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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 currently progressing 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 drift estimates for each gyro, as well as refined daily drift estimates. In this phase, the focus remains on individual gyro performance.

In Phase III, which is scheduled to run from late August 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 gyro drift rates 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 drift 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. At the same time, we will be working 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 740 days (105.7 weeks/24.3 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
212 days (30.3 weeks/7.0 months)
Current Orbit # 10,032 as of 4:00 PM PDT
Spacecraft General Health Good
Roll Rate 0.04 rpm (25 minutes per revolution)
Gyro Suspension System (GSS) All four gyros digitally suspended
Gyro Spin Rates ~0.52 rpm (nominal spacecraft roll rate before roll down)
Dewar Temperature ~240.3 kelvin (and rising ~0.19 kelvin/day)
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; 2 in GSS computers; 0 in SRE computer


On Mission Day 740, both the GP-B space vehicle and payload continue to be 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. We have reduced our communications with the spacecraft to TDRSS satellite status communications only, in preparation for placing the spacecraft in a hibernation state.

Two weeks ago, we celebrated the 2nd anniversary of our spacecraft launch on 20 April 2004. It's hard to believe that GP-B has now been in orbit for over two years. Towards the end of May, the spacecraft will enter its 5th full-sun "season." During this two-week period, the plane of the spacecraft's orbit will be orthogonal to the sun's position and the sun will shine broadside on the spacecraft throughout each orbit around the Earth.

The temperature inside the Dewar has now warmed to ~240.3 kelvin, and its rate of temperature rise has slowed to ~0.19 kelvin per day. The temperature inside the Dewar is approaching thermal equilibrium with the temperature of the dewar's outer shell, which has been averaging approximately 249 kelvin (-24.2 degrees centigrade). However, as the spacecraft approaches its full-sun season, the temperature of the dewar's outer shell will will rise a bit, causing a continued temperature increase inside the dewar.

During the month of April, three multi-bit computer memory errors (MBEs) occurred: one in the CCCA (main) computer and one in each gyro suspension computer (GSS1 and GSS2). All three of these memory locations have been patched via commands sent from our Mission Operations Center (MOC).

In coordination with our NASA management office at the Marshall Space Flight Center (MSFC) in Huntsville, AL, we are now in the process of preparing the GP-B spacecraft to enter a hibernation state. In this state, all of the payload computers and electronics will be turned off—this includes turning off the Gyro Suspension System (GSS) computers, the SQUID Readout (SRE) computers, the Telescope Readout (TRE) electronics, and the Experiment Control Unit (ECU). The gyros will be placed in analog backup suspension, which does not require GSS computer control. The CCCA (main) computer will remain on, but its timeline (queue of commands to execute) will be empty.

The on-board GPS system will also be turned off, and we will begin using the NORAD Two-Line Element (TLE) data set for GP-B to determine the spacecraft's orbit. Likewise, the spacecraft's Attitude and Translation Control system (ATC) will be placed in an "auto-pilot" mode, utilizing the magnetometers and magnetic torque rods on-board to maintain the spacecraft's correct attitude on orbit.

Finally, the spacecraft's communication system will be turned off and safeguarded from automatically turning itself on. Thus it will only be possible to communicate with the spacecraft by explicitly sending commands from the ground to power on its communications system. In order to monitor the spacecraft's general health, we are planning to power on the communications system for a short time once a week. We are in the process of discontinuing communications with NASA ground tracking stations, so we will only be able to retrieve spacecraft status data via the NASA Tracking & Data Relay Satellite System (TDRSS).

We expect to complete the transition to spacecraft hibernation by the end of May. Once in hibernation, the spacecraft can remain in this low-maintenance state indefinitely. Should funding become available for one or more post-mission experiments, we can re-activate any on-board systems required. Ultimately, if it is determined that no further experiments will be performed with the spacecraft, we will simply stop communicating with it. The spacecraft's orbit will slowly decay, and in about 70-80 years, it will enter the Earth's lower atmosphere and disintegrate.


As has been the case periodically throughout the GP-B mission, the GP-B spacecraft will be visible in the evening skies over North America for a few days this week. For satellite trackers living in California, the following table provides the evening times when the spacecraft will be visible for the next few days, rising over the southern horizon and visible for about 13 minutes as it travels overhead to the northern horizon. (All rising times shown are Pacific Daylight Time zone.)

Date Rise Set Duration SunLit Max. El.
5/01 09:32:35pm S N 13:41 12:00 55 W
5/02 08:20:28pm SE N 12:30 12:15 23 E
5/02 09:57:38pm SSW NNW 12:56 12:15 26 W
5/03 08:44:25pm SSE N 13:35 13:14 48 E

For people who have satellite-tracking software installed on a computer or handheld PDA, here is the latest Two-Line Element (TLE) data set for the GP-B spacecraft:

1 28230U 04014A 06120.22042468 .00000161 00000-0 21345-4 0 6030
2 28230 89.9779 162.9281 0001799 90.3397 269.8068 14.75757116109050

You can track the GP-B satellite on the Web using NASA's Java-based J-Pass satellite tracking application. In addition, you can track the GP-B satellite on Personal Digital Assistants (PDAs) using either the Palm OS or Pocket PC operating systems with software from Big Fat Tail Productions.

In viewing areas where there are no street lamps or city lights, you can see the GP-B spacecraft with the naked eye. In urban areas, you will probably have more luck viewing it through binoculars.

To the right of this paragraph, we have brought back a photo of the GP-B spacecraft in orbit that was taken by Swiss amateur astronomer and physics teacher, Stefano Sposetti, on 6 December 2004 (during one of the spacecraft’s full-sun seasons). Mr. Sposetti used a 20mm f2.8 photo lens, coupled with a CCD camera on a fixed tripod, with a 60-second exposure to capture a beautiful time exposure of the GP-B spacecraft, rising over a rooftop. You can view other astronomical photos taken by Mr. Sposetti, including photos of the GP-B spacecraft on Mr. Sposetti’s Web page in the Astronomical Image Data Archive (AIDA).



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

Photos & Drawings: The GP-B data collection collage, the composite photo of the GP-B spacecraft orbiting above the Earth, the pre-launch and launch photo collages, and the composite "Seasons of GP-B" diagram were created by GP-B Public Affairs Coordinator, Bob Kahn, using images and photos from the GP-B image archive here at Stanford. The photos of the dewar and magnetic torque rods also came from the GP-B image archive. Finally, amateur astronometer, Stefano Sposetti, provided the two black & white photos of the GP-B spacecraft in orbit. Click on the thumbnails to view these images at full size.


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