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

7.5.7 Star Tracker Magnitude UpdatesThe star tracker has worked well after the initial setup and configuration. The majority of the updates to the startracker operations have been updates to the magnitudes in the onboard star catalog. The variation of themeasured magnitudes of the stars identified by the trackers is larger than the magnitude threshold that has beenchosen in order to minimize the probability of mis-identification. Some stars have been identified on the orderof once per day. Upon further investigation, it was determined that the magnitude in the catalog did not matchthe measured magnitude and the star was only identified when the outliers landed in the acceptable rangeaccording to the magnitude threshold. Updates to the magnitude in the onboard catalog have solved theproblem of infrequent star identifications for a number of the stars that were seen regularly, but infrequently.7.5.8 Dewar Slosh ControlThe drag-free control system and control efforts began to grow in an unstable manner at one point during themission. The cause of this behavior is believed to be slosh occurring in the liquid helium in the dewar. Thebehavior observed was a slowly growing magnitude in the control efforts at the estimated slosh frequency andwas likely caused by an attempt to change the roll rate of the vehicle too quickly. After a period of time out ofdrag-free and some changes to the drag-free controller, the system settled down, and the vehicle was returned todrag-free operations. Since the occurrence, a slower rollup/rolldown rate has been used, and no further sloshproblems have been observed.Gravity Probe B — Post Flight Analysis • Final Report March 2007 199