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

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Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 0, Single-bit errors (SBE): 8 (dailyaverage)The GP-B spacecraft is beginning to move out its full-sun “season,”and it is once again being eclipsed from sunlight by the Earth for aportion of each orbit. Because of the geometry of the spacecraft's polarorbit around the Earth, and the Earth's orbit around the sun, thespacecraft experiences two “seasons” when the plane of the spacecraft'sorbit is perpendicular with respect to the sun's position for about twoweeks. Thus, the sun shines broadside on the spacecraft throughouteach orbit around the Earth. During these full-sun periods, theAttitude Reference Platform (ARP) containing the star trackers andother equipment heats up, causing it to move slightly, andconsequently we must adjust certain parameters to counteract thismotion. Then, as the full-sun season comes to an end, we re-adjustthese parameters back to their normal settings.A final heat pulse test was run on the dewar this past Monday, 13 June2005. The results of this test indicate that the liquid helium in thedewar will run out towards the beginning of September. This result isconsistent with previous heat pulse measurements, so this was the lastheat pulse measurement we will perform.Preliminary SQUID calibration signal tests described in previousMission Director's Summaries are continuing. These tests do not affectthe science data collection, but they help us evaluate the performanceof the four SQUIDS relative to each other.is currently not in use. Thus, a memory patch was prepared anduploaded to the spacecraft, and this event had no further impact onthe mission.On Wednesday, 22 June 2005, as part of our ongoing instrumentcalibration tests, we switched the spacecraft's drag-free control fromgyro #3 to gyro #1. The purpose of this switch is to determine whetheror not certain gyro control data values that have been uniquelyobserved on gyro #3 are a function of the drag-free system or gyro #3itself. The process of moving drag-free operation from gyro #3 to gyro#1 went very smoothly, and we intend to continue drag-free operationaround gyro #1 for the remainder of the science phase of the mission.1 JULY 2005—GRAVITY PROBE B MISSION UPDATEMission Elapsed Time: 437 days (62 weeks/14.32 months)Science Data Collection: 308 days (44 weeks/10.10 months)Current Orbit #: 6,449 as of 3:00 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 1 (in GSS computer on 6/30), Singlebiterrors (SBE): 15 (daily average)This past week was relatively quiet for GP-B mission operations.Several routine adjustments were made in various spacecraft systems,and preliminary instrument calibration tests are continuing.24 JUNE 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 430 days (61 weeks/14.01 months)Science Data Collection: 301 days (43 weeks/9.87 months)Current Orbit #: 6,345 as of 3:00 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 1 (in main CCCA computer on 6/20),Single-bit errors (SBE): 9 (daily average)Last Monday, 20 June 2005, a multi-bit error (MBE) occurred in the B-Side (backup) main computer (CCCA). Our resident computer experttraced the location of this error to a thruster initialization routine thatDuring the mission up to this point, we have been cycling certainapplications on and off in the SQUID Readout Electronics (SRE) andTelescope Readout Electronics (TRE) computers during Guide StarValid (GSV) and Guide Star Invalid (GSI) periods to ensure that thesecomputers have adequate margin for memory scrubbing, whichautomatically corrects single-bit errors (SBEs). Based on recentengineering tests, we have determined that leaving these applicationson all the time, instead of cycling them on and off, doubles the lengthof the memory scrub period, which doubles the likelihood of thesecomputers sustaining a multi-bit error (MBE) composed of two SBEsin a single memory word during a scrub cycle. However, based on theengineering tests and on-orbit experience, we have determined thatthe probability of sustaining an MBE composed of two SBEs wasdeemed to be an acceptable level of risk. The advantage of keepingthese applications enabled all the time is that it provides simultaneousdata on trapped flux and the London moment, in addition to thescience pointing (relativity) data.506 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission
On Wednesday 29 June 2005, members of our Anomaly Review Boardpresented information about our GP-B anomaly review process to aNASA group studying anomaly resolution best practices at NASA'sAmes Research Center here in Mountain View, CA.On Thursday, 30 June 2005, a multi-bit error (MBE) occurred in abenign location in the Gyro Suspension (GSS) computer for gyro #2.This memory location was subsequently patched, restoring its propervalue.8 JULY 2005—GRAVITY PROBE B MISSION UPDATEMission Elapsed Time: 444 days (63 weeks/14.56 months)Science Data Collection: 315 days (45 weeks/10.33 months)Current Orbit #: 6,553 as of 5:30 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 0, Single-bit errors (SBE): 6 (dailyaverage)As of Mission Day 444, the Gravity Probe B vehicle and payload are ingood health. All four gyros are digitally suspended in science mode.The spacecraft is flying drag-free around Gyro #1.Last Saturday, 2 July 2005, a GPS channel alignment error flag wastriggered in the spacecraft's B-side (backup) Global PositioningSystem (GPS) as it passed over the Earth's north magnetic pole. Wehave seen this same behavior twice before with the A-side (main) GPSreceiver, and in both previous cases there was no discernible effect onthe GPS system performance. An analysis of this latest event is inprogress, and if necessary, we will reboot the GPS system.Otherwise, all was quiet and nominal on-board the GP-B spacecraftduring this year's 4th of July holiday weekend. Because its orbit did notpass over North America last Monday evening, the telescope detectorson-board the spacecraft did not register any brilliant bursts ofpyrotechnics in the skies over U.S cities.Yesterday, we performed a calibration test on gyro #3 (formerly thedrag-free gyro) in which the gyro rotor was electrically “nudged” tovarious pre-defined positions within its housing. About 45 minutesinto this test, the Gyro Suspension System (GSS) for gyro #3automatically switched from digital to analog suspension mode.Analog suspension is used primarily as a backup mode that holds agyro rotor securely to keep it from striking the housing wall and allowsonly coarse positioning of the rotor. By contrast, digital suspensionmode is computer-controlled; it puts less torque on the rotor andenables its position within the housing to be controlled with extremelyhigh precision. Yesterday evening, we sent commands to re-suspendgyro #3 in digital mode, and the calibration tests are continuing todayand into the weekend. Due to the nature of this calibration test and theperformance history of gyro #3, we will not be surprised if ittransitions to analog mode again during the second phase of thiscalibration test.performance stabilizes during this period because the Sun is no longerheating up the spacecraft's attitude reference platform (ARP), wherethe navigational rate gyros and star trackers are mounted, on theforward dome of the dewar.As we have reported recently, our measurements indicate that thesuperfluid helium in the dewar will be exhausted in approximatelyeight weeks—during the period of maximum ATC stabilization. Thissituation has raised some questions about how to proceed through thefinal two months of the mission. Our original plan was to stopcollecting relativity data 3-5 weeks before the helium runs out and tospend those final weeks of the mission exclusively running calibrationtests of the science instruments. Some of these tests involve placingtorques (forces) on the gyros, and we cannot use the science datacollected from a gyro while it is undergoing such a test. Anothercalibration test involves purposely moving the telescope's pointingaxis away from our guide star, IM Pegasi, to a different star and thenback again. Clearly, we cannot collect any science data during thistelescope pointing test.However, our plan for spending the final 3-5 weeks of the missionexclusively running calibration tests is not cast in stone, and we arecurrently at a point of making some trade-off decisions in this regard.For example, we have determined that it is possible to perform thegyro torquing calibration tests on an individual gyro, while the othergyros continue to collect science data. (We are performing such acalibration test on gyro #3 right now.) To address these issues andtrade-offs, our GP-B management and science teams spent the entireday today at an off-site meeting. The decisions resulting from this offsitemeeting will determine our course of action for the remainder ofthe mission, and we will report on these decisions in an upcomingMission News story.In conjunction with today's off-site meeting, yesterday we had a visitfrom Tony Lyons, our NASA Program Manager at NASA's MarshallSpace Flight Center (MSFC) in Huntsville, Alabama and his boss,Tony Lavoie, Director of Space Systems Programs at MSFC. TonyLavoie assumed his current role at the Marshall Center last October,and he had never before visited GP-B. So, instead of our usual dailyall-hands meeting yesterday, we began the day with a briefpresentation to both of our MSFC visitors by GP-B PrincipalInvestigator, Francis Everitt, GP-B Program Manager, Gaylord Green,and several GP-B staff members.ANALYZING END OF MISSION TRADE-OFFSTowards the beginning of June, the spacecraft transitioned out of itstwo-week full sun season to once again being eclipsed from the Sun forpart of each orbit. Furthermore, the position of the Earth relative tothe Sun has been changing so that the Sun's light is now movingtowards the rear of the spacecraft, and this orientation will continuethrough August and September. During this period, the spacecraft'sAttitude and Translation Control (ATC) performance will be the moststable, giving us the “cleanest” data of the entire mission. The pointingThe presentation included an overview of the program for the benefitof Tony Lavoie, followed by a status update on the spacecraft and datacollection. Following the presentations, Francis Everitt, GaylordGreen, and GP-B co-founder Bob Cannon took both of our MSFCGravity Probe B — Post Flight Analysis • Final Report March 2007 507
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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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Description of the GP-B experience:
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Figure 16-1. Six interacting transl
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16.1.3.2 Training and certification
Page 484 and 485: Description of the GP-B experience:
Page 486 and 487: Below is an edited summary of six a
Page 488 and 489: 460 March 2007 Chapter 16 — Lesso
Page 490 and 491: 462 March 2007 Appendix A — Gravi
Page 492 and 493: 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 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 544 and 545: Gyro Suspension System (GSS): All 4
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
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ReqIDLevel 1CSCLevel2 CSC Level 3 C
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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
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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
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AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
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AcronymDefinitionAcronymDefinitionT
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588 March 2007 Appendix F — Acron
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