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

Figure 7-12. Drag-Free system diagramSince the GP-B vehicle is rolling at 0.77 rpm, cross track control forces at the roll rate frequency (0.0128 Hz) arethe dominant long-term gyro drift contributor. Cross track forces at frequencies other than the roll ratefrequency will average out due to the vehicle roll and will not cause gyroscope drift. Cross track control forces atthe roll rate frequency must be small enough so that resulting torques do not cause significant gyroscopeprecession or drift.During the Science Mission phase of the Gravity Probe B mission, one of the four Science Gyroscopes inside thespace vehicle acts as a drag-free sensor. In primary drag-free, this gyroscope is following a near perfectgravitational free-fall orbit around the Earth, and is spinning inside its housing a mere millimeter from the edge.GP-B uses sensors inside the housing and proportional micro thrusters to keep the space vehicle perfectlyoriented around the spinning gyroscope therefore, following the drag-free sensor around the Earth in a nearperfectorbit.For the most part, the space vehicle stays on course by following the same free-fall orbit. However, outside thespace vehicle, two factors can alter its path: solar radiation pressure streaming from the Sun and atmosphericdrag.GP-B uses extremely sensitive proportional micro thrusters to reorient itself and keep it on its proper path. Thethrusters utilize escaping helium gas that slowly boils off from the liquid helium inside the dewar as propellant.This minute amount of gas, roughly equivalent to 1/10 th of a human breath or a few millinewtons of force,provides just the right amount of thrust necessary to adjust the space vehicle’s position.The Gravity Probe B drag-free control system has two control configurations. The first is primary drag-freecontrol which senses the science gyro’s relative position inside its housing, and the second is back up drag-freecontrol which measures the control forces, known as “control efforts” applied by the GSS.Since the back up drag-free control configuration attempts to minimize the gyroscope control efforts and notthe measured position of the gyroscope, drag-free performance must be based on the size of the forces appliedto the proof mass gyroscope instead of the proof mass’ position relative to its housing. Furthermore, controlforce levels are an especially relevant performance metric because the primary purpose of the GP-B drag-freecontrol system is to minimize gyro suspension forces that cause gyro torques.There has been one major change to the control algorithm used in the drag-free system. Since the controller iscoordinated in the nadir frame, the body-fixed roll components are shifted to lower frequency. By increasing thegain at orbit, improved performance has been observed.Gravity Probe B — Post Flight Analysis • Final Report March 2007 185