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

Figure 16-1. Six interacting translational degrees of freedomAdditional coupling between the translational DOF comes from orientation changes. The center of mass of thespacecraft is about 20 centimeters from the center of the drag-free sensor’s housing. Thus, any change inorientation tends to be about the center of mass of the vehicle. The drag-free action nulls these orientationinduced forces and thus moves the effective center of mass of the vehicle to the center of the drag-free sensor’shousing. Again, the orientation control loop bandwidth is the same as the translation control bandwidth andthus this control loop, as well, interacts significantly with the gyro suspension control loop.This brief overview shows, in outline form, that the 9 degrees of freedom are strongly coupled in normaloperation and thus cannot be easily separated into simpler and isolated control subsystems.Lessons:Gravity Probe B has learned a number of important lessons about the design, test, and operation of asophisticated 9 DOF space vehicle, and these can be extended to other similarly complex missions (ST7/GRS,STEP, LISA, ConX)1. Interacting control systems must be tested together. While design is done using best engineeringpractices, often the full performance and complexity of the instrument can be assessed only through ajoint test of the equipment. For missions such as GP-B, a representative ground test is not possible. Thus,final performance is assessed and adjusted on orbit. This was proven out on GP-B with the drag-freesystem, the attitude control system, and the gyro charge management systems.2. Complex mission operations. Tuning up the space vehicle pointing and translation control loopsrequired significantly more time than originally expected (4 months actual vs. an estimated two weeks),though significant effort was expended prior to launch to develop and test in simulation the on-orbitoperations plan. Unmodelled vehicle dynamics required in-place system identification to allow thevehicle to perform to science-compatible levels while both the translation and orientation control was inoperation. This mission complexity stems directly from the coupled nature of the 9 degrees of freedom.3. Prepare for unanticipated behavior. The GP-B gyroscopes exhibited some unexpected behavior once onorbit, significantly: 1) the initial rotor charge was 450mV rather than the expected 100mV or less, 2) thepolhode period of the rotor changed over time when it was expected to be constant, 3) patch charge452 March 2007 Chapter 16 — Lessons Learned and Best Practices