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

7.4 Conclusions and Performance SummaryThe Attitude and Translation Control system of the Gravity Probe B space vehicle has performed well over theScience Mission phase. The drag-free control system operating in back up drag-free control mode has proven tomaintain control forces applied to the gyroscopes within acceptable limits for the Science Mission. Drag-freecontrol has been shown to reduce rms control forces by at least a factor of 10. During drag-free, the rms controlefforts applied to the proof mass gyroscope are on the order of 10 nanonewtons.The attitude control system has kept the pointing accuracy during Guide Star Valid periods within the limitsrequired for the Science Mission. The rms normalized telescope signal is consistently around 0.1, or at 10% ofthe total range of S N , and the telescope signal at the roll rate frequency is consistently around 0.01, or a mere 1%of the total range of S N . The attitude control system has proven to keep the normalized telescope signal withinthe linear range 95% of the time allowing for accurate pointing determination.7.5 Flight Challenges/Solutions and AdditionalAccomplishmentsThe following sub-sections of this chapter detail eight major accomplishments of the GP-B ATC system.7.5.1 Quick vehicle stabilization off Delta II w/ failed thrusterThe vehicle rotation rates were damped out quickly even with a failed thruster, shortly after separation from theDelta II. Figure 7-16 below is a plot of the vehicle pitch and yaw rates (RF_SensRate_X and RF_SensRate_Yrespectively).Figure 7-16. Vehicle rotation rate damping after separation from the Delta IIGravity Probe B — Post Flight Analysis • Final Report March 2007 189