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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 331 days (47 weeks/11.00 months) as of 3/17/05
Science Data Collection 202 days (29 weeks/6.75 months) as of 3/17/05
Current Orbit # 4,883 as of 2:00 PM PST on 3/17/05
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 1.80 kelvin, rising slowly
Global Positioning System (GPS) lock Greater than 98.8%
Attitude & Translation Control (ATC)

X-axis attitude error: 347.3 marcs rms as of 3/17/05
Y-axis attitude error: 353.4 marcs rms as of 3/17/05

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 3 as of 3/17/05
Single-bit errors (SBE): 7 (daily average) as of 3/17/05
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential -0.1 mV as of 3/17/05
Gyro #2 rotor potential +8.8 m as of 3/17/05V
Gyro #4 rotor potential -5.1 mV as of 3/17/05
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 331, 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.

On Monday evening, 14 March 2005 at 11:38PM PST, the spacecraft's safemode system triggered another computer reboot—the second flight computer reboot within the past two weeks. Telemetry indicated that this most recent safemode response was activated when a memory checkout procedure detected at least three multi-bit errors (MBE's) within a 0.2 second interval in the B-side computer.

Since the switchover from the A-Side (main) computer two weeks ago, the spacecraft has remained in control of the B-Side (backup) flight computer. In this configuration, if a new series of MBEs occurs, the spacecraft does not automatically switch back to the A-side computer. Rather, the B-side computer reboots itself, and the mission operations team follows a pre-defined set of procedures to restore the normal systems configuration for science data collection-as was the case this past Tuesday and Wednesday.

In response to the reboot, the mission operations team uploaded and ran a set of pre-approved recovery commands on the B-side computer. The spacecraft's roll rate, which had automatically decreased during the reboot, was quickly restored to the science value of 0.7742 rpm. The team also sent commands to reboot the SQUID Readout Electronics (SRE) as part of the initial recovery response, prior to re-acquisition of the guide star and return to drag free flight. This accelerated the overall recovery process by a full day. We returned to drag-free operation on Gyro #3 at 5:34PM PST on Tuesday, less than 18 hours after the B-side reboot event. After several guide star search maneuvers Tuesday night, the guide star was successfully acquired at 4:41AM PST on Wednesday, 29 hours after the reboot event.

Because this is the second MBE-triggered event in a two-week period, we have changed the action response of the sequential MBE safemode test from “rebooting the B-side computer” to “stopping the mission timeline.” We have made this configuration change because we believe that the reboot response overreacts to the reporting of sequential MBEs. This change will minimize unnecessary adverse impact to science data collection that occurs when the GP-B flight computer reboots. The cause of the last two safemode response activations is under investigation, and a fault tree is being developed to identify the root cause of these events.

Each time we experience a switchover or reboot of the on-board flight computer, a small amount of science data is lost. If the reboot events of the past two weeks have not placed any non-relativistic torques (forces) on the gyros, this small data loss will not have any significant effect on the outcome of the experiment. Analysis to determine whether this is the case is in progress.


Consider the following: GP-B is a unique, once-in-a-lifetime experiment. Its payload, including the gyroscopes, SQUID readouts, telescope and other instrumentation took over four decades to develop. Once launched, the spacecraft is physically out of our hands, and typically, we only communicate with it about every two hours via scheduled telemetry passes. Thus, we entrust the on-board flight computer (and its backup) with the task of safeguarding the moment-to-moment health and well-being of this invaluable cargo. How does the GP-B flight computer accomplish this critically important task? The answer is the safemode subsystem.

Most autonomous spacecraft have some kind of safemode system on-board. In the case of GP-B, Safemode is an autonomous subsystem of the flight software in the GP-B spacecraft's on-board flight computers. The GP-B Safemode Subsystem is comprised of three parts:

  1. Safemode Tests—Automatic checks for anomalies in hardware and software data.
  2. Safemode Masks—Scheme for linking each safemode test with one or more safemode response sequences.
  3. Safemode Responses—Pre-programmed command sequences that are activated automatically when a corresponding test fails to provide an expected result.

These tests and response commands are designed to safeguard various instruments and subsystems on the spacecraft and to automatically place those systems in a known and stable configuration when unexpected events occur.

For example, one of the tests in the GP-B Safemode Subsystem checks to ensure that the communications link between the on-board flight computer and the GP-B MOC here at Stanford is alive and active. The requirement for this test is that the flight computer must receive some command from the MOC at least once every 12 hours. If the flight computer does not receive a command within that time frame, we assume that normal telemetry is not working, and the pre-programmed response commands cause the computer to automatically reboot itself and then re-establish communication. This test is a variation of the “deadman” test used on high-speed bullet trains. To ensure that the train's engineer is alive and awake while the train is traveling at high speed, the engineer is required to press a button or switch at regular intervals. If the train does not receive the expected human input within each time interval, the train automatically throttles down and comes to a halt.

Not all of the tests and responses in the GP-B Safemode Subsystem are currently enabled. Some of the tests were specifically created for use during the launch and/or during the Initialization and Orbit Checkout (IOC) phase and are no longer needed during the science phase of the mission. Also, active tests and responses can be re-programmed if necessary. For example, the response commands for the safemode that triggered last Monday's computer reboot were changed this past week to simply stop the mission timeline, rather than rebooting the flight computer, as described in the mission director's summary above.

When anomalous events occur, automatic responses ensure that the spacecraft and its subsystems remain in a safe and stable condition. This enables our mission operations team to become aware of the issue on-board, identify and understand the root cause, and take appropriate action to restore normal operations. GP-B has a formal process, called “anomaly resolution,” for dealing with unexpected situations that occur in orbit. This process was thoroughly tested and honed during a series of seven pre-flight simulations over the course of two years.

GP-B anomalous events are evaluated and classified into one of four categories:

  1. Major Space Vehicle Anomalies—Anomalies that endanger the safety of the spacecraft and/or the payload. Response to these anomalies is time-critical.
  2. Medium Space Vehicle Anomalies—Anomalies that do not endanger the safety of the space vehicle but may impact the execution of the planned timeline. Response to these anomalies is not time-critical if addressed within a 72-hour window.
  3. Minor Space Vehicle Anomalies—Anomalies that do not endanger the safety of the space vehicle. These are low risk problems with the vehicle that are resolved by taking the appropriate corrective action.
  4. Observations—In addition to formal anomaly categories, the space vehicle may exhibit off-nominal or unexpected behavior that does not appear initially to be an operational or functional issue and does not violate any limits, but may warrant attention over time. Observation items may be elevated to an anomaly category if it is judged to be serious enough to warrant a high-priority investigation. 

A special room in the GP-B Mission Operations, called the Anomaly Room, is the home of the GP-B Anomaly Review Board (ARB), a select group of senior GP-B team members from Stanford, NASA, and Lockheed Martin, who manage the troubleshooting of anomalies and observations. The Anomaly Room, which is located across the corridor from the GP-B MOC, contains a set of spacecraft status monitors, communications and teleconference equipment, computer and voice hookups, a documentation library, white boards, a computer projection system, and an oval discussion table.

Whenever an anomaly is in the process of being resolved, the Anomaly Room is staffed 24 hours a day, 7 days a week; at other times, it is staffed during normal working hours, with team members on call. When major events such as last week's computer reboot occur outside normal working hours, the Mission Director on duty activates the Anomaly Room and issues a series of pager and cell phone calls via computer, summoning key staff members on the ARB, along with a selected anomaly team, comprised of resident engineers and engineering specialists, to come in and work through the issue. The group uses a technique called "fault tree analysis" to evaluate and determine the root cause of unexpected events.

The GP-B Safemode Subsystem and anomaly resolution process have worked very well throughout the mission. Thus far, the ARB has successfully worked through more than 150 issues while the spacecraft has been in orbit. Most of these issues have been classed as observations or medium to minor anomalies. But a handful have been classified as major anomalies, including the B-Side computer switchover and the stuck-open valve problems with two micro thrusters early in the mission, as well as several subsequent computer and subsystem reboot problems due to radiation strikes. In each case, the established anomaly resolution process has enabled the team to identify the root causes and provide recovery procedures.


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 diagram of the GP-B experiment created by GP-B Public Affairs Coordinator, Bob Kahn. The photos of the GP-B daily all-hands meeting, the Anomaly Room, and the Mission Operations Center were also taken by Bob Kahn. 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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