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

eported in the Anomaly Review Board (ARB) Observation #71. (See Appendix D, Summary Table of FlightAnomalies.) In analyzing this situation, the ATC team, in consultation with the ARB, identified three factorsthat most likely contributed to the star trackers renewed failure to correctly identify star patterns:1. Gyro bias errors—at the time we had rolled up to 0.9 RPM, roll and attitude errors are greatly magnifiedat this high roll rate, making the precise setting of control gyro hardware biases difficult. We did notspend enough time collecting data at this roll rate, in order to obtain an accurate prediction of gyrobiases.2. Incorrect gate strategy—this involves setting a software “gate,” in the flight database, that tells the startrackers (in degrees) how large a “window” to put around a potential star, and how big a region to searchin the star catalog. At 0.9 rpm, it appears we had too large a gate.3. Star attitude correction weighting gain—this sets how far the vehicle's attitude reference “steers” awayfrom a misidentified star, thereby preventing further star updates. Basically, the gain was set too low.During IOC, the vehicle was in mode 2A, using star trackers to help update the attitude error estimate.During the science mission, the star trackers update only the spacecraft roll; they have no effect on X/Yattitude. This is not an issue for the science mission, where the science telescope alone provides X/Yattitude error estimates.Following the mass trim and liquid helium bubble wrap operations, the spacecraft’s roll rate was decreased to0.52 rpm, and revised ATC software that addressed both star tracker and malfunctioning thruster issues wasuploaded to the spacecraft and installed on the on-board computer. The revised software resulted in asignificant improvement in attitude control, and on 3 July 2004, the science telescope re-locked onto the guidestar, with the spacecraft rolling at 0.52 rpm.Throughout the month of July, the ATC team continued to fine tune the ATC parameters in an attempt tooptimize drag-free performance at 0.52 rpm, and also to decrease the amount of time required for the telescopeto re-lock onto the guide star each time the spacecraft emerged from its eclipsed state behind the Earth. By thelast week in July, ATC parameter tuning had reduced the time required to re-lock onto the guide star from asmuch as 15 minutes to less than one minute. In addition, the number of reference stars “seen” by the startrackers was increased from three to eight.Finally, during the second week in August 2004, the spacecraft roll rate was increased from 0.52 rpm to 0.75rpm (and eventually to 0.7742 rpm) in order to improve the signal-to-noise ratio of the science data with thegyros spinning more slowly than anticipated. This final increase in spacecraft roll rate was well tolerated by therevised ATC parameters, and since they were implemented, the telescope was more consistently able to re-lockonto the guide star as it emerged over the North Pole each orbit for the remainder of the mission.2.4.3.3 Mass Trim SequencesAfter launch, the center of mass and products of inertia of the space vehicle can be corrected by moving some orall of the seven mass trim mechanisms (MTM). The process to operate the MTMs includes analysis on theground of ATC data (to see what forces and torques are required to hold attitude), generation of commands,and then execution of special mass trim sequences. The mass trim procedures were conducted during the thirdweek in June.The mass trim procedure is somewhat similar to dynamically spin balancing an automobile tire. Weightsmounted on long screw shafts are attached in strategic locations around the spacecraft frame, and small motors,under control of the spacecraft’s ATC, can turn these screw shafts in either direction, causing the weights tomove back and forth by a specified amount. Based on feedback from the GSS, the spacecraft’s center of mass canthus be precisely positioned to place the vehicle’s center of mass on the desired vehicle roll axis. (A very limitedcapability exists to position the fore/aft mass center, but none of this trimming was performed)Gravity Probe B — Post Flight Analysis • Final Report March 2007 51