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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 353 days (50 weeks/11.57 months)
Science Data Collection 224 days (32 weeks/7.34 months)
Current Orbit # 5,210 as of 4:00 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 97.3%
Attitude & Translation Control (ATC)

X-axis attitude error: 143.4 marcs rms
Y-axis attitude error: 88.1 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 7 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential 2.1 mV
Gyro #2 rotor potential 10.6 mV
Gyro #4 rotor potential -2.8 mV
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 353, 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 #3.

For a number of weeks now, the spacecraft has been experiencing degraded attitude performance in the South Atlantic Anomaly (SAA) region. As a short-term fix for this problem, we have been commanding the Attitude and Translation Control system (ATC) to switch from the science telescope to the spacecraft's navigational gyroscopes for controlling the spacecraft's attitude while in the SAA region. This has increased the spacecraft's stability in the SAA, but since we only use relativity data collected when the science telescope is in control of the ATC, we have effectively been losing science data in those orbits where the spacecraft passes through the SAA under navigational gyro control.

Over these past few weeks, our Attitude and Translation Control (ATC) group has been working very hard to fine tune the ATC system so that the science telescope can remain in control of the spacecraft's attitude in the SAA region. Three weeks ago, we made some progress on this issue by shrinking our defined boundaries of the SAA region, thus reducing the number of orbits affected. Two weeks ago, the ATC group made further progress by changing the ATC gain settings (similar to adjusting the volume control on a radio). Finally, this past week, the ATC group adjusted the gain settings on the Telescope Readout Electronics (TRE). This past week's adjustments, in combination with those previously made, have mitigated this issue. The science telescope is now able to remain locked on the guide star in all passes through the SAA, and we are once again collecting science data in all orbits.

Also this past week, we started making detailed plans for the post-science instrument re-calibration phase, estimated to begin early in July.


In recent Mission News sections of these weekly updates, we have discussed safemodes and the anomaly resolution process and computer error detection and correction techniques employed on the GP-B spacecraft. Underlying these topics is a philosophy of redundancy that has been built into the GP-B spacecraft and its subsystems, as well as the experiment itself. This week, we take a closer look at all of the redundancy built into GP-B.

At the highest level of the GP-B experiment, redundancy begins with the four gyroscopes. We actually only need one gyroscope to perform the GP-B experiment, but since it is unlikely that this experiment will ever be repeated, it seemed prudent to build redundancy into the experiment itself. Having four gyros obviously provides backup in case one, two, or even three gyros should fail during the mission. More important, however, having multiple gyroscopes provides built-in crosschecks on the relativity data. In other words, a high degree of correlation in data collected from four gyros provides greater confidence in the results than data collected from a single gyro, or even two gyros--especially if the results should differ from the predictions of general relativity.

An independent Gyro Suspension System (GSS) computer controls each of the gyros. Each GSS computer is connected to a single gyro, so they cannot be used interchangeably. Each of the GSS computers has two gyro suspension modes--analog and digital--that provide a form of redundancy in the gyro rotor suspension. The analog suspension mode is used primarily as a backup or safe mode for suspending the gyros. The digital suspension mode is computer-controlled; it puts less torque on the gyros than analog mode and enables their position to be controlled with extremely high precision. A failsafe mechanism, called the arbiter, is hard-coded into the GSS firmware (programming at the chip level) that automatically switches the suspension system from digital to analog mode under certain pre-set conditions. The GSS system is a marvel of electronics and engineering in its own right, and we will devote a future GP-B Mission News story just to the GSS.

The GSS computers comprise four of the eight computers on-board the spacecraft. In addition there are two flight computers and two SQUID Readout (SRE) computers. The main flight computer and its twin backup are called the A-Side (main) and B-Side (backup) CCCA (Command & Control Computer Assembly). Only one of these computers is running at any given time. If the A-side computer fails certain safemode tests, the B-side computer automatically takes over, as was the case a few weeks ago. However, if the B-side computer fails these safemode tests, it is rebooted. We can only switch back to the A-side computer through manual commands.

Each of the twin SRE computers can control all four SQUID readouts, showing the spin axis orientations of all four gyros. Like the main computers, only one of the SRE computers is running at any given time. When the main CCCA computer automatically switched over to the B-side a few weeks ago, the SRE computer did not switch with it. However, in order to synchronize the timing between the B-side CCCA computer and the A-side SRE computer, we commanded the SRE computer to reboot. Also, to ensure that we remain on the A-side SRE computer, we have disabled the safemode response that automatically switches from the A-side SRE computer to the B-side SRE computer.

Redundancy is also built into the Telescope Readout Electronics (TRE). The science telescope has two sets of detectors, designated A-side (primary) and B-side (backup). These detector packages, which are about the diameter of a dime, are comprised of pairs of silicon photo diodes that basically count photons from each half of the telescope's split beam, in both the X-axis (side-to-side) direction and the Y-axis (top-to-bottom) direction. Beam splitters and mirrors are used to redundantly direct all of these split light beams to the B-side detectors as well as the A-side detectors. For comparison sake, we collect the data from both the primary and backup detectors during telemetry communications passes, although we only use the data from one set of detectors to control the spacecraft's pointing direction through the ATC system.

The ATC uses the navigation control gyros (called “rate” gyros) and the star trackers to control the spacecraft's attitude when the telescope detectors are not controlling the spacecraft's attitude. The spacecraft contains two pairs of rate gyros and two star trackers, again designated A-side (primary) and B-side (backup). The two rate gyros in each pair work together, independently controlling the X-axis and Y-axis position of the spacecraft; the redundant Z-axis position control from one of the gyros is not used. The star trackers are essentially pattern-matching cameras, located on opposite sides of the spacecraft frame.

Both star trackers have been running since the beginning of the mission, but only one pair of rate gyros was in use. We have since activated the other set of rate gyros, as well. Data from one set is sent to the ATC to control the spacecraft's position; data from the other set is collected during telemetry communications sessions and is used for precise roll attitude calibrations to the science data. The B-side switchover of the CCCA computer a few weeks ago also triggered a switch to the B-side rate gyros and star tracker. We have since commanded the spacecraft to switch back to the A-side rate gyros and the A-side star tracker, both of which had been fine tuned during the Initialization and Orbit Checkout (IOC) phase of the mission and were performing slightly better than their backup counterparts.

The final redundant spacecraft system is the micro-thruster system. The spacecraft is outfitted with 8 pair of opposing micro-thrusters, arranged in four clusters--two at the top of the spacecraft frame and two at the bottom. One thruster in each cluster is redundant, and a set of valves in the thruster system enables individual thrusters to be isolated and effectively disabled. This is precisely what we did early in the mission with two micro-thrusters, whose nozzles became stuck open, apparently due to particle contamination shortly after launch.



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.


If you're going to be in Los Angeles anytime before 30 May 2005, and if you’re interested in Einstein’s life and work, the Einstein Exhibition at the Skirball Cultural Center (just north of the Getty Museum on Interstate 405) is the most comprehensive presentation ever mounted on the life and theories of Albert Einstein (1879-1955). It explores his legacy not only as a scientific genius who re-configured our concepts of space and time, but also as a complex man engaged in the social and political issues of his era. It examines the phenomenon of his fame and his enduring status as a global icon whose likeness has become virtually synonymous with genius.

In this exhibit, you can examine Einstein's report card, inspect his FBI file, and enjoy his family photographs, love letters, and diary entries. Exhibition highlights include scientific manuscripts and original correspondence—including original handwritten pages from the 1912 manuscripts of the special theory of relativity and his 1939 letter to President Roosevelt about nuclear power—and a wealth of other documents from the Albert Einstein Archives at the Hebrew University of Jerusalem.

In addition to these displays of Einstein memorabilia, the exhibit also features a number of interactive components that help provide an understanding of Einstein's revolutionary theories. Furthermore, several “explainers,” identified by their red aprons, are on hand to discuss various aspects of the exhibit and to explain and demonstrate difficult concepts, such as time dilation and warped spacetime. At the end of the exhibit, you’ll find one of GP-B’s gyro rotors on display.

The Einstein exhibition was jointly organized by the American Museum of Natural History (AMNH), the Hebrew University of Jerusalem, and the Skirball Cultural Center. It was designed by the AMNH under the supervision of Dr. Michael Shara, curator of the exhibit and chairman of the museum’s Astrophysics Department. It opened in November 2002 at the AMNH in New York and then traveled to Chicago and Boston, spending about 8 months in each location. It will remain at its final U.S. stop at the Skirball Center in Los Angeles through 29 May 2005, after which time it will move permanently to the Hebrew University in Jerusalem.

Information about the Einstein exhibition is available on the Skirball Center Web site. If you can’t make it to Los Angeles, you can visit the AMNH’s virtual Einstein exhibit on the Web.

Drawings and photos: The diagrams of the GP-B experiment and the GP-B telescope Image Divider Assembly optics were created by GP-B Public Affairs Coordinator, Bob Kahn. The drawings of the South Atlantic Anomaly (SAA) region and the Science Instrument Assembly, as well as the photos of the various GP-B instruments and spacecraft components are from the GP-B Photo & Graphics Archive here at Stanford. The photos from the Einstein Exhibit are courtesy of the Skirball Cultural Center. Click on the thumbnails to view these images at full size.


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