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

2.4.3.2 Guide Star AcquisitionAbout 24 hours after launch, the flight software, and specifically the ATC system, automatically began theprocess of acquiring the guide star, which, for a number of reasons, turned out to be more difficult and timeconsumingthan anticipated. As noted in section “Micro Thruster & Star Tracker Issues” above, it becameapparent shortly after launch that the conventional star trackers, located on either side of the spacecraft, werenot properly identifying reference stars, resulting in erratic spacecraft attitude control. This problem was loggedby the ARB as anomaly #5 on Mission Day #1, and it was assigned a medium level of severity. (See Appendix D,Summary Table of Flight Anomalies.)This problem persisted throughout the first month of the mission. Through considerable analysis and testing,the ATC and ARB teams determined that space vehicle conditions (mass imbalance, alignments, and startracker settings) were generating data that the star tracker processing algorithm was unable to successfullyprocess. Actions taken to correct this problem included all of the following:1. Switching to a magnetometer-based attitude determination algorithm.2. Adjusting magnitude and error gate thresholds.3. Tuning control gyro biases.4. Updating star tracker alignment matrices.5. Updating the guide star acquisition procedure based on ATC performance in orbit.In addition, the process of locking onto the guide star was delayed by various other issues that arose on orbit,such as proton radiation that caused a switch over to the B-Side on-board computer, mechanical problems withtwo of the 16 micro thrusters, and thruster instability resulting from a pressure build-up in the dewar followinga flux reduction procedure.By the beginning of June 2004, the pointing error of the spacecraft had been reduced to within 385 arcseconds(0.11 degrees) of the guide star, allowing the science telescope to track IM Pegasi over a portion of a spacecraftroll cycle. Further fine tuning of the ATC continued over the following two weeks, and during the second weekin June, the telescope finally locked onto IM Pegasi.The process of locking the science telescope onto IM Pegasi started with star trackers on either side of thespacecraft locating familiar patterns of stars. Feedback from the star trackers was used to adjust the spacecraft'sattitude so that it was pointing to within a few degrees of the guide star. The telescope's shutter was then opened,and a series of increasingly accurate “dwell scans” were performed to home in on the star. Since the spacecraftwas rotating along the axis of the telescope, imbalance in the rotation axis could cause the guide star to move inand out of the telescope's field of view. Feedback from the telescope was sent to the ATC system, which adjustedthe spacecraft's attitude until the guide star remained in view of the telescope throughout multiple spacecraftroll cycles. The ATC was then commanded to “lock” onto the guide star.Finally, to verify that the telescope was locked onto the correct guide star, the spacecraft was pointed at a knownneighboring star, HD 216635 (SAO 108242), 1.0047 degrees north of IM Pegasi. When the telescope waspointed at this location, the neighboring star appeared with anticipated brightness, and there were no otherstars in the immediate vicinity. Thus, the sighting of the star, HD 216635, confirmed the correct relationshipbetween the locations of the two stars, ensuring that the telescope was indeed locked onto the correct guide star.In a similar maneuver, the star HR Peg (HR 8714), a brighter and redder star, located less than half a degree tothe west of IM Pegasi, was also seen in the telescope, further confirming that we had properly locked onto IMPegasi.During the week of 20 June 2004 (Mission Week #10), we increased the spacecraft roll rate from 0.3 rpm to 0.9rpm in order to carry out mass trim and liquid helium bubble wrap procedures that balance the spacecraftaround its center of mass. (See the following two sections for descriptions of these procedures.) Primarily as aresult of the significantly increased spacecraft roll rate, the telescope came unlocked from the guide star, as50 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission