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

6. Using the Gyro Suspension System (GSS), many tests were made to calibrate the positions of the fourscience gyro rotors in their housings.7. In three spin-up stages, spread over a period of about a month, the spin rates of the gyros were graduallyincreased to their final science experiment levels, with many tests and calibrations performed at eachstage.8. A Low-Temperature Bakeout (LTB) procedure was performed to remove stray helium molecules fromwithin the gyro housings and science probe.9. The spin axes of the four gyros were aligned with the guide star,10. Electrostatic charging of each gyroscope was controlled throughout the period using an ultraviolet lightdischarge system.2.4.1.2 The Need to Extend The IOC Time LinePrior to launch, the estimated time line for the IOC phase, based on many tests and simulations conducted atStanford University and Lockheed Martin, was between 42 and 60 days—42 days of tasks, plus 18 dayscontingency. An additional 15 days contingency, if needed, was also budgeted into the overall mission time line.In the actual mission, IOC required a little over twice the amount of time originally anticipated—the IOC phaseofficially concluded on Mission Day #129 (27 August 2004), with gyros #1, #2, and #3 in science mode, and gyro#4 continuing to undergo alignment of its spin axis. spin axis alignment of gyro #4 was completed on MissionDay #149 (16 September 2004), and it too then began collecting relativity data, along with the other three gyros.Two main factors were responsible for most of the extension of the IOC phase of the mission:1. Tuning the ATC system to perform correctly—complicated by the loss of two micro thrusters and acommanded increase in the spacecraft roll rate to more than twice its originally anticipated value2. Full-speed gyro spin-up and gyro spin axis alignment—complicated by the gyros spinning at less thanhalf the anticipated full-speed rates2.4.1.3 Tuning The ATC SystemGP-B is the first spacecraft ever launched with a requirement to control all 6 degrees of freedom of the vehicle. Itrequires three axes of rotational control—pitch, yaw, and roll—and it requires three axes of translational dragfreepositioning. Many other spacecraft, like the Hubble Space Telescope, control pitch, yaw, and roll, but theydo not roll continuously. Also, GP-B is only the second satellite ever launched to achieve 3-axis drag-freecontrol. In retrospect, these requirements were more difficult to achieve on orbit than had been anticipated.2.4.1.4 Micro Thruster & Star Tracker IssuesThe initial IOC time line called for locking onto the guide star within the first four days in orbit. Successfullocking onto the guide star was somewhat delayed due, in part, to the unexpected malfunction of twoproportional helium-gas thrusters, which had to be identified, isolated, and taken out of service. These thrustersfailed individually after two separate events 1) launch, and 2) Science Instrument Assembly (SIA) flux flushingoperations. The likely root cause for both these failures has been identified as foreign particle contamination ofthe thruster mechanism. This problem was mitigated by revising the ATC software programming to performwith 14, instead of 16 thrusters and then uploading the revised software to the spacecraft’s on-board computer.(The remaining 14 thrusters performed flawlessly throughout the entire experiment.)The two conventional star trackers and associated processing software had difficulty properly identifying starfields while the spacecraft was rolling from the first day in orbit and this was formally declared Anomaly #5 bythe Anomaly Review Board (ARB). (For a description of this anomaly, see Appendix D, Summary Table ofGravity Probe B — Post Flight Analysis • Final Report March 2007 45