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

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indicate that we have reduced the time it takes to re-lock onto theguide star from as much as 15 minutes to less than a minute. We havealso increased the number of reference stars seen by the star trackerfrom 3 to 8.Next week we will be performing a few final operations prior tobeginning taking science data. The most significant operation left to beperformed is the spin axis alignment of each gyro. This is performedusing the Gyro Suspension System in a special oscillation mode toslowly align each gyro spin axis to be in nearly perfect alignment withthe guide star. Having the gyros aligned at the beginning of themission makes any precession caused by general relativity simpler tomeasure. We are nearly complete aligning the spin axis of gyros #2 and#4 and plan to align #3 and #1 next week. Another operation for nextweek is to perform a final bake-out operation to remove any residualhelium molecules from around the gyros. This operation is performedby heating up the instrument area slightly (to approximately 6 degreesKelvin), which will cause residual helium to migrate to a collectordevice in the Probe. And finally, the final operation prior to science isto command the spacecraft to begin drag-free operations.6 AUGUST 2004—GRAVITY PROBE B MISSIONUPDATE: DAY 108Gravity Probe B has nearly completed the Initial On-Orbit CalibrationPhase of the Mission. Last week we completed the fast speed spin-upof all four gyros. This week we completed the low temperature bakeoutof the Probe and expect to complete spin axis alignment of the finaltwo gyroscopes this weekend. Prior to starting the Science Phase of theMission, we decided to increase the roll rate of the spacecraftfrom.52rpm to .75rpm which will improve the accuracy of theexperiment and minimize the risk of achieving a good measurement inthe event of a shortened mission. We plan to increase the roll rate earlynext week. At that point we will transition to the Science Phase of themission.The low temperature bakeout operation was done in order tominimize the pressure inside the vacuum probe. The resulting lowpressure, or ultra-high vacuum, is required to minimize the spindownrate and torques on the gyroscopes. The low temperature bakeoutprocess involved heating the Probe to approximately 6-7 degreesKelvin while the Probe was open to the vacuum of space. This allowedexcess helium gas to escape to space. The probe was then closed tospace and the heaters turned off. The probe temperature then returnedto its nominal value and the remaining helium was adsorbed into thecryopump. The cryopump is a passive device made of sinteredtitanium, which has a very large adsorbing area. GP-B’s lowtemperature bakeout process created pressure inside the vacuumprobe less than one thousandth that of space.We continued to perform spin axis alignment of the gyroscopes thisweek. This is performed using the Gyro Suspension System in a specialoscillation mode to slowly align each gyro spin axis to be in nearlyperfect alignment with the guide star. Having the gyros aligned at thebeginning of the mission makes any precession caused by generalrelativity simpler to measure. Thus far, we've completed the alignmentof gyro #1 and #2. Gyro's #3 and #4 should be completed this weekend.After much analysis and discussion this week we decided to increasespacecraft roll rate during the science mission from .52 rpm to .75rpm. A .75 rpm roll rate minimizes the risk of events which mightoccur that could cause degradation to the accuracy of the mission. Inaddition, by combining the factors that influence gyro accuracy andguide star acquisition, this relatively small increase in roll rate shouldproduce a ~20% improvement in the overall accuracy of theexperiment. We expect to perform this operation beginning Monday.Assuming success, we then will begin drag-free operation with thestart of the Science Mission Phase later next week.13 AUGUST 2004—GRAVITY PROBE B MISSIONUPDATE: DAY 115On day #115 of the mission, Gravity Probe B is within a week ofcompleting Initialization and Orbit Checkout (IOC) and making thetransition into the Science Phase of the mission. The spacecraft is inexcellent health, and all subsystems are performing well. Thespacecraft's roll rate is stable at 0.75 rpm, and final testing of drag-freeorbital flight and fine-tuning of the Attitude and Translation Controlsystem (ATC) is nearing completion. All four gyros are digitallysuspended and spinning at science mission speeds, ranging from61.8Hz (3,708 rpm) to 82.1 Hz (4,926 rpm). The spin axes of Gyros #1and #3 have been aligned with the science telescope's sighting axis,which is locked onto the guide star, IM Pegasi, and these gyros havecompleted the transition to science data collection mode. Spin axisalignment is continuing on Gyros #2 and #4, which are expected totransition to science mode over the next few days.This past Monday, August 9, 2004, we successfully increased thespacecraft's roll rate from 0.52 rpm to 0.75 rpm. In last week'shighlights, we noted that the decision to do this was made “after muchanalysis and discussion,” triggering a number of email inquiries aboutthe risks of increasing the roll rate. Three main issues influenced thisdecision: First, increasing the roll rate requires more fine-tuning of theATC system, which would delay the transition into Science mode by afew more days. Second, because the star trackers used by the ATC aremounted on the sides of the spacecraft, an increase in roll rate meansthat the star trackers must perform their pattern-matching on a fastermoving field of stars. Third, there was a concern that any effects ofmass imbalances in the spacecraft might be exaggerated at a faster rollrate. In the end, we decided that the ~20% improvement inexperimental accuracy outweighed these risks. All systems areperforming correctly at the increased roll rate, and the only downsidewas a small delay in our transition to the Science phase of the mission.Also this past week, we have flown the spacecraft in both primary andback-up drag-free modes, using Gyro #1 as the proof mass(spacecraft's center of mass). Primary drag-free mode relies solely onthe micro thrusters to create a drag-free orbit around the proof massgyro, whereas in back-up drag-free mode, the ATC uses suspensionforce data from the Gyro Suspension System (GSS) of the proof massgyro to “steer” the spacecraft so that residual GSS forces are nullifiedor canceled on this gyroscope. Either primary or back-up drag-freemode can be used in the Science Phase of the mission. However, theresidual voltages applied by the GSS to suspend the drag-free gyro adda very small, but acceptable, amount of noise to the gyro signal inback-up drag-free mode, so the primary mode is preferable, all otherthings being equal.In fact, early in July, we had selected back-up drag-free mode as thenominal mode for the Science Phase of the mission because at thattime, when the gyros were spinning very slowly, back-up mode wasyielding better results than the primary mode. However, now that thegyros are spinning at full speed, primary drag-free mode is yielding thebest results, as originally anticipated, so it will be the nominal modefor drag-free operation during the Science Phase of the mission. Thereason for this performance difference is that while the GP-B gyrorotors (spheres) are the roundest objects ever manufactured, they arenot perfectly spherical. The GSS is capable of controlling the positionof the gyros to a level of one nanometer. However, some of the482 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission
“bumps” or deviations on the rotor surfaces are as large as 20nanometers. When the rotors are spinning slowly, these larger“bumps” are detected by the GSS, and they decrease its positioningaccuracy. However, when the rotors are spinning fast, these deviationsaverage out. Thus, at full speed, the average surface deviation on thegyro rotors is less than the one-nanometer precision of the GSS, sothere is no degradation in positioning the gyro rotors.solution for ensuring that the helium mass flow through the ATCsystem remains within required levels. By adjusting thruster biases,and by reducing the commanded helium flow, the amount of heliumconsumed by the ATC system is now less than the helium boiled awayby the natural heat in the dewar. As a result, the helium requirementsof the ATC system will have no impact on the mission lifetime,thereby maximizing the science performance.The final steps in the IOC, prior to beginning data collection, involvethe transition of each gyro into science mode. There are three mainsteps in this transition. First, the spin axis of each gyro is aligned withthe science telescope/spacecraft roll axis. Then, ultraviolet light,beamed from lamps on the spacecraft frame through fiber optic cablesinto the gyro housings, are use to remove any residual static chargefrom the gyro rotor surfaces. Finally, the GSS voltage is reduced, inorder to obtain the best signal on the SQUID readouts. Currently,Gyros #1 and #3 have completed this transition to science mode.Gyros #2 and #4 are finishing spin axis alignment, and they areexpected to complete the transition to science mode over the next fewdays.20 AUGUST 2004—GRAVITY PROBE B MISSIONUPDATE: DAY 122A little over four months into the mission, the spacecraft remains inexcellent health, and all subsystems continue to perform well. Thespacecraft’s roll rate remains stable at 0.75 rpm, and final testing ofdrag-free orbital flight and fine-tuning of the Attitude and TranslationControl system (ATC) is continuing. All four gyros are nowsuspended in low-voltage science mode. All of their spin axes havebeen aligned with the science telescope’s sighting axis, which is lockedonto the guide star, IM Pegasi. These alignments are in the process ofbeing fine-tuned.Last weekend, we completed coarse spin axis alignment on gyros #2and #4. On Monday, August 16th, ultraviolet light was beamedthrough fiber optic cables into the housing of gyro #2 to remove excessstatic charge on the gyro #2 rotor (sphere). Then, the suspensionvoltage for gyro #2 was reduced to 200 millivolts, the level required forscience data collection. The following day, the same procedures werecompleted on gyro #4. Thus, by the middle of this past week, all fourgyros were in the science mode configuration.Furthermore, in the process of performing these tests, we havedetermined that back-up drag-free mode is working very reliably, andthus we have re-selected back-up drag-free operation as the baselinemode for the Science Phase of the mission. We anticipate that ourinvestigation and problem-solving efforts around this roll-axis forcewill take a few more days. During that time, we will continue testingback-up drag-free mode around gyro #3, as well as further fine-tuningthe spin axis alignment of the four gyros.In addition this past week, while optimizing the performance of thetelescope system, we have found a small source of bias noise in thetelescope detector output signal. Although it would probably have noeffect on the science outcome, it seems prudent to eliminate this biasnoise, especially since a simple adjustment will take care of it. We areplanning to make this adjustment over the next two days.Finally, analysis of the gyroscope spin-down rate performance overthis past week has shown that, under low-voltage, science modesuspension, the spin-down rate for all four gyros is less than 0.000001Hz (0.00006 rpm) per hour. This means that it would take more than7,000 years for one of our gyros to completely spin down. On Earth,we could only test the spin-down rates of the gyros in a high-voltagesuspension mode, and based on these ground tests, the orbital spindownrequirement for the gyros was set at 2,300 years. We now seethat the spin-down performance of the GP-B gyros in science mode ismore than three times better than this requirement.About ten days ago, we began to notice that while in primary drag-freemode, the ATC was requiring more helium propellant than planned tocounter an unexpected force along the spacecraft’s roll axis—that is, inthe direction of the guide star. This past weekend, we switched toback-up drag-free mode on gyro#3, which gave a similar result. Overthe past two days, we have completed two tests that have provided aGravity Probe B — Post Flight Analysis • Final Report March 2007 483
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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
Page 460 and 461: Figure 15-8. A slide from the GP-B
Page 462 and 463: Figure 15-9. A poster on the effect
Page 464 and 465: electrostatic patches, located at o
Page 466 and 467: Last summer (2006), Mac Keiser devi
Page 468 and 469: 440 March 2007 Chapter 15 — Preli
Page 470 and 471: 442 March 2007 Chapter 16 — Lesso
Page 472 and 473: 16.1.1.3 Flight science downlink da
Page 474 and 475: Description of the GP-B experience:
Page 476 and 477: Lessons:1. Perform all tests with a
Page 480 and 481: Figure 16-1. Six interacting transl
Page 482 and 483: 16.1.3.2 Training and certification
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 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 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
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DateSeveritySubtypeTitle Descriptio
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DateSeverity68 16-Jun-04 Medium Dec
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DateSeverity84 13-Jul-04 Obs F Slig
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DateSeveritySubtypeTitle Descriptio
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DateSeverity116 26-Sep-04 Obs F SG3
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DateSeverity132 20-Dec-04 Obs F Unc
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DateSeverity149 5-Mar-05 Obs T Few
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DateSeverity169 04-Jun-05 Obs F MBE
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DateSeverity191 04-Oct-05 Obs A GSS
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550 March 2007 Appendix E — Fligh
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ReqIDLevel 1CSCDMP DataMgmtProcessi
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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
Page 596 and 597:
ReqIDLevel 1CSCGUP GSS Processing g
Page 598 and 599:
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
Page 610 and 611:
AcronymPMPMAPMCPMEPMETPMSPMSUPNPoPO
Page 612 and 613:
AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
Page 614 and 615:
AcronymDefinitionAcronymDefinitionT
Page 616:
588 March 2007 Appendix F — Acron
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