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

“bumps” or deviations on the rotor surfaces are as large as 20nanometers. When the rotors are spinning slowly, these larger“bumps” are detected by the GSS, and they decrease its positioningaccuracy. However, when the rotors are spinning fast, these deviationsaverage out. Thus, at full speed, the average surface deviation on thegyro rotors is less than the one-nanometer precision of the GSS, sothere is no degradation in positioning the gyro rotors.solution for ensuring that the helium mass flow through the ATCsystem remains within required levels. By adjusting thruster biases,and by reducing the commanded helium flow, the amount of heliumconsumed by the ATC system is now less than the helium boiled awayby the natural heat in the dewar. As a result, the helium requirementsof the ATC system will have no impact on the mission lifetime,thereby maximizing the science performance.The final steps in the IOC, prior to beginning data collection, involvethe transition of each gyro into science mode. There are three mainsteps in this transition. First, the spin axis of each gyro is aligned withthe science telescope/spacecraft roll axis. Then, ultraviolet light,beamed from lamps on the spacecraft frame through fiber optic cablesinto the gyro housings, are use to remove any residual static chargefrom the gyro rotor surfaces. Finally, the GSS voltage is reduced, inorder to obtain the best signal on the SQUID readouts. Currently,Gyros #1 and #3 have completed this transition to science mode.Gyros #2 and #4 are finishing spin axis alignment, and they areexpected to complete the transition to science mode over the next fewdays.20 AUGUST 2004—GRAVITY PROBE B MISSIONUPDATE: DAY 122A little over four months into the mission, the spacecraft remains inexcellent health, and all subsystems continue to perform well. Thespacecraft’s roll rate remains stable at 0.75 rpm, and final testing ofdrag-free orbital flight and fine-tuning of the Attitude and TranslationControl system (ATC) is continuing. All four gyros are nowsuspended in low-voltage science mode. All of their spin axes havebeen aligned with the science telescope’s sighting axis, which is lockedonto the guide star, IM Pegasi. These alignments are in the process ofbeing fine-tuned.Last weekend, we completed coarse spin axis alignment on gyros #2and #4. On Monday, August 16th, ultraviolet light was beamedthrough fiber optic cables into the housing of gyro #2 to remove excessstatic charge on the gyro #2 rotor (sphere). Then, the suspensionvoltage for gyro #2 was reduced to 200 millivolts, the level required forscience data collection. The following day, the same procedures werecompleted on gyro #4. Thus, by the middle of this past week, all fourgyros were in the science mode configuration.Furthermore, in the process of performing these tests, we havedetermined that back-up drag-free mode is working very reliably, andthus we have re-selected back-up drag-free operation as the baselinemode for the Science Phase of the mission. We anticipate that ourinvestigation and problem-solving efforts around this roll-axis forcewill take a few more days. During that time, we will continue testingback-up drag-free mode around gyro #3, as well as further fine-tuningthe spin axis alignment of the four gyros.In addition this past week, while optimizing the performance of thetelescope system, we have found a small source of bias noise in thetelescope detector output signal. Although it would probably have noeffect on the science outcome, it seems prudent to eliminate this biasnoise, especially since a simple adjustment will take care of it. We areplanning to make this adjustment over the next two days.Finally, analysis of the gyroscope spin-down rate performance overthis past week has shown that, under low-voltage, science modesuspension, the spin-down rate for all four gyros is less than 0.000001Hz (0.00006 rpm) per hour. This means that it would take more than7,000 years for one of our gyros to completely spin down. On Earth,we could only test the spin-down rates of the gyros in a high-voltagesuspension mode, and based on these ground tests, the orbital spindownrequirement for the gyros was set at 2,300 years. We now seethat the spin-down performance of the GP-B gyros in science mode ismore than three times better than this requirement.About ten days ago, we began to notice that while in primary drag-freemode, the ATC was requiring more helium propellant than planned tocounter an unexpected force along the spacecraft’s roll axis—that is, inthe direction of the guide star. This past weekend, we switched toback-up drag-free mode on gyro#3, which gave a similar result. Overthe past two days, we have completed two tests that have provided aGravity Probe B — Post Flight Analysis • Final Report March 2007 483