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

Low temperature bakeout was first practiced in May, during the third week of the mission. This practice LTBprocedure had the added benefit of imparting a very small amount of spin-up helium gas to the gyros. Followingthe practice LTB, the SQUID gyro read-out data revealed that gyro #1, gyro #3, and gyro #4 were slowlyspinning at 0.001, 0.002, and 0.010 Hz, respectively (1 Hz = 60 rpm). The Gyro Suspension Systems (GSS) wereable to measure gas spin-up forces at the level of approximately 10 nano-newtons. This meant that the GP-Bscience team is able to interpret data from gyro spin rates four to five orders of magnitude smaller than whatwas planned for the GP-B science experiment.The final low temperature bakeout procedure was performed during the first week in August, following the finalhigh-speed spin up of all four gyros. This process yielded a post-LTB pressure inside the vacuum probe of1x10 –11 Torr, or 100,000 times less than 1x10 -6 Torr in the external space vehicle low Earth orbit (LEO)environment.2.4.3.8 Spin Axis AlignmentAfter gyro spin-up, the spin axis direction of each gyro had to be closely aligned to the guide star. This wasperformed using the Gyro Suspension System (GSS) in a special mode to slowly align each gyro spin axis to bein a certain orientation nearly parallel to the line of sight to the guide star. The gyroscopes were oriented anumber of arc-seconds away from the apparent line of sight to the guide star so that over the mission, with thepredicted relativistic precession and expected annual pointing aberration taken into account, the averagemisalignment of the gyroscopes would be zero. This minimized the possibility of disturbance torques on therotors and enhanced the accuracy of the spin axis measurement process.There were two phases to this alignment process—coarse and fine. During the coarse alignment, as much as onedegree of spin axis orientation correction was achieved. The fine axis alignment then provided the finalorientation accuracy of better than 10 arcseconds relative to the target orientation.At the end of July 2004, after all four gyros had been spun-up to full speed, coarse spin axis alignmentcommenced. As noted in section 2.4.1.6 above, the lower than anticipated spin rates of the gyros, coupled withthe nearly perfect sphericity of the gyro rotor surfaces, made the process of aligning the spin axes of some of thegyros—particularly gyro #4— more difficult and more time consuming.Coarse spin axis alignment was completed on gyros #1 and #3 during the first week in August 2004, and the spinaxes of gyros #2 and #4 were coarsely aligned by the middle of August. Fine spin axis alignment thencommenced on all four gyros. Gyros #1, #2, and #3 completed their alignment and transitioned into science(data collection) mode during the last week in August. Gyro #4 required a little over two extra weeks tocomplete fine spin axis alignment and transition to science mode.2.4.3.9 Electrostatic Discharge of the Gyro RotorsAn ongoing concern during the Gravity Probe B mission has been to minimize the build-up of electrostaticcharges on the gyro rotors. The rotors build up a charge in two ways: 1) the process of suspending the rotorstransfers some change between the housing and rotor surface, and 2) protons from the sun are constantlybombarding the spacecraft, especially over the South Atlantic Ocean—the so-called “South AtlanticAnomaly”—and some of these protons strike the rotors and leave a deposit charge on them. Furthermore,various tasks such as gyro spin-up may impart a charge to the rotors.Thus, periodically during the IOC phase, the team exercised an electrostatic discharge procedure, which usesultraviolet light to reduce the electrostatic charge on the rotors. Each gyro housing is fitted with two fiber-opticcables that run from the gyro housings, up through the top hat of the Probe and out to an ultraviolet light sourcein the Experiment Control Unit (ECU) box, mounted on the spacecraft frame. UV light was beamed throughthe fiber optics onto the gyro rotors to discharge them. As a result of the light shining on metal surfaces in therotor housing, electrons are liberated from the surface of the metal and create a cloud of electrons between the56 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission