Hi,

Here is the Gravity Probe B Mission Update

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GRAVITY PROBE B MISSION UPDATE FOR 8 APRIL 2005
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GP-B STATUS AT A GLANCE
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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 4:00PM 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 avg.)
Telescope Readout (TRE): Nominal
SQUID Readouts (SRE): Nominal
Gyro #1 rotor potential: 2.1 mV (as of 3/21)
Gyro #2 rotor potential: 10.6 mV (as of 3/21)
Gyro #4 rotor potential: -2.8 mV (as of 3/21)
Gyro #3 Drag-free Status: Backup Drag-free mode (normal)

MISSION DIRECTOR'S SUMMARY
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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.


MISSION NEWS-REDUNDANCY IN THE GP-B SPACECRAFT
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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.

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UPDATED NASA/GP-B FACTSHEET
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A recently updated NASA Factsheet on the GP-B mission and experiment is now available on our Web site in Adobe Acrobat PDF format. You can download this 6-page fact sheet at: http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf

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PREVIOUS GP-B UPDATES
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If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

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OTHER LINKS THAT MAY INTEREST YOU
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* Our GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.


* Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html


* PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.


* NASA's Marshall Space Flight Center also has a series of Web pages devoted to GP-B: (http://www.gravityprobeb.com )


* Photo, taken through a telescope by Swiss physics teacher and amateur astronomer Stefano Sposetti, of GP-B spacecraft in orbit, passing near IM Pegasi: http://aida.astronomie.info/sposetti.



* The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/


* In addition, you'll find information in the Guide Star FAQ on our Web site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar and on pages 18-20 of the Gravity Probe B Launch Companion: http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf.


* Track the GP-B satellite on the Web using NASA's Java-based J-Pass satellite tracking application at: http://science.nasa.gov/realtime/JPass/ Also, 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: http://www.bigfattail.com.


* The Einstein Exhibition at the Skirball Cultural Center in Los Angeles through May 2005: Information about the Einstein exhibition is available on the Skirball Center Web site: http://www.skirball.org/index.asp?s=exhibit&p=einstein.asp. If you can't make it to Los Angeles, you can visit the AMNH's virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html.


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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu

Bob Kahn
Public Affairs Coordinator

Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,

LelandJ