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

housing and rotor surface. A charge control electrode, controlled via the Gyro Suspension System is biased to acertain voltage level to either drive electrons onto or off of the rotor surface, thus affecting the overall charge onthe rotor. The suspension system then measures the charge on the rotor via a force balance technique, and thecharging/discharging process stops when the rotors are within 15 mV of zero. Typically, the rotor charge is heldto within 5 mV of ground potential during normal science operations. Reducing the level of charge wasimportant because it increased the sensitivity and accuracy of the Gyroscope Suspension System (GSS) andreduced some disturbance torques on the rotors.2.5 Science PhaseThis section provides an overview and summary of the 5-week science phase of the mission which began on 27August 2004 and ran through 14 August 2005.2.5.1 Overview of the Science PhaseFollowing IOC, the mission transitioned into the science phase, where the essential science data were collected.This phase officially began on 27 August 2004 (Mission Day #129, Mission Week #20) with the transition ofgyros #1, #2, and #3 into science mode. The gyroscopes were fully configured for the science phase once theiraxes had been aligned with the guide star, as described in section 2.4.3.8 and their rotors had beenelectrostatically discharged as described in section 2.4.3.9. Gyroscopes #1, #2, and #3 were all discharged andaligned by 27 August, 2004. However, gyro #4 required an additional 2 1/2 weeks to complete its spin axisalignment, which occurred on 16 September 2004 (Mission Day #149). Since each of the gyroscopes isindependent of the others, gyros #1, #2, and #3 began providing science data at the end of August, and gyro #4followed in mid-September.2.5.1.1 Mission Lifetime DeterminationsDuring the week of 20 September 2004 (Mission Week #23), we performed a heat pulse meter operation on thedewar to determine its cryogenic lifetime, and in turn, the anticipated length of the science and subsequentinstrument calibration phases of the mission. The results of this test, and several more performed at varioustimes throughout the science phase, indicated that the helium remaining in the dewar would supportapproximately 10 months of science data collection and one month of instrument calibrations.In fact, all of the heat pulse meter helium lifetime predictions that we made during the science phase of themission turned out to be based on an incorrect assumption about the thermal connection between the liquidand gas phases of helium inside the dewar, leading to overly conservative helium lifetime predictions. We wereactually able to collect science data for 11.5 months, followed by 1.5 months of instrument calibration testing.2.5.1.2 Transitioning from IOC to Data CollectionUpon commencement of the science phase, the science team began storing data with a “SCI” prefix so that itwould be clear to all users this was valid science data. The anomaly team also separated its IOC record keepingfrom the new science documentation. Our daily 10 am meeting was moved to 11 am to allow the science teamextra time to analyze the data, thereby making the meeting more meaningful.The science phase was much quieter than the very busy IOC phase, although there still was important work eachday requiring completion in a timely manner. The daily work load generally decreased as the science phaseprogressed. By the end of this phase, operations ran very smoothly, and were punctuated only by the occasionalanomaly.Gravity Probe B — Post Flight Analysis • Final Report March 2007 57