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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-September 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.

Phase II, which lasted from March through last month (August 2006), focused on understanding and compensating for certain long-term systematic effects in the data that spanned weeks or months. Three important milestones were achieved during Phase II:

  1. Our telescope team completed a careful analysis of data collected from the science telescope over the course of the mission, providing us a thorough understanding of the telescope system performance and enabling us to address some subtle systematic errors introduced into the science data by the telescope.
  2. Using SQUID measurements of the trapped magnetic flux on the gyro rotors, the team made significant progress modelling the polhode motion of the gyroscopes. The term, "polhode," which was coined by the 19th Century French geometer, Louis Poinsot, is a natural, periodic exchange of rotational energy among the inertial axes of a spinning body. The polhode motion of the gyro rotors does not affect the ability of the gyroscopes to measure relativistic precessions, but it does introduce some subtle systematic effects that needed to be removed to obtain the most accurate measurements of gyro spin axis precession.
  3. Towards the end of Phase II, our data analysis team developed some novel methods of looking at the data from a geometrical perspective. These geometrical interpretations highlight key features in the data and are most useful in showing how the data change over time.

The results of the Phase II analysis enabled the team to improve the accuracy of the analysis, yielding increased precision for gyro precession rates over short intervals. Phase II culminated with the 15th meeting of our GP-B Science Advisory Committee (SAC) here at Stanford last Friday and Saturday (8-9 September 2006). During this important meeting, our data analysis team presented a complete progress report to the SAC. (See the GP-B Mission News Story below for more details about the SAC #15 meeting.)

We are now beginning Phase III—the final phase-of the data analysis—which will last until January-February, 2007. Whereas in Phases I and II the focus was on individual gyro performance, during Phase III, the 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 early in the spring of 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.

At the end of Phase III, 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 the SAC, which has been closely monitoring our experimental methods, data analysis procedures, and progress for the past eight years, to obtain its independent review. Moreover, we will seek independent reviews from a number of international experts.

In addition to analyzing the data, members of our team are now in the process of preparing scientific and engineering papers for publication in late 2006-2007. We have also 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 874 days (124.9 weeks/28.7 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
346 days (49.4 weeks/11.3 months)
Current Orbit # 12,877 as of 10:00 AM 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 ~256.6 kelvin (and falling)
Dewar Outer Shell Temperature ~254.5 kelvin
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):
3 in CCCA Backup computer
5 in GSS computers
0 in SRE computer (turned off)


On Mission Day 874, 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 in mid July has been completed, and preparations for the U. S. Air Force Academy to begin controlling the spacecraft are in the final stages.

During August, our small mission operations team finished working through a series of procedures to recover from the 10 July 2006 reboot of the backup CCCA flight computer on-board the spacecraft. Spacecraft recovery efforts, which would have been accomplished in approximately 12 hours during the flight mission, take much longer now because the GP-B team is primarily focused on data analysis rather than spacecraft operations, coupled with the fact that NASA has downgraded post-mission GP-B spacecraft communications to low priority.

An issue with the spacecraft's high-speed communications electronics that arose following the computer reboot has been resolved, and we are once again able to communicate with the spacecraft through the high-speed NASA Ground Station communications network, in addition to low-speed communications via TDRSS (NASA's Tracking and Data Relay Satellite System). Consequently, we now have updated status data on the spacecraft (e.g., current dewar temperatures) that we were unable to provide last month.

One notable status item is that the dewar's inner temperature (~256.6 K or ~2.21 degrees F) has essentially reached thermal equilibrium with the dewar's outer shell temperature (~254.5 or -1.57 degrees F). From now on, both the inner temperature of the dewar and its outer shell temperature will continue to fluctuate slightly, due to “seasonal” changes in the spacecraft's position in relation to the Sun.

Having now recovered from the on-board computer reboot and patched several memory locations affected by Multi-Bit Errors (MBEs) in both the CCCA flight computer and the GSS computer, our mission operations team has returned to the task of completing the spacecraft 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 autonomously begin sending out un-monitored communication signals that could interfere with other spacecraft and missions.

Arrangements for the U.S. Air Force Academy (USAFA) to use the spacecraft part time—shared with our use here at Stanford—as a space operations training vehicle are now nearing completion. Members of the USAFA team are continuing to work with NASA to set up the necessary communications network that will enable them to control the spacecraft from the Academy. It is anticipated that the USAFA will begin controlling the GP-B spacecraft in mid-late October.



This past Friday and Saturday, 8-9 September 2006, we hosted the 15th meeting of our GP-B Science Advisory Committee (SAC). This committee is external to GP-B and is comprised of seven distinguished scientists, with expertise in various areas of relativistic and gravitational physics, low-temperature physics, and astrophysics/cosmology. Several members of the SAC are associated with other NASA and NSF missions whose results relate to GP-B, including WMAP (Wilkinson Microwave Anisotropy Probe), LIGO (The Laser Interferometer Gravitational-Wave Observatory) and GRACE (Gravity Recovery and Climate Experiment). The SAC Chairman is Clifford Will, James S. McDonnell Professor of Physics at Washington University in St. Louis and a world-renowned expert in relativity and gravitational physics.

At this meeting, SAC committee members were joined by three representatives from NASA: Tony Lyons, NASA GP-B Program Manager from Marshall Space Flight Center, Alan Smale, Program Executive for Mission Operations and Data Analysis for Astro-E2, Chandra, and GP-B at NASA Headquarters, and Michael Salamon, Program Scientist at NASA Headquarters.

The purpose of the SAC is to review and advise on all aspects of the GP-B program, including the functioning of the probe and spacecraft, possible sources of error, and the final data analysis. The knowledge and experience garnered by SAC members on other NASA missions has enabled SAC members to provide valuable advice and feedback to GP-B, both during the flight mission and during the current data analysis period.

SAC meeting #15 was the culmination of the second of three GP-B data analysis phases. Beginning the day-long meeting last Friday, GP-B Principal Investigator, Francis Everitt, presented a 90-minute overview of the progress accomplished by the GP-B team during Phase II of the data analysis, and he also described the issues that the team will be addressing during the final data analysis phase, which has just begun. Phase II focused on understanding and compensating for certain long-term systematic effects in the data that spanned weeks or months. Two data analysis sub-teams have been working in parallel, using different methodologies to analyze the same data. Following Everitt's overview, the head of each sub-team presented detailed reviews of their findings to the advisory committee. As was the case in SAC meeting #14 at the end of Phase I last February, there continues to be a high correlation in the results of both sub-teams. GP-B Program Manager, Bill Bencze, then wrapped up the formal presentations with an updated plan for the final data analysis phase.

Among the most interesting developments presented to the SAC was the geometrical approach to the data analysis developed by GP-B Chief Scientist, Mac Keiser, in late July and August. This approach employs a geometric representation of gyro motion and will be integrated with existing filter-based, "algebraic" analysis of the data.

As in previous SAC meetings, on Saturday morning, SAC members had the opportunity to meet informally with members of our science team and discuss details of the data analysis process in more depth. The SAC meeting officially ended Saturday afternoon, and early this week, GP-B Principal Investigator, Francis Everitt, reported that the SAC and NASA representatives alike were pleased with the clarity of the presentations and impressed with the progress that has been achieved.

We are now entering the final phase of the data analysis, which will last 5-6 months. During this phase, the data from all four gyros will be integrated over the entire experiment. The results of this phase will include both individual and correlated changes in gyro spin axis orientation covering the entire 50-week experimental period for all four gyros. Phase III will culminate in a final, extended SAC review process in the January-February 2007 time frame and will involve a careful and critical review of the complete analysis and results. It is expected that other international experts will participate in the final review process, as well.

There has been some discussion in both SAC #14 and SAC #15 about the optimum and most objective method for implementing a blind or double-blind test of the final results, including incorporation of the Harvard-Smithsonian Center for Astrophysics/York University measurements of guide star proper motion. Decisions on these and other end-around tests will be developed in conjunction with NASA and the SAC towards the end of Phase III.

In addition to their Phase III analysis work, members of our science team have already begun preparing a number of scientific and engineering papers for publication, and we are currently working with NASA to plan a formal public announcement of the results of this unprecedented test of General Relativity. We currently anticipate announcing the results both at a scientific conference and at a special NASA event in April 2007.



Our next regularly scheduled update will be at the beginning of October. 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.

Click here to view/download a PDF file containing Professor Everitt's slide presentation from this 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, the composite photo of the GP-B spacecraft orbiting above the Earth, the "seasons" of GP-B, and the photos of our new Mission Operations Center, the SAC #15 meeting, 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. All other photos are part of the GP-B Image Archive here at Stanford. 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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