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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 444 days (63 weeks/14.56 months)
Science Data Collection 315 days (45 weeks/10.33 months)
Current Orbit # 6,553 as of 5:30 PM PST
Spacecraft General Health Good
Roll Rate Normal at 0.7742 rpm (77.5 seconds per revolution)
Gyro Suspension System (GSS) All 4 gyros digitally suspended in science mode
Dewar Temperature 1.82 kelvin, holding steady
Global Positioning System (GPS) lock Greater than 98.3%
Attitude & Translation Control (ATC)

X-axis attitude error: 221.9 marcs rms
Y-axis attitude error: 221.2 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 6 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +2.8 mV (as of 7/5)
Gyro #2 rotor potential +2.6 mV (as of 7/5)
Gyro #3 rotor potential +8.4 mV (as of 7/5)
Gyro #4 rotor potential +2.7 mV (as of 7/5)
Gyro #1 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 444, the Gravity Probe B vehicle and payload are in good health. All four gyros are digitally suspended in science mode. The spacecraft is flying drag-free around Gyro #1.

Last Saturday, 2 July 2005, a GPS channel alignment error flag was triggered in the spacecraft's B-side (backup) Global Positioning System (GPS) as it passed over the Earth's north magnetic pole. We have seen this same behavior twice before with the A-side (main) GPS receiver, and in both previous cases there was no discernable effect on the GPS system performance. An analysis of this latest event is in progress, and if necessary, we will reboot the GPS system.

Otherwise, all was quiet and nominal on-board the GP-B spacecraft during this year's 4th of July holiday weekend. Because its orbit did not pass over North America last Monday evening, the telescope detectors on-board the spacecraft did not register any brilliant bursts of pyrotechnics in the skies over U.S. cities.

Yesterday, we performed a calibration test on gyro #3 (formerly the drag-free gyro) in which the gyro rotor was electrically "nudged" to various pre-defined positions within its housing. About 45 minutes into this test, the Gyro Suspension System (GSS) for gyro #3 automatically switched from digital to analog suspension mode. Analog suspension is used primarily as a backup mode that holds a gyro rotor securely to keep it from striking the housing wall and allows only coarse positioning of the rotor. By contrast, digital suspension mode is computer-controlled; it puts less torque on the rotor and enables its position within the housing to be controlled with extremely high precision. Yesterday evening, we sent commands to re-suspend gyro #3 in digital mode, and the calibration tests are continuing today and into the weekend. Due to the nature of this calibration test and the performance history of gyro #3, we will not be surprised if it transitions to analog mode again during the second phase of this calibration test.


Towards the beginning of June, the spacecraft transitioned out of its 3-week full sun season to once again being eclipsed from the Sun for part of each orbit. Furthermore, the position of the Earth relative to the Sun has been changing so that the Sun's light is now moving towards the rear of the spacecraft, and this orientation will continue through August and September. During this period, the spacecraft's Attitude and Translation Control (ATC) performance will be the most stable, giving us the "cleanest" data of the entire mission. The pointing performance stabilizes during this period because the Sun is no longer heating up the spacecraft's attitude reference platform (ARP), where the navigational rate gyros and star trackers are mounted, on the forward dome of the Dewar.

As we have reported recently, our measurements indicate that the superfluid helium in the Dewar will be exhausted in approximately eight weeks--during the period of maximum ATC stabilization. This situation has raised some questions about how to proceed through the final two months of the mission. Our original plan was to stop collecting relativity data 3-5 weeks before the helium runs out and to spend those final weeks of the mission exclusively running calibration tests of the science instruments. Some of these tests involve placing torques (forces) on the gyros, and we cannot use the science data collected from a gyro while it is undergoing such a test. Another calibration test involves purposely moving the telescope's pointing axis away from our guide star, IM Pegasi, to a different star and then back again. Clearly, we cannot collect any science data during this telescope pointing test.

However, our plan for spending the final 3-5 weeks of the mission exclusively running calibration tests is not cast in stone, and we are currently at a point of making some trade-off decisions in this regard. For example, we have determined that it is possible to perform the gyro torquing calibration tests on an individual gyro, while the other gyros continue to collect science data. (We are performing such a calibration test on gyro #3 right now.) To address these issues and trade-offs, our GP-B management and science teams spent the entire day today at an off-site meeting. The decisions resulting from this off-site meeting will determine our course of action for the remainder of the mission, and we will report on these decisions in an upcoming Mission News story.


In conjunction with today's off-site meeting, yesterday we had a visit from Tony Lyons, our NASA Program Manager at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama and his boss, Tony Lavoie, Director of Space Systems Programs at MSFC. Tony Lavoie assumed his current role at the Marshall Center last October, and he had never before visited GP-B. So, instead of our usual daily all-hands meeting yesterday, we began the day with a brief presentation to both of our MSFC visitors by GP-B Principal Investigator, Francis Everitt, GP-B Program Manager, Gaylord Green, and several GP-B staff members. The presentation included an overview of the program for the benefit of Tony Lavoie, followed by a status update on the spacecraft and data collection. Following the presentations, Francis Everitt , Gaylord Green, and GP-B co-founder Bob Cannon took both of our MSFC visitors on a tour of the GP-B facilities, including our Mission Operations Center (MOC), the Anomaly Resolution Board Room (ARB), and the display of GP-B technology in the lobby of our building.


We recently updated our NASA Factsheet on the GP-B mission and experiment. You'll now find this 6-page document (Adobe Acrobat 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.

Drawings & Photos: The two layered composite photos of the GP-B spacecraft orbiting the Earth, the diagram of the GP-B seasons were created by GP-B Public Affairs Coordinator, Bob Kahn using Adobe Photoshop and Adobe Illustrator. Mr. Kahn also took the photos of the visit by Tony Lyons (NASA GP-B Program Manager) and Tony Lavoie (Director of Space Systems Programs) at NASA's Marshall Space Flight Center in Huntsville, AL. The photo of the gyroscope housing is from the GP-B Image Archive here at Stanford. Click on the thumbnails to view these images at full size.


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