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

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17 JULY 2004—MISSION UPDATE: DAY 88At just under 3 months in orbit, Gravity Probe B is nearing the end ofthe Initialization and Orbit Checkout (IOC) phase of the mission. Thespacecraft remains in excellent health, and all subsystems arecontinuing to perform well. All four gyros are digitally suspended,with gyros #2 and #4 spinning at science mission speed—greater than80Hz (4,800 rpm)—and gyros #1 and #3 spinning at approximately 1.5Hz (90 rpm), ready for full-speed spin-up. The updated drag-freethruster control software that was uploaded to the spacecraft threeweeks ago to optimize performance of the Attitude and TranslationControl system (ATC) is continuing to perform nominally. Thespacecraft’s roll rate is 0.52 rpm, and the science telescope is being relockedonto the guide star, IM Pegasi, following the full-speed spin-upof gyro #2 yesterday.This past Tuesday, July 13th, Gravity Probe B achieved a majormilestone with the successful spin-up of gyro #4 to a science-readyspeed of 105.8 Hz (6,348 rpm). Second to the launch, the full-speedspin-up of the gyros has been the next most long-awaited event in thehistory of GP-B. Members of the team were very attentive at theirstations in the Mission Operations Center (MOC) here at Stanford for3 hours and broke into applause when the final announcementboomed over the MOC intercom that the gyro #4 full spin-up hadbeen completed successfully.Yesterday, July 16th, gyro #2 underwent the same spin-up procedure,reaching a final spin rate of 87 Hz (5,220 rpm). Spinning up the gyrosto science-ready speed is a complex and dynamic operation thatexercises the full capabilities of the Gyro Suspension System (GSS) andrequires a high level of concentration and coordination on the part ofthe GP-B Team. Following is an overview of the process.First, commands are sent to the GSS to move the gyro rotor (sphere)very close to the spin-up channel (about 1/100th of the edge of a sheetof paper) in one half of the gyro’s housing. Ultra-pure (99.99999%)helium gas is streamed from the Gas Management Assembly (GMA),mounted in a bay on the spacecraft frame, through tubing that entersthe “top hat” (the thermal interface at the top of the Probe) and travelsdown to the gyro housings in the Science Instrument Assembly (SIA)at the bottom of the Probe. As the helium gas descends into the Probe,which is at a temperature of approximately 1.8 Kelvin, the gas coolsdown from 273 Kelvin to around 12 Kelvin.Before entering the spin-up channel in one of the gyro housings, thegas is passed through a combination filter/heater. The filter, which ismade of sintered titanium, removes any impurities that may have beenimparted to the helium on its journey into the Probe. The heaterenables the helium to be warmed slightly, which increases its adhesionto the ultra-smooth surface of the gyro rotor. The filtered and warmedhelium then passes through the spin-up channel in one half of the gyrohousing, and most of the gas evacuates into space through an exhaustsystem. However, some of the helium leaks into the housings of theother gyros, causing their spin rates to decrease up to 20% over a fullspin-up period of 2-3 hours.480 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission
For this reason, the order in which the gyros are spun up is veryimportant. Earlier in the IOC phase, the 3 Hz (180 rpm) spin-upprovided information on the helium leakage rate of each gyro. Gyro#4, which had the highest leakage rate, was spun-up to full speed first,so that helium leaked from its spin-up would not affect other gyrosthat were already at science mission speed. The remaining gyros arethen spun-up in decreasing order of their helium leakage rates—gyro#2, gyro #1, and finally, gyro #3.Each full-speed spin-up takes most of a day. In the morning, heliumgas is flowed over the gyro for 90 seconds, and tests are run to ensurethat the helium leakage rate for that gyro corresponds to previousmeasurements. If everything checks out, the full-speed spin-up, inwhich helium gas is flowed over the rotors for 2-3 hours, commencesearly in the afternoon. The GP-B team controls the spin-up process bysending commands from the Mission Operations Center (MOC) hereat Stanford to the spacecraft in real-time. For example, they sendcommands to open or close the GMA valves to flow helium throughthe gyro’s spin-up channel. They also control the amount of heatapplied to the gas before it enters the gyro spin-up channel, and theycontrol opening and closing of exhaust valves. Real-time telemetryprovides immediate feedback on the progress of the spin-up so thatvarious parameters can be adjusted as necessary.The successful spin-up of gyro #4 to full speed enabled us to spin-upgyro #2. Gyro #2 topped out at 87 Hz (5,220 rpm), which is onlyslightly above the minimum spin rate of 80 Hz (4,800 rpm) requiredfor the science experiment. Also, the helium gas leakage from the Gyro2 spin-up slowed gyro #4 down to 91Hz (5,460 rpm). We are takingsome time to evaluate the data from the first two spin-up proceduresand to perform some ground-based tests before spinning up gyros #1and #3 to full speed.Meanwhile, we are fine-tuning the drag-free software used by theAttitude and Translation Control system (ATC) to optimize itsperformance at the current and final spacecraft roll rate of 0.52 rpm.For part of each orbit, the spacecraft passes behind the Earth, causingthe telescope to lose visual contact with the guide star. During this“eclipse” period, two standard external rate gyroscopes, plus the startrackers on either side of the spacecraft, enable the ATC to keep thespacecraft/telescope pointed towards the guide star. When thespacecraft emerges over the North Pole and the guide star becomesvisible again, the telescope must be re-locked onto it. This re-lockinghas been taking up to 15 minutes, and the fine-tuning will speed upthis process considerably.23 JULY 2004—GRAVITY PROBE B MISSIONUPDATE: Day 94One day #94 of the mission, Gravity Probe B is poised to enter thehome stretch of the Initialization and Orbit Checkout (IOC) phase ofthe mission. The spacecraft is in excellent health, and all subsystemsare continuing to perform well. All four gyros are digitally suspended,with gyros #2 and #4 spinning at science mission speed—greater than80Hz (4,800 rpm). Gyros #1 and #3 are spinning at less than 1.5 Hz (90rpm) and are ready for full-speed spin-up next week. Fine-tuning ofthe Attitude and Translation Control system (ATC) is still in progress,and the ATC is performing well. The spacecraft’s roll rate is 0.52 rpm,and the science telescope is locked onto the guide star, IM Pegasi.Last Friday, the full-speed spin-up of gyro#2 went smoothly, with afinal spin rate of 87 Hz (5,220 rpm). Helium gas leakage from the spinupof gyro #2 caused gyro #4 to slow down from 105.8 Hz (6,348 rpm)to 91 Hz (5,460 rpm). We had hoped that gyro #2 would achieve a spinrate above 100 Hz (6,000 rpm), with less leakage effect on gyro #4.Thus, rather than spinning up gyros #1 and #3 as originally planned,we spent the past week doing analysis and running tests—both on thespacecraft and here at Stanford—in order to ensure that the upcomingspin-up of gyros #1 and #3 will result in higher speeds, with lessleakage effect on the remaining gyros.The spin rate of the gyros during the Science Phase of the missionaffects the signal-to-noise ratio in the SQUID readouts of theexperimental data. The noise level is quite small, but constant. Thehigher the gyro spin rate, the larger the London moment (magneticfield created by a spinning superconductor), and thus, the greater thesignal-to-noise ratio. In ground testing prior to launch, we determinedthat a spin rate of 80 Hz (4,800 rpm) or greater for each gyro wouldprovide a good signal-to-noise ratio for the science mission. However,the threshold of 80 Hz (4,800) rpm is not a hard and fast limit, so if thefinal spin rate of one or more gyros falls slightly below this value, thiswill not appreciably compromise the science data.One way to potentially increase the spin-up rate of the remaining twogyros, while reducing the amount of helium gas leakage during spinupis to use the Gyro Suspension System to position the gyro rotorscloser to the spin-up channel in the gyro housing. Tests and analysisperformed this past week indicate that we can move the rotors of gyros#1 and #3 up to 30% closer to the spin-up channels than gyros #2 and#4, and still have a safe margin of clearance from the suspensionelectrodes and the gyro housings. We have also determined thatopening a second exhaust valve during spin-up may help to reduce thepressure in the Probe caused by helium leakage, thereby reducing thespin-down effects on the remaining gyros. Both of these changes willbe implemented in the spin-up of gyros #1 and #3 next week.Also, this past week, we continued fine-tuning the drag-free softwareused by the Attitude and Translation Control system (ATC) tooptimize its performance at the current and final spacecraft roll rate of0.52 rpm. Tests from parameter changes we made to the ATC systemindicate that we have reduced the time it takes to re-lock onto theguide star from as much as 15 minutes to less than 2 minutes.31 JULY 2004—GRAVITY PROBE B MISSIONUPDATE: Day 102Gravity Probe B successfully accomplished a major milestone thisweek as the final two gyroscopes were spun up to high spin speed. Allfour gyros are now spinning at high spin speed. The spacecraft is inexcellent health, and all subsystems are continuing to perform well.With this major accomplishment behind us the Program is enteringthe home stretch of the Initialization and Orbit Checkout (IOC) phaseof the mission.This week's high speed spin-up operations were performed on gyro #3(Tuesday) and gyro #1 (Friday). There were no anomalies during thespin up. The SQUID readout FFT tracked the spin up in real-time. Asexpected, the other gyros spun down approximately 15% with eachhigh speed spin-up operation due to pressurization of the Probecaused by gas leakage during the spin-up. Final spin speeds of thegyros are now: gyro #1 (80.0 Hz - 4800 rpm), gyro #2 (62.3 Hz - 3738rpm), gyro #3 (82.7 Hz - 4962 rpm) and gyro #4 (65.5 Hz - 3930 rpm).At these spin speeds, GP-B’s relativity measurement is expected to besignificantly better than specification. Therefore, no further gyro highspeed spin-up operations are planned.Fine-tuning of the Attitude and Translation Control system (ATC) isnearly complete, and the ATC is performing well. This past week, wecontinued fine-tuning the drag-free software used by the ATC tooptimize its performance at the current spacecraft roll rate of 0.52rpm. Tests from parameter changes we made to the ATC systemGravity Probe B — Post Flight Analysis • Final Report March 2007 481
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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
Page 458 and 459: 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 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 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
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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
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ReqIDLevel 1CSCGUP GSS Processing g
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570 March 2007 Appendix F — Acron
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AcronymDefinitionAcronymDefinitionB
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AcronymDefinitionAcronymDefinitionD
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AcronymDefinitionAcronymDefinitionF
Page 606 and 607:
AcronymDefinitionIRUInertial Refere
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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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GP-B Post-Flight AnalysisÃ¢Â€Â”Final Report - Gravity Probe B - Stanford ...

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