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

2.6.1 Overview of the Calibration PhaseThe purpose of the post-science calibration phase was to perform tests to allow the placing of tight limits onsystematic errors and gyroscope torques. A natural tension existed in choosing the correct moment to end thescience phase and to begin the calibration phase. The technical underpinnings of this tension can be understoodas follows. The total experiment error is the sum of three different types of error (plus the exact knowledge ofthe proper motion of the guide star). These three error sources are:1. Statistical measurement error,2. Systematic errors3. Gyroscope torques.The overall experiment measurement error improves as the duration of the science data taking is increased.Systematic errors, however, do not. Furthermore, some systematic errors and gyroscope torques may not bedirectly evident from the science data and may require on-orbit tests to set upper limits on their effect.Therefore a longer calibration phase reduces the impact of systematic errors and gyroscope torques at theexpense of statistical measurement error. The purpose of the calibration phase was therefore to enhance variouspossible systematic error sources so that tighter limits could be placed on each, thereby reducing the overallexperiment error. This technical balance had to be blended with an important programmatic issue; riskmanagement.During the science phase each of the gyroscopes had a spin rate of more than 60 Hz. or a stored kinetic energyof more than 1 joule. Based upon years of ground test, there was a concern that a de-levitation of a gyroscopewould damage not only that gyroscope, but put the other gyroscopes at some risk as well. Since the calibrationphase required a number of new commands to be performed, the sequence of testing was adjusted to performthe low risk operations first. Another risk involved the possibility that calibration phase would not be completedprior to the depletion of the liquid helium. The final schedule for this phase involved performing a number ofGSS tests followed by a 2 week break. The purpose of the break was to allow the team to assess the adequacy ofthe tests and to allow time to generate new commands that were determined to be needed to support the scienceteam.In choosing to perform the GSS tests first, some of the gyroscopes could be kept in science mode while theothers were GSS tested. Gyroscopes 2 and 3 entered the post-science calibration phase on July 7, 2005.Gyroscopes 1 and 4 entered the post-science calibration phase on August 15, 2005.A series of ATC tests followed the GSS tests. (GSS tests on gyroscopes 1 and 4 were also completed during thisfinal stage of the mission.) The most important of these ATC tests involved intentionally mis-pointing the spacevehicle to allow investigation of resulting gyroscope torques. A series of these tests were performed. These testswere not believed to be of high hardware risk (i.e. the risk of a gyroscope de-levitation was thought to be low,there was a consensus that the risk of schedule slippage was significant. For many of the mis-pointingoperations, the vehicle was to acquire a star other than IM Pegasi. There was a very real concern that findingthese other stars could be problematic and time consuming. In practice, just like the GSS phase which precededit, the operations went off without a hitch. By the end of this period, moving back and forth between stars hadbecome almost routine. The team had learned a tremendous amount during the IOC and science phases.2.6.2 Weekly Calibration Phase HighlightsTable 2-5 below provides a month-by-month overview of the science phase of the mission. For a more detailed,weekly description of activities that occurred during the final calibration phase, see Appendix C, WeeklyChronicle of the GP-B Mission.62 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission