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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 472 days (67 weeks/15.53 months)
Science Data Collection 343 days (50 weeks/11.28 months)
Current Orbit # 6,962 as of 1:00 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 98.1%
Attitude & Translation Control (ATC)

X-axis attitude error: 162.8 marcs rms
Y-axis attitude error: 211.4 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 8 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +5.9 mV
Gyro #2 rotor potential - 2.8 mV
Gyro #3 rotor potential +5.2 mV
Gyro #4 rotor potential +1.8 mV
Gyro #1 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 472, 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.

With more than eleven months of science data captured, the mission is proceeding well. Over the past week, we completed preliminary calibration tests and prepared for the final calibration phase in late August. Preliminary calibration testing of gyros #2, #3, and #4 included a modulation of the Gyroscope Suspension System (GSS) preloads at roll rate, a SQUID configuration test, and a SQUID off test.

On July 28, we performed the final heat pulse meter test to determine the amount of liquid helium remaining in the Dewar. The results support previous estimates for the helium lifetime. Based on this final test, we are planning on running final calibrations from August 15 to September 1, 2005. If any time remains after that, further calibrations will be performed at a different roll rate.


Last week, when NASA's Space Shuttle Discovery lifted off from Cape Canaveral, Florida for an 11-day mission, the GP-B team had to do more than stand up and applaud - we had to give the shuttle some elbow room on the satellite communications superhighway.

Normally, GP-B Mission Operations Control (MOC) communicates with our satellite by scheduling “passes” on the Tracking and Data Relay Satellite System (TRDSS). During these communications passes, we send commands to the spacecraft and receive data about the spacecraft's status. We typically complete about 8 passes every day scheduled at our choosing.

The presence of the Space Shuttle in orbit changes things. When the Space Shuttle is flying, TDRSS is primarily dedicated to providing communication links to Johnson Space Center in Houston, especially when astronauts are conducting critical operations like EVA's (extravehicular activities). What this means for GP-B is that we must constantly adjust our communications passes to times when the shuttle is not using the primary TDRSS satellites.

Adjusting these passes is not a simple thing. A complex mission like GP-B must be very detailed in planning and scheduling our procedures, and some of the GP-B operations require specific amounts of time. If we lose a scheduled pass to Space Shuttle operations, it is complicated to reschedule it.

For example, we performed a heat pulse meter test last week to determine the amount of liquid helium remaining in the Dewar (see February 18 Highlight for test description). During the test, it was critical that the MOC be able to communicate with the spacecraft for a four-hour period. Given this requirement, the Mission Planning team then had to find a four hours in the schedule when the GP-B spacecraft and the TDRSS satellites were in the right locations and when TDRSS was not being used by the Space Shuttle.

This was especially challenging for our mission planning team during the shuttle launch in late July. To prepare for the launch, NASA dedicates most of the TDRSS communication times in a two-week window to the shuttle mission. Since the launch date for the shuttle kept shifting, this two-week window shifted as well. Each day GP-B mission planning had to check the revised shuttle launch schedule and re-schedule our GP-B contact times.

Fortunately, the communications schedule for the shuttle landing is more predictable. NASA reserves approximately one day before and after the planned re-entry for TDRSS communications instead of two weeks. Currently, Space Shuttle Discovery is scheduled to land very early Monday morning (August 8), so GP-B is adjusting the communications times over the weekend.

For more information about TDRSS and the Space Shuttle mission (STS-114), go to these links.
STS-114 --


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 layered composite photos of the GP-B spacecraft orbiting the Earth and starlight from IM Pegasi entering the spacecraft's telescope, as well as the GP-B experiment diagram were created by GP-B Public Affairs Coordinator, Bob Kahn using Adobe Photoshop and Adobe Illustrator. Mr. Kahn also took the photos of the Greenwich Observatory and the photos of Dr. Anne Kinney from NASA Headquarters visiting GP-B. The photo of the gyroscope housing and the SQUID readout diagrams are from the GP-B Image Archive here at Stanford. The sky chart image, showing the guide star, IM Pegasi was generated by the Voyager III Sky Simulator from Carina Software. Click on the thumbnails to view these images at full size.


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