GP-B Post-Flight AnalysisÃ¢Â€Â”Final Report - Gravity Probe B - Stanford ...

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Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: Not AvailableGlobal Positioning System (GPS) lock: Greater than 99.8%Attitude & Translation Control (ATC): 137.8 marcs rms (as of 9/15)Y-axis error: 176.4 marcs rms (as of 9/15)Command & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 0Single-bit errors (SBE): Not AvailableGyro #1 Drag-free Status: Backup Drag-free mode (OFF during somecalibration maneuvers)As of Mission Day 514, the Gravity Probe B vehicle and payload are ingood health, with all subsystems performing nominally. We still havehelium remaining in the dewar, and the spacecraft is currently flyingdrag-free around Gyro #1.This coming weekend, if we still have helium remaining, we will visitsimilar virtual star locations, or possibly real stars, within a 4-degreeradius of IM Pegasi, each time visiting a neighboring location forseveral hours and then returning to IM Pegasi. And, as long as there isstill helium, we will continue similar tests into next week…andbeyond.GP-B MISSION NEWS—THE HELIUM JUST KEEPS ONFLOWING…Like Energizer Bunny® that “keeps on going,” the helium in the GP-Bdewar just keeps on flowing. Our dewar specialists assure us that weare, indeed, well into the final days—or possibly hours—of helium.But when will the helium actually run out? That's anybody's guess (wehave a pool going amongst the remaining team members here atStanford).Estimating the amount of liquid helium remaining in a 650 gallonThermos bottle, orbiting 400 miles above the Earth, requires complexcalculations, and there are many variables at play. Also, there is aconsiderable margin of error in these calculations. Our GP-B dewarspecialists, who modeled the dewar's life cycle and have been trackingits performance, have purposely remained conservative in theirlongevity estimates of the helium, so as to ensure that there would beenough helium to perform the most important calibration tests.Following the initial tests, we have been working our way through aprioritized list of further tests that will be beneficial in analyzing thescience data. As long as the helium continues to flow, we will continueworking our way through this list. The consensus here is that thehelium will very likely run out by the beginning of next week. But, if itlasts longer, we will simply continue performing these tests.The helium in the dewar has now surpassed its estimated lifetime bymore than two weeks. We have thus continued working our waythrough a prioritized list of calibration tests. Most of these tests haveinvolved slewing the telescope (and spacecraft) to “visit” stars (bothreal and virtual) in the neighborhood around the guide star, IM Pegasi.In each case, we visit a neighboring location for a period of time andthen lock back onto IM Pegasi for a number of hours. The purpose ofthese excursions to neighboring stars is to precisely calibrate thetorques imparted onto the science gyroscopes by purposelymisaligning the telescope axis (and spacecraft roll axis) by varyingamounts from the direction of the gyros' spin axes.Last Friday, we visited a virtual star (place where no stars visible to thetelescope exist) located 0.1 degrees west of our guide star, IM Pegasi, inthe direction of neighboring star HR Pegasi. We remained fixed onthat virtual star location for 48 hours and then returned to IM Pegasi,where we remained for 24 hours.While locked onto IM Pegasi, we switched drag-free control from gyro#1 to gyro #3 in order to perform some calibration tests on gyro #1.During this calibration test, the pressure in the dewar dropped, andthe spacecraft experienced thruster instability in the z-axis direction(along the spacecraft/telescope roll axis). This behavior was similar toinstabilities that we had previously seen in June 2004, during theInitialization and Orbit Checkout (IOC) phase of the mission. Theroot cause is under investigation, but there is some speculation thatthis may be the result of a micrometeorite striking the spacecraft. Torectify this situation, we switched the operating mode of our microthrusters to “open loop”—that is, turning off the thruster pressuresensors and relying instead on pre-defined thruster calibration curvesto control thruster flow. We will leave the micro thrusters in open loopmode until the helium is depleted.When the helium actually does run out, we will post a notice on ourWeb site and send out a message to the subscribers of our GP-BUpdate email list. NASA will also issue a news release, and we will thenpost the content of that release on our Web site and send it to ouremail subscribers. Stay tuned….16 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 514 days (73 weeks/ 16.85 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 33 daysCurrent Orbit #: 7,586 as of 5:30PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)516 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission
Alpha Pegasi, Zeta Pegasi, HR Pegasi, IM Pegasi, and HD 216635—allthe stars in the neighborhood of IM Pegasi that we have visitedrecently.Last Tuesday, 13 September, we visited the very bright (magnitude2.45) neighboring star Alpha Pegasi (also known as Markab or HD218045), located about 3.25 degrees south of IM Pegasi. We remainedlocked on Alpha Pegasi for 12 hours. Upon returning to returning toIM Pegasi. We switched drag-free control back to gyro #1.Then, last Wednesday, we sent commands to the spacecraft todecrease its roll rate from 0.7742 rpm (77.5 seconds per revolution) to0.4898 rpm (122.5 seconds per revolution). This resulted in someinterference with a SQUID calibration signal, and the frequency of thecalibration signal was subsequently changed, eliminating this issue.The spacecraft has since been performing nominally at the slower rollrate.Finally, today, we initiated procedures to de-fluxing the SQUIDS—that is, removing electromagnetic flux from the SQUIDs by heatingthem up a few kelvins, and then allowing them to cool back down totheir normal cryogenic operating temperature of 1.8 kelvin. Assumingthat we still have helium remaining this weekend and next week, thisprocess is a prelude to performing several gyro housing explorationtests, followed by a switch from backup to primary drag-free mode.GP-B MISSION NEWS—RUNNING ON EMPTY, AND VISITINGTHE BRIGHTEST STAR IN THE CONSTELLATION PEGASUSIf you drive a car, you have likely experienced—at least once—thesituation where the needle on your car's gas gage moves into the redzone, and the light comes on indicating that the gas tank is nearlyempty. But, unless you've been in this situation a few times with anyparticular car, you don't know for sure how much farther you candrive before the engine finally sputters and stops running. This isessentially the situation with our GP-B spacecraft right now. We knowthat we're in the “red zone” with our helium very nearly depleted, buthow many more orbits will actually be sustained is basically anyone'sguess.And so, continuing our prioritized calibration tests, last week wevisited the star Alpha Pegasi (aka Markab/HD 218045/ HR 8781),located about 3.25 degrees southward of IM Pegasi. Alpha Pegasi isone of the defining stars in the constellation Pegasus, the flying horse.It is approximately 110 light years away, and its luminosity is about 95times greater than our Sun. In fact, the designation of “Alpha” Pegasimeans that it is the brightest star in the constellation Pegasus. A set ofcomposite photos from the Palomar Observatory Sky Surveys (POSSIand POSSII) accompanying this week's Mission News story on ourWeb site clearly show the comparative luminosity or brightness ofAt this time of year in North America, the constellation Pegasus isclearly visible above the Eastern horizon during the evening hours, asshown in the sky charts accompanying this week's Mission News onour GP-B Web site.As we noted last week, as long as we still have helium in the dewar, wewill continue working our way through our prioritized list ofcalibration tests. When the helium actually does run out, we will post anotice on our Web site and send out a message to the subscribers ofour GP-B Update email list. NASA will also issue a news release, andwe will then post the content of that release on our Web site and sendit to our email subscribers.23 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 521 days (74 weeks/ 17.08 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 40 daysCurrent Orbit #: 7,688 as of 4:00PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: Not AvailableGlobal Positioning System (GPS) lock: Greater than 99.4%Attitude & Translation Control (ATC): 128.0 marcsY-axis error: 170.8 marcsCommand & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 1 (in GSS#3 computer on 9/23)Single-bit errors (SBE): 7 (daily average)Telescope Readout (TRE): NominalGyro #1 Drag-free Status: Backup Drag-free mode (OFF during somecalibration maneuvers)On Mission Day 521, the Gravity Probe B vehicle and payload remainin good health, with all subsystems performing nominally. We stillhave some helium in the dewar, and the spacecraft is flying drag-freearound Gyro #1.Gravity Probe B — Post Flight Analysis • Final Report March 2007 517
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Post Flight Analysis — Final Repo
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Signatures & ApprovalsPrepared byPu
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Table of ContentsPreface . . . . .
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6.8 Sources and References . . . .
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11 Telescope Readout Subsystem (TRE
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14 Data Collection, Processing & An
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xiv March 2007 Table of Contents
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Figure 3-4. The GP-B dewar—one of
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Figure 8-5. The Seasons of GP-B . .
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Figure 11-20. Pointing angle differ
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Figure 14-9. In the Anomaly Room, t
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xxiv March 2007
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Table 13-3. GP-B payload magnetomet
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xxviiiMarch 2007
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2 March 2007 Chapter 1 — Executiv
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facility, it was necessary to recas
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spacetime around with them as they
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After years of work and the inventi
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axis of a gyroscope rotor changes i
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Figure 1-8. Clockwise, from top lef
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“management experiment.” This w
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Attitude-Control Gyroscopes. Two pa
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Any significant deviation from “g
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established, ranging from Level 1 (
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1.14 The Broader Legacy of GP-BAt l
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24 March 2007 Chapter 2 — Overvie
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Figure 2-1. GP-B Historical Time Li
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Applied Physics Laboratory, of the
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William Fairbank once remarked: “
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In Einstein’s view, space and tim
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y a mere 1.1 inches. You can see th
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Figure 2-10. Schematic diagram of t
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Figure 2-12. Final assembly of the
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Sun Shield. The sun shield is a lon
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2.1.8 The Broader Legacy of GP-BWhe
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2.3 Spacecraft SeparationThe Solar
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Flight Anomalies.) Furthermore, a d
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Table 2-2. Weekly summary of IOC ac
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2.4.3.2 Guide Star AcquisitionAbout
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A second mass trim operation had be
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On Friday, 2 July 2004, the spin ra
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Low temperature bakeout was first p
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2.5.1.3 Lockheed Martin Team Phase-
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Monthly Highlights of the GP-B Scie
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2.6.1 Overview of the Calibration P
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Weekly Highlights of the GP-B Final
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66 March 2007 Chapter 3 — Accompl
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It was in this pristine, near-zero
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Each quartz rotor is coated with a
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Figure 3-3. Functional diagram of a
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Based on data from the on-board tel
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Ideally, the telescope should have
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Figure 3-9. Schematic diagram of st
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used. The star trackers are essenti
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Result: By using the porous plug, w
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Solution: Create a tetrahedral lapp
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Constraint 2: The exhaust tube must
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Result: The on-orbit gyroscope spin
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spacecraft's subsystems for the dur
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Constraint 3: Keep the spacecraft p
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Technology Category Specific Implem
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96 March 2007 Chapter 4 — GP-B On
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Any significant deviation from “g
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packages, one for each Ping load an
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In addition to these software packa
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Table 4-6. Features & benefits of W
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This software added considerable ef
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It is said that “an experiment is
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Table 4-10. Significant Events/Sour
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Figure 4-5. Pictorial depiction of
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Figure 4-7. Eta-Average Error Assoc
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4.4.4.2 OD Using SLR DataFigure 4-9
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4.4.5 ConclusionsFigure 4-11. Compa
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Overall, the storage issues did not
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Table 4-11. ITF equipment listEQUIP
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124 March 2007 Chapter 5 — Managi
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5.2 Overview of GP-B Anomalies in O
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Figure 5-1. Anomaly Review Team Org
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5.3.3.1 Anomaly Investigation and R
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5.3.5 Anomaly Identification and Re
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5.4.3 Risk Management ApproachThe r
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Table 5-1. Summary of Open GP-B ris
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138 March 2007 Chapter 5 — Managi
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140 March 2007 Chapter 6 — The GP
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3. Payload Integration, Testing and
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Figure 6-1. Clockwise from top left
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MSFC conducted an in-house Phase A
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It is important to note that from t
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6.3.1 Incremental PrototypingThe pe
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alignment, and act as an accelerome
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Figure 6-9. The Lockheed Martin GP-
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Furthermore, a fourth electronic sy
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positive thermal connections betwee
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astrophysics/cosmology. Many of the
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Figure 6-10. MSFC Program Managers/
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As described in Chapter 2, the flig
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6.7 Some Observations on the Manage
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168 March 2007 Chapter 6 — The GP
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170 March 2007 Chapter 7 — Attitu
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7.1.2 Vehicle ATC ModesThere are es
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The output of the pressure transduc
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Figure 7-3. Science Telescope Compo
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Figure 7-5. Pitch and Yaw Pointing
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Figure 7-7. Magnitude of the roll r
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Changing the method by which the gy
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7.3.3 Drag-Free Control SystemFigur
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7.3.3.1 DFS - PerformanceThe GP-B d
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Figure 7-15. Mass flow effects of a
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7.5.2 Successful recovery from mult
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If the vehicle control system were
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accomplish this, the ARPs are mount
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7.5.6.1 South Atlantic AnomalyProto
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Figure 7-23. Effects on telescope p
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200 March 2007 Chapter 7 — Attitu
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202 March 2007 Chapter 8 — Other
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Figure 8-2. Block Diagram of CDH co
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8.1.1.4 WATCH DOG TIMERFigure 8-4.
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of eclipses each day (approximately
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Figure 8-8. GSS1 Temperature Trends
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Figure 8-10. Forward Dewar Vacuum S
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The Gravity Probe B spacecraft has
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Figure 8-13. Forward -X Thruster Te
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gaFigure 8-15. Aft -X Thruster Temp
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Because there are only two SQUID br
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Because the specifications for SRE
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8.3.2 Critical Mission RequirementT
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Figure 8-19 is a plot of the power
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Battery Performance Summary:Figure
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Figure 8-23. Solar Array Temperatur
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8.3.10 ConclusionThe electrical pow
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8.4.1 TDRSS OperationsFigure 8-28.
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Figure 8-30. Xpndr-A STDN (green),
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Figure 8-32. Plot of TDRS AGC vs Te
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The FSW also met its subsystem leve
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Appendix E, Flight Software Applica
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Table 8-4. SCRs Addressed in On-orb
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The software and macro changes resu
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Figure 8-38 below is an excerpt fro
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Figure 8-40. A-side CCCA Single Bit
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Figure 8-43. A-side CCCA Single Bit
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When single MBEs did not result in
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256 March 2007 Chapter 9 — Gyro S
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9.1 GSS Hardware DescriptionFigure
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significantly reduces and thus pres
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9.1.11 Clock synchronizationThe aft
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Figure 9-8. Block diagram of the LQ
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direction (transverse to the space
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9.2.2.2 Spin-up SuspensionDuring sp
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Figure 9-16. Predicted and measured
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Figure 9-18. The GSS passes rotor c
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Figure 9-20. Representative drag-fr
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Table 9-1. GSS Science Mission Mode
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Table 9-3. GSW application source l
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The Design and Testing of the Gravi
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282 March 2007 Chapter 10 — SQUID
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ate uncertainty). Although we have
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All of the GP-B electronics have be
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10.3 Pre-Launch Ground-Based TestsW
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Figure 10-6. AC Magnetic Shielding
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Figure 10-8. Temperature Error of S
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Figure 10-10. Quiescent SQUID Noise
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Analog control loops on the electro
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The science support applications ru
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Table 10-7. SSW application source
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308 March 2007 Chapter 11 — Teles
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Figure 11-1. Block diagram of TRE a
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Additionally, the warm electronics
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Figure 11-3. CLL switch states duri
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Table 11-3. Dates and UTC times whe
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Figure 11-7. Low Gamma Angle Data S
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s N+w i= sign+w i++−+−w i−w i
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expression, which otherwise on a po
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Figure 11-14. Y axis, B side RMS po
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Figure 11-17. X axis, B side curren
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Figure 11-20. Pointing angle differ
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Figure 11-24. Scaled summed current
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Figure 11-26 through Figure 11-29 s
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334 March 2007 Chapter 12 — Cryog
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12.2 Temperature / pressure control
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control the space vehicle without t
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Figure 12-3. Helium flow rate predi
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helium at 1.8 K. It does not appear
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Figure 12-6. Flow meter and ATC flo
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were generally within a day or so o
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shields) and subsequent instability
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350 March 2007 Chapter 12 — Cryog
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352 March 2007 Chapter 13 — Other
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Figure 13-2. ECU-operated heaters i
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13.1.3.1 Vatterfly Valve Operations
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13.1.3.7 Payload MagnetometersThe E
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Figure 13-11. ECU performance durin
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As we entered the second month of o
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During the last month of IOC, prepa
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Figure 13-19. ECU heater activity d
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Table 13-1. Proton monitor channel
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Figure 13-21. Proton Monitor data i
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In November, 2004 there was a large
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Figure 13-28 shows a correlation th
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13.3.1 About the MagnetometersFigur
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13.3.2 Other Applications for Paylo
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Figure 13-35. GPS Antennae Field of
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Figure 13-37. Master Antenna Switch
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Figure 13-39. Time difference betwe
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13.4.6 On-Orbit ResultsThe GPS comp
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and velocity values erroneously cal
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E. G. Lightsey, C. E. Cohen, B. W.
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Figure 13-45. A Bottom View of the
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Figure 13-48. The GMA configuration
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Table 13-10. Summary for Helium Gas
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398 March 2007 Chapter 14 — Data
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a relatively slow data rate, so we
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Figure 14-4. NASA ground stations a
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electronic components to recover fr
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By a process of elimination, the GP
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A special room in the GP-B Mission
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Starting on 3 December 1725, Bradle
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seemed that aberration of starlight
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14.1.6 Telescope Dither—Correlati
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14.1.6.3 The Telescope Dither Patte
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amplifications. Thus, in addition t
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14.2.2 Independent Data Analysis Te
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monthly time scales. Though only sh
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424 March 2007 Chapter 15 — Preli
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Figure 15-2. A diagram of the GP-B
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15.4 The Two Surprises and Their Im
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Near Zeroes.) The exception that we
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Figure 15-8. A slide from the GP-B
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Figure 15-9. A poster on the effect
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electrostatic patches, located at o
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Last summer (2006), Mac Keiser devi
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440 March 2007 Chapter 15 — Preli
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442 March 2007 Chapter 16 — Lesso
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16.1.1.3 Flight science downlink da
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Description of the GP-B experience:
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Lessons:1. Perform all tests with a
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Figure 16-1. Six interacting transl
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16.1.3.2 Training and certification
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Below is an edited summary of six a
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460 March 2007 Chapter 16 — Lesso
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462 March 2007 Appendix A — Gravi
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Apogee altitude659.1 km (409.6 mile
Page 494 and 495: 466 March 2007 Chapter B — Spacec
Page 496 and 497: Gravity Probe B — Post Flight Ana
Page 498 and 499: 470 March 2007 Appendix C — Weekl
Page 500 and 501: 16 April 2004—Vehicle is Prepared
Page 502 and 503: The electrical power system is full
Page 504 and 505: 4 JUNE 2004—MISSION UPDATE: DAY 4
Page 506 and 507: SQUIDs to detect their rotation spe
Page 508 and 509: 17 JULY 2004—MISSION UPDATE: DAY
Page 510 and 511: indicate that we have reduced the t
Page 512 and 513: C.4 Science Mission Phase: 8/27/04
Page 514 and 515: • GP-B also has an independent da
Page 516 and 517: free suspension parameters to de-tu
Page 518 and 519: We have received inquiries about a
Page 520 and 521: One of the effects of geomagnetic s
Page 522 and 523: egan transmitting, one-by-one, stat
Page 524 and 525: 28 JANUARY 2005—GRAVITY PROBE B M
Page 526 and 527: magnetic pole. This event triggered
Page 528 and 529: 8 APRIL 2005—GRAVITY PROBE B MISS
Page 530 and 531: On Tuesday afternoon (19-April), GP
Page 532 and 533: Norway or through the NASA space ne
Page 534 and 535: Dewar Temperature: 1.82 kelvin, hol
Page 536 and 537: visitors on a tour of the GP-B faci
Page 538 and 539: test, we are planning on running fi
Page 540 and 541: contractor at the Goddard Space Fli
Page 542 and 543: On Wednesday, we visited the star H
Page 546 and 547: The helium in the dewar has now sur
Page 548 and 549: the all the spacecraft status data.
Page 550 and 551: 522 March 2007 Appendix D — Summa
Page 552 and 553: Figure D-2 below shows the distribu
Page 554 and 555: DateSeveritySubtypeTitle Descriptio
Page 556 and 557: DateSeverity24 26-Apr-04 Obs F Nois
Page 558 and 559: DateSeverity40 11-May-04 Obs T Dwel
Page 562 and 563: DateSeverity68 16-Jun-04 Medium Dec
Page 564 and 565: DateSeverity84 13-Jul-04 Obs F Slig
Page 568 and 569: DateSeverity116 26-Sep-04 Obs F SG3
Page 570 and 571: DateSeverity132 20-Dec-04 Obs F Unc
Page 572 and 573: DateSeverity149 5-Mar-05 Obs T Few
Page 574 and 575: DateSeverity169 04-Jun-05 Obs F MBE
Page 576 and 577: DateSeverity191 04-Oct-05 Obs A GSS
Page 578 and 579: 550 March 2007 Appendix E — Fligh
Page 580 and 581: ReqIDLevel 1CSCDMP DataMgmtProcessi
Page 582 and 583: ReqIDLevel 1CSCLevel2 CSC Level 3 C
Page 588 and 589: ReqIDLevel 1CSCSRM Solid StateRecor
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ReqIDLevel 1CSCSRP SafemodeResponse
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ReqIDLevel 1CSCGUP GSS Processing g
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570 March 2007 Appendix F — Acron
Page 600 and 601:
AcronymDefinitionAcronymDefinitionB
Page 602 and 603:
AcronymDefinitionAcronymDefinitionD
Page 604 and 605:
AcronymDefinitionAcronymDefinitionF
Page 606 and 607:
AcronymDefinitionIRUInertial Refere
Page 608 and 609:
AcronymDefinitionAcronymDefinitionM
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AcronymPMPMAPMCPMEPMETPMSPMSUPNPoPO
Page 612 and 613:
AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
Page 614 and 615:
AcronymDefinitionAcronymDefinitionT
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588 March 2007 Appendix F — Acron
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