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

from the dewar through the micro thrusters continues to be withinexpected limits. We are now in our seventh week of data collection,which is going very smoothly, and a recent test on the dewar indicatesthat we have enough helium to continue collecting data forapproximately eight more months. The quality of the data gatheredthus far is excellent.This past week has been a relatively quiet one for the GP-B spacecraft.The oscillations in the drag-free control force, which we have beenreporting on for the past two weeks, have diminished to aninsignificant level. Adjustments that we made to the drag-freesuspension parameters to de-tune a harmonic coupling between thedrag-free control system and helium sloshing in the dewar, combinedwith reducing the gain in the drag-free controller to its initial level,seem to have mitigated this instability. To be certain, we arecontinuing to monitor the drag-free control system with regards tothis issue.Furthermore, gyro #3, which we have been monitoring over the pastthree weeks, since it unexpectedly transitioned from digital to analogback-up suspension, has remained digitally suspended, withoutfurther incident this past week. We suspect that the issue with gyro #3is not related to the gyro rotor (sphere) itself, but rather with somenoise spikes in its suspension system that have exceeded pre-set limits,automatically triggering a transition into analog back-up suspensionmode. Increasing the signal-to-noise ratio of this gyro’s positionreadout seems to have mitigated this problem. However, we arecontinuing to monitor gyro #3, and we are still analyzing the rootcause of this behavior.The short answer is that neither of these events has had any significanteffect on the data or the experimental results. The SQUID readouts foreach gyro continue to provide spin axis orientation data in both digitaland analog suspension modes, so no data was lost when gyro #3transitioned to analog mode. Also, even in those few moments whenthe spacecraft is not in drag-free mode, we continue to collect data, butin this case, we must carefully evaluate any potential torques (forces)placed on the gyros to ensure that they did not alter the gyro’s spin axisalignment.22 OCTOBER 2004—GRAVITY PROBE B MSSIONUPDATE: Day 185GP-B continues to perform well. We have switched back to flying thespacecraft drag-free around gyro #3, and all four gyros are digitallysuspended and generating science data. The spacecraft’s roll rateremains constant at 0.7742 rpm (77.5 seconds per revolution), and allsubsystems are continuing to perform well. As we near two months ofdata collection, the flow of helium from the dewar through the microthrusters remains within expected limits, and the dewar temperature isstable at 1.82 Kelvin. We have approximately seven months of datacollection remaining, and the quality of the data received thus farcontinues to be excellent.The two issues mentioned above have prompted several inquiriesabout the effects of such events on the health of our data and theultimate results of the experiment.This past Tuesday, October 19th, gyro #1, which has been serving asthe “drag-free” gyro for the past few weeks, transitioned into analogbackup suspension mode. Having already worked through this samescenario with gyro #3 last month, we were able to smoothly switchdrag-free operations back to gyro #3 and restore digital suspension togyro #1 within three hours of this event. Because gyro #3 has beenperforming well after we increased the signal to noise ratio in its GyroSuspension System (GSS) position readout, we have made the sameadjustment to gyro #1. We are now monitoring the performance ofboth gyros #3 and #1 to determine if the analog suspension transitionwill recur, or if this issue has been resolved.The oscillations in the drag-free control force, which we have beenreporting on in previous highlights, have remained at an insignificantlevel over the past week. We are performing further analysis andadjustments on the drag-free suspension parameters to ensure the detuningof any harmonic coupling between the drag-free control systemand helium sloshing in the dewar. We are continuing to monitor thedrag-free control system with regard to this issue.Gravity Probe B — Post Flight Analysis • Final Report March 2007 489