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Gravity Probe B

Testing Einstein's Universe



During the 50-week science phase of the GP-B mission and the 7-week instrument calibration phase, which lasted from August 2004 - Septermber 2005, we collected over a terabyte of experimental data. Analysis has been progressing through a 3-phase plan, each subsequent phase building on those preceding it.

In Phase I, which lasted from the end of September 2005 through February 2006, the analysis focused on a short term—day-by-day or even orbit-by-orbit—examination of the data. The overall goals of this phase were to optimize the data analysis routines, calibrate out instrumentation effects, and produce initial "gyro spin axis orientation of the day" estimates for each gyro individually. At this stage, the focus was on individual gyro performance; there was no attempt to combine or compare the results of all four gyros, nor was there even an attempt to estimate the gyro drift rates.

We are continuing to progress through Phase II of the data analysis process, which began at the beginning of March and is scheduled to run through mid-August 2006. During Phase II, our focus is 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 orientation 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.

In May, our telescope team completed a careful analysis of data collected from the science telescope over the course of the mission. We now have a thorough understanding of the telescope system performance. Consequently, some subtle systematic errors introduced into the science data by the telescope have now been addressed in the data analysis process. Likewise, we studied the performance of the SQUID gyro readout system, the gyro rotor dynamics, and the gyro suspension system.

During June, the team made significant progress modelling the polhode motion of the gyroscopes. This polhode motion—a natural, periodic exchange of rotational energy among the inertial axes of a spinning body—does not affect the ability of the gyroscopes to measure relativistic precessions, but does introduce some subtle systematic effects that need to be removed to obtain the most accurate measurements. Using SQUID measurements of the trapped magnetic flux on the rotor, a very precise measurement of the polhode period history was identified. This information, together with the history of the spin speed of the gyroscope allowed the team to build accurate physical models of the polhode motion and how it evolved for each gyroscope over the mission. These models now form the base from which the effects of this class of systematic errors can be largely eliminated.

In July, our data analysis team continued to make progress on analyzing and modeling the polhode motion of the gyroscopes, described above. In addition, 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.

In Phase III, which is scheduled to run from early September 2006 through December 2006, data from all four gyros will be integrated over the entire experiment. The results of this phase will be both individual and correlated changes in gyro spin axis orientation covering the entire 50-week experimental period for all four gyros. These results will be relative to the position of our guide star, IM Pegasi, which changed continually throughout the experiment. Thus, the final step in the analysis, currently scheduled to occur in January 2007, will be to combine our gyro spin axis orientation results with data mapping the proper motion of IM Pegasi relative to the unchanging position of a distant quasar. The proper motion of IM Pegasi has been mapped with unprecedented precision using a technique called Very Long Baseline Interferometry (VLBI) by Irwin Shapiro and his team at the Harvard-Smithsonian Center for Astrophysics (CfA), in collaboration with Norbert Bartel at York University in Toronto and French astronomer Jean-Francois Lestrade.

Playing the role of our own harshest critic, our science team will then perform a careful and thorough final review of the analysis and results, checking and cross-checking each aspect to ensure the soundness of our procedures and the validity of our outcomes. We will then turn the analysis and results over to our GP-B Science Advisory Committee (SAC), that has been closely monitoring our experimental methods, data analysis procedures, and progress for 11 years, to obtain its independent review. In addition, we will seek independent reviews from a number of international experts.

Throughout phases II and III, members of our team will be preparing scientific and engineering papers for publication in late 2006-2007. In addition, we have already begun discussions with NASA to plan a formal public announcement of the results of this unprecedented test of General Relativity. We expect to make this announcement of the results in April 2007.



Item Current Status
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 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 Inside Temperature As of 7 July: ~257.0 kelvin (and rising ~0.13 kelvin/day); current temperature data not available
Dewar Outer Shell Temperature As of 7 July: ~264.0 kelvin (average); current temperature data not available.
Global Positioning System (GPS) lock Nominal
Attitude Control System (ATC)

Nominal for post-mission operation
Pointing Error (XY/Pitch-Yaw): 2.0 degrees rms
Roll Phase (Z Axis) Error: 5.8 degrees rms

Telescope Readout (TRE) 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 computer (turned off)


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.



Our next regularly scheduled update will be at the beginning of September. Of course, we will post 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.


For a two-page, up-to-date overview of GP-B in Adobe Acrobat PDF format, click here to view/download "Gravity Probe B in a Nutshell." In addition, you'll now find our 6-page NASA/GP-B Fact Sheet (PDF format) listed as the last navigation link under "What is GP-B" in the upper left corner of this Web page. You can also click here to download a copy.


On Thursday evening, May 18, 2006, GP-B Principal Investigator, Francis Everitt, gave a 90-minute free public lecture entitled: “Testing Einstein in Space: The Gravity Probe B Mission.” The lecture was sponsored by the Stanford Continuing Studies program, as part its Brainstorms: New Frontiers in Science & Engineering lecture series.

Click here to view an MPEG4 streaming video of Professor Everitt's May 18th lecture.

Both audio only and video versions of this lecture are also available on the Stanford on iTUNES U Web site. This Web page automatically launches the Apple iTunes program on both Macintosh and Windows computers, with a special Stanford on iTunes U "music store," containing free downloads of Stanford lectures, performances, and events. Francis Everitt's "Testing Einstein in Space" lecture is located in the Faculty Lectures section. People with audio-only iPods can download the version under the Audio tab; people with 5th generation (video) iPodfs can download the version under the Video tab.

Photos, Drawings, and Video: The GP-B data collection collage and the composite photo of the GP-B spacecraft orbiting above the Earth, as well as the photos of our new Mission Operations Center and Francis Everitt's lecture were created/taken by GP-B Public Affairs Coordinator, Bob Kahn. The group photo of the team from the U.S. Air Force Academy was taken by former GP-B Program Manager, Gaylord Green. The MPEG-4 video of Francis Everitt's lecture was created by Stanford Video. Click on the thumbnails of any photo or graphic to view these images at full size.


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