Hi,
Here is the Gravity Probe B Mission Update for October 15, 2004.
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GRAVITY PROBE B MISSION UPDATE FOR 15 OCTOBER 2004:
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Six months into the mission, GP-B is performing remarkably well. The spacecraft is in fine health, flying drag-free around gyro #1. All four gyros remain digitally suspended, and they are all generating science data. The spacecraft is rolling at 0.7742 rpm (77.5 seconds per revolution), and all subsystems are performing well. The Dewar temperature remains nominal at 1.82 kelvin, and the flow of helium from the Dewar through the micro thrusters continues to be within expected limits. We are now in our seventh week of data collection, which is going very smoothly, and a recent test on the Dewar indicates that we have enough helium to continue collecting data for approximately eight more months. The quality of the data gathered thus 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 been reporting on for the past two weeks, have diminished to an insignificant level. Adjustments that we made to the drag-free suspension parameters to de-tune a harmonic coupling between the drag-free control system and helium sloshing in the Dewar, combined with reducing the gain in the drag-free controller to its initial level, seem to have mitigated this instability. To be certain, we are continuing to monitor the drag-free control system with regards to this issue.
Furthermore, gyro #3, which we have been monitoring over the past three weeks, since it unexpectedly transitioned from digital to analog back-up suspension, has remained digitally suspended, without further incident this past week. We suspect that the issue with gyro #3 is not related to the gyro rotor (sphere) itself, but rather with some noise spikes in its suspension system that have exceeded pre-set limits, automatically triggering a transition into analog back-up suspension mode. Increasing the signal-to-noise ratio of this gyro's position readout seems to have mitigated this problem. However, we are continuing to monitor gyro #3, and we are still analyzing the root cause of this behavior.
The two issues mentioned above have prompted several inquiries about the effects of such events on the health of our data and the ultimate results of the experiment. The short answer is that neither of these events has had any significant effect on the data or the experimental results. The SQUID readouts for each gyro continue to provide spin axis orientation data in both digital and analog suspension modes, so no data was lost when gyro #3 transitioned to analog mode. Also, even in those few moments when the spacecraft is not in drag-free mode, we continue to collect data, but in 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 axis alignment. For readers who are interested, we will provide a more detailed explanation of our data collection process in a future update.
Please Note: We will continue sending out this GP-B email update on a weekly basis at least through the first few weeks of the Science Phase of the mission. As mission operations become more routine, we may reduce the frequency of these updates to biweekly. However, from time to time, we intend to post special reports and special updates, as warranted by mission events.
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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.eduBob Kahn
Public Affairs Coordinator
Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,
LelandJ
