Here is the update on the Gravity Probe B mission:

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GRAVITY PROBE B MISSION UPDATE FOR 7 AUGUST 2006
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GP-B DATA ANALYSIS STATUS
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Note: A more complete status overview of the GP-B data analysis process & results announcement is posted at the top of our GP-B Home page: http://einstein.stanford.edu. Following is a brief summary of our activities and accomplishments during the past month.

As of August 7, we are continuing with Phase II of the data analysis process, which began at the beginning of March and will conclude at the end of this month. During Phase II, our focus has been on understanding and compensating for certain long-term systematic effects in the data that span weeks or months. The primary products of this phase will be monthly spin axis precession estimates for each gyro, as well as refined daily spin axis orientation estimates. In this phase, the focus remains on individual, rather than correlated gyro performance.
During July, our data analysis team continued to make progress on analyzing and modeling the polhode motion of the gyroscopes--a natural, periodic exchange of rotational energy among the inertial axes of a spinning body. Polhode motion does not affect the ability of the gyroscopes to measure relativistic precessions, but it does introduce some subtle systematic effects that must be removed in order to obtain the most accurate measurements.

Also during July, our data analysis team came up with some novel methods of looking at the data from a geometrical perspective. These geometrical interpretations highlight key features in the data and show how the data change over time. The results of the Phase II analysis have enabled the team to improve the accuracy of the analysis, yielding increased precision for gyro precession rates over short intervals. The team is now beginning to wrap up the Phase II analysis and has begun preparing presentations that will be made to the GP-B Science Advisory Committee during a meeting scheduled here at Stanford on 8-9 September 2006.


GP-B SPACECRAFT & PAYLOAD STATUS AT A GLANCE
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Mission Elapsed Time: 839 days (119.9 weeks/ 27.5 months)
--IOC Phase: 129 days (4.2 months)
--Science Phase: 352 days (11.6 months)
--Final Calibration Phase: 43 days (1.3 months)
--Extended Science Phase: 4 days
--Post Mission Phase: 311 days (44.4 weeks/ 10.2 months)
Current Orbit #: 12,479 as of 2:00 PM PDT
Spacecraft General Health: Good
Roll Rate: Normal at 0.04 rpm (25 minutes per revolution)
Gyro Suspension System (GSS): All four gyros in analog backup suspension mode
Gyro Spin Rates: ~0.52 rpm ("tumbling" at nominal spacecraft roll rate)
Dewar Temperature: As of 7 July: ~257.5 K and rising ~0.13 K/day; Outer shell temperature: ~264 K; current temperatures not available)
Global Positioning System (GPS) lock: Nominal
Attitude Control System: Magnetic Sensing System (MSS) control
Pointing Error: (XY/Pitch-Yaw Axes) 2.0 degrees RMS;
Roll Phase (Z Axis) Error: 5.8 degrees RMS
Telescope Readout: Pointing performance too low to lock onto guide star
Command & Data Handling (CDH): B-side (backup) computer in control
Multi-bit errors (MBE): 1+ in CCCA Backup computer, causing reboot; 2+ in GSS computers; 0 in SRE computers (turned off)

MISSION DIRECTOR'S SUMMARY
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On Mission Day 839, both the GP-B space vehicle and payload remain in good health. All active subsystems, including solar arrays/electrical power, Experiment Control Unit (ECU), flight computer, star trackers, magnetic sensing system (MSS) and magnetic torque rods, gyro suspension system (GSS), and telescope detectors, are performing nominally. Recovery from a computer reboot and other preparations to ready the spacecraft for use by the U. S. Air Force Academy are in progress.
On 10 July 2006, the backup CCCA flight computer on-board the spacecraft rebooted itself. The root cause is most likely stray protons from the Sun striking the spacecraft and triggering one or more multi-bit errors (MBEs) in mission-critical memory locations. One of the areas affected during this reboot was the spacecraft's high-speed communications electronics, and until this issue is resolved, the mission operations team has been limited to slow-speed communications with the spacecraft via the NASA TDRSS (Tracking and Data Relay Satellite System) space network. Consequently, some of the spacecraft status data (e.g., dewar temperatures) that we usually collect during high-speed ground network communications and report on in these updates has not been available, as reflected in the Spacecraft and Payload Status Chart, shown above.

Our now-much-reduced mission operations team has been communicating with the spacecraft to reset all affected on-board systems and to restore the spacecraft to its normal operating configuration. However, since data analysis rather than spacecraft operations is now our main priority here at GP-B, the recovery is proceeding much more slowly than it would have during the actual flight mission.

In July, the United States Air Force Academy (USAFA) in Colorado Springs, CO, decided to begin using the GP-B spacecraft, starting around late September. The USAFA plans to use the spacecraft part time--shared with our use here at Stanford--as a space operations training vehicle. In addition, physicists at the USAFA are evaluating the possibility of performing some experimental studies involving GPS on-orbit monitoring and using the science gyros as accelerometers for aeronomy experiments. Members of the USAFA team are currently working with NASA to set up the necessary communications network that will enable them to control the spacecraft from the Academy, as well as finalizing other details in preparation for their use of the spacecraft.

In preparation for sharing control of the spacecraft with the USAFA, our team here at Stanford must first finish recovering from last month's computer reboot, restoring nominal operation of all on-board systems. In addition, we must also finish implementing hibernation upgrades to various on-board systems and software. Among other things, these hibernation preparations will ensure that in the event of future computer reboots, the spacecraft will not begin sending out spurious communications that could interfere with other spacecraft and missions.

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NEXT SCHEDULED GP-B UPDATE IN EARLY SEPTEMBER, 2006
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Our next regularly scheduled update will be at the beginning of September. Of course, we will send out a timely update if there are any important changes in the spacecraft's status, or if noteworthy events occur here at GP-B in the meantime.
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PREVIOUS GP-B UPDATES
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If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

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OTHER LINKS THAT MAY INTEREST YOU
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* Our GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.


* Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html


* PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.


* NASA's Marshall Space Flight Center also has a series of Web pages devoted to GP-B: http://www.gravityprobeb.com


* The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/


* In addition, you'll find information in the Guide Star FAQ on our Web site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar and on pages 18-20 of the Gravity Probe B Launch Companion: http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf


* The Einstein Exhibition at the Skirball Cultural Center in Los Angeles has closed.However, you can visit the American Museum of Natural History's virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html


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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu

Bob Kahn
Public Affairs Coordinator

Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,

LelandJ