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

oversight groups (e.g. NASA) are able to draw on their organizations for expertise as needed. Thisleadership is distinct from program management; it is primarily focused on the technical issuessurrounding the anomaly.3. Employ flexible and easy-to-use record keeping tools. This provides flexibility in the method ofrecording anomalies and attacking problems, it does not significantly limit the team, and it requiresminimal training required. They provide for easy dissemination of data by using popular, established fileformats. Does not (generally) require special software applications installed on each users’ system. Aformal investigation process provides the structure needed to ensure relatively uniform record keeping.4. Develop “custom” computer tools only after true mission needs are known. Experience in pre-launchsimulations with tools developed a-priori found the tools lacking or utterly useless; they did not addressthe practical needs of anomaly investigation. Tools developed following detailed simulations withrealistic anomalies fall closer to the mark. Some of the tools’ features address large time intervals,features not generally brought out during a typical 1 to 3 day simulation; this should be taken intoaccount during tool development. The ops, engineering, and analysis teams should recognize that someof their true needs will be clear only after launch. Ensure that the tools are flexible enough to be modifiedin short order.16.1.2.8 Complexities of a 9-DOF satelliteIssue Summary: Design, test, and operation of a 9-DOF satellite must take into account the additionalcomplexities and challenges that are inherent in such a system.Description of the GP-B experience:The Gravity Probe B satellite actively monitors and controls 9 interacting degrees of freedom during normaloperation:• 3 in orientation of the space vehicle to keep the tracking telescope pointed at the guide star and maintaina constant roll rate and fixed roll phase with respect to the orbital plane• 3 in translation of the space vehicle to keep the vehicle in a drag-free orbit about the geometric center ofthe one of the science gyro’s housing cavities (nominally Gyro 3)• 3 in translation of the housing cavity with respect to the gyroscope rotor (nominally Gyro 3)The other 9 degrees of translation freedom of the remaining 3 gyroscopes are slaved to the first 9 describedabove and do not significantly interact with the dynamics of the vehicle as a whole. Note that by design, therotational degrees of freedom of the gyroscopes are not controlled.Gyro 3 serves as the drag-free sensor (accelerometer) for the satellite. The three control efforts required to keepthe housing centered around the gyroscope rotor are fed to the spacecraft. The spacecraft, in turn, appliescompensating forces via its thruster set to null the forces felt by the gyroscope rotor, as shown in Figure 16-1.These six degrees of freedom significantly interact. The gyroscope position control loop bandwidth (GSS) ismade small to minimize disturbance torques on the rotor, and is on the order of 2 Hz. The spacecraft translationand pointing (ATC) bandwidth is of the same order and is limited by thruster authority and ATC processing.These two control loops interact significantly during operation.Gravity Probe B — Post Flight Analysis • Final Report March 2007 451