attitudecorrection weighting gain.Insufficient time allowed insafemode test for receiver tolock onto GPS constellationTemplate schduling error.The excess mass flow wentnegative for substantialportions of the roll downperiod, causing the vehicleto abort the roll downduring those times. Sensedrates on X and Y were largerthan expected.Reboot of unit cleared theproblem; GPS continues toperform as expectedfollowing reboot.Schedule templates incorrect order.Adjust pressure sensor biasto eliminate bottom-outRemaining actions: Updateprocedure for re-ID andlock on after possiblefuture loss of stars (in work)Increase first failure testtimeout duration.Shedule templates incorrect order.Command slower roll downrates.534 March 2007 Chapter D — Summary Table of Flight Anomalies