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

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• GP-B also has an independent data analysis team that will beanalyzing the data in parallel with the GP-B science team tocorroborate our results. Any differences in the results of these twoteams will be reconciled before the results are announced.• The incoming raw data is fed directly into a database and is onlyavailable on a “read-only” basis to the scientists who will beanalyzing it.During the week, GP-B continued to tune-up the ATC system byimproving the quality of “roll phase data” of the spacecraft. This dataincreases the precision with which we can monitor the alignment ofthe gyroscopes. The “roll phase data” refers to the precise orientationof the spacecraft as it rolls around its axis (360 every 77.5 seconds).• Multiple cross checks are performed to identify any potentialsystematic experimental errors. For example, the results of all fourgyros are compared and correlated for consistency.• During the GP-B experiment, the spin axis deflections of thegyroscopes are measured relative to the guide star, IM Pegasi. But,in order to determine the actual relativistic drift rate of the gyros,we must also measure the proper motion of IM Pegasi relative toquasars (the most distant objects in the universe). The Harvard-Smithsonian Center for Astrophysics (CfA) is measuring theproper motion of IM Pegasi with respect to quasars to anunprecedented level of precision. However, by design, the CfA willnot release this proper motion data to the GP-B science team untilafter the scientific data collection, post-science instrumentcalibrations, and the data analysis have all been completed. In otherwords, the relativistic drift rates of the gyros may only bedetermined once the GP-B gyro drift results and CfA propermotion results are combined.Regarding the second question of why it is necessary to wait so longbefore releasing any results, one reason is now apparent: the guide starproper motion safeguard requires that all the GP-B data analysis becompleted before we can receive and combine the CfA proper motiondata and release/publish the final result. A related technical reason isthat while we should have a very good measure of gyro drift after 2-3months of data collection, it is essential that this data be cross checkedand calibrated to ensure its accuracy. We use the annual aberration ofour guide star signal (the difference between the actual position of IMPegasi and its apparent position due to the Earth’s revolution aroundthe Sun) as one means of calibrating the telescope/gryo readout scalefactor. Collecting this aberration data over a substantial portion of ayear will ensure the greatest accuracy of this calibration.In summary, we are using utmost care and rigor in our experimentalmethods and analysis, and only after the analysis has been completedand thoroughly checked will we announce and publish the results. Atthat time, all of the GP-B data will become available to the publicthrough the National Space Sciences Data Center (NSSDC), located atNASA’s Goddard Space Flight Center in Greenbelt, MD.17 SEPTEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 150After 22 weeks in orbit, GP-B finally has all four gyroscopes generatingrelativity data. Gyro #4 joined Gyro’s #1, #2, and #3 in science modethis week after its spin axis was successfully aligned with the guide star,IM Pegasi, on Tuesday. The official transition occurred on Thursdaymorning (PDT), marking the first time that GP-B is conductingscience on all four gyros.The spacecraft continues to be in excellent health, rolling at a rate of0.7742 rpm (one revolution every 77.5 seconds). The Attitude andTranslation Control (ATC) system is maintaining a drag-free orbitaround gyro #3, and it is properly tracking the guide star, IM Pegasi.The dewar temperature is nominal (1.82 Kelvin), and the flow ofhelium, venting from the dewar through the micro thrusters hasremained within expected limits.When the spacecraft rolls, so do the pickup loops encircling eachgyroscope. These superconducting wire loops are used to monitor theorientation of the magnetic field around each gyroscope. They arelocated in the same plane as the spacecraft’s roll axis, so when thespacecraft rolls the loops turn with it. As a result, the loops perceive a“change” in each gyroscope’s orientation as the wire turns through thegyroscope’s magnetic field.If GP-B knows the exact orientation of the spacecraft (and, in turn, thepickup loops) at each moment in time, we can better determine theorientation of each gyroscope. The spacecraft’s orientation isdetermined primarily through the two star trackers located on theforward portion of the spacecraft on opposite sides. These star trackersimage the starry sky through an 8x8 field, providing the spacecraftwith a reference map to measure its orientation. If the stars in eachtracker’s field of view are in the right location, then the spacecraft ispointing in the right direction.In last week’s highlights, we noted that a double-bit error in computermemory was caused by “a proton hit”. An astute reader asked, “Howdo we know that the spacecraft was hit by proton?”Onboard the spacecraft is a proton detector, specially made for theGP-B mission. Its purpose is to monitor the high-energy particles thatimpact the spacecraft and the science instrument. This proton flux canlead to the accumulation of charge on the gyroscopes, which in turncould lead to unacceptable torques on the gyroscopes. So far, thecharge accumulated on the gyroscopes is much less than expected.One ancillary benefit of GP-B’s proton monitor has been its ability toaccurately locate the South Atlantic Anomaly, a region above theSouth Atlantic Ocean where the fluxes of particles trapped in theEarth’s geomagnetic field are much greater than anywhere else. GP-B’sorbit takes it through this region regularly, allowing it to produce amore precise map of this magnetic anomaly.24 SEPTEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 157GP-B had an eventful week. We began the 4th week of science with allfour gyroscopes in the science mode. Then on Thursday we had ananomaly with gyro #3.At 8:30 PM local time on Thursday, the drag-free gyro #3 transitionedto analog mode resulting in safemode that stopped the timeline. TheGP-B vehicle and science gyros were safe. Gyro's #1, #2 and #4486 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission
emained in the digital suspension science mode. A preliminaryassessment by the science team indicated minimal impact to anyexperiment accuracy. An investigation is under way to determine thecause of the drag-free gyro #3 transition to analog mode and safemodeactivation. The operations and engineering teams did an excellent jobworking through the night to safe the vehicle and quickly develop arecovery plan.By Friday afternoon the spacecraft was back in science collecting modewith gyro #1 performing the drag-free operation. On Saturday theoperations team successfully transitioned Gyro #3 from analogsuspension to the digital suspension science mode. All other spacecraftsystems are in excellent health. The spacecraft is rolling at a rate of0.77419 rpm (one revolution every 77.5 seconds). The Attitude andTranslation Control (ATC) system is maintaining a drag-free orbitaround gyro #1, and it is properly tracking the guide star, IM Pegasi.The dewar temperature is nominal (1.82 Kelvin), and the flow ofhelium, venting from the dewar through the micro thrusters hasremained within expected limits.spacecraft was back in science mode, with gyro #1 performing thedrag-free operation. The effect of this event on the science experimentwas insignificant. The most likely cause of this event was a data spikein the Gyro Suspension System (GSS) position sensor for gyro #3.Gyro #3 was returned to digital suspension as part of the recoveryplan, and it has remained digitally suspended, with no furtherproblems, since that time. However, we are continuing to monitor itsperformance and investigate the root cause. Thus, for the foreseeablefuture, the spacecraft will continue to fly drag-free around gyro #1.During the week the team performed a heat pulse test on thespacecraft dewar to obtain further information to help predict lifetime.Early indications are that the helium in the dewar will last another 10months, which should be sufficient to satisfy the missionrequirements.1 OCTOBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 164The spacecraft is in good health, flying drag-free around gyro #1. Allfour gyros are digitally suspended, and their SQUID readouts arecollecting relativity data during the portion of each orbit when thescience telescope is locked onto the guide star, IM Pegasi. Thespacecraft’s roll rate remains at 0.7742 rpm (one revolution every 77.5seconds). The dewar temperature is nominal (1.82 Kelvin), and theflow of helium, venting from the dewar through the micro thrusters iswithin expected limits.At this time of year in North America, at around 8:30 PM PacificDaylight Time, the constellation Pegasus, in which the guide star IMPegasi resides, is clearly visible above the Eastern horizon. With amagnitude ranging from 5.6 – 5.85, IM Pegasi is too dim to see withthe naked eye, but in a good viewing location, it should be visible withbinoculars.In last week’s update, we reported that at 8:30 PM local time onThursday, 23 September 2004, gyro #3—which was then serving as thedrag-free gyro—suddenly transitioned to analog backup suspensionmode, automatically triggering a safemode that stopped the missiontimeline. The GP-B operations and engineering teams quicklydeveloped a recovery plan, and by the following afternoon, theAlso last week, we reported that the team had performed a heat pulsetest on the spacecraft dewar in order to determine the mass of liquidhelium remaining inside, which determines the remaining lifetime ofthe mission. The heat pulse test works in the following way: Theamount of heat that it takes to warm an object by a specified amountdepends on the type of material, its temperature, and its mass. Thus, ifthe “specific heat” (the amount of heat needed to warm a kilogram ofmaterial by one degree kelvin) is known, it is a simple matter tomeasure the mass by applying a known amount of heat (usually withan electric heater) and measuring the resultant temperature rise. In thecase of the GP-B dewar, the situation is simplified by the fact that heatdistributes itself virtually instantaneously throughout superfluidhelium. The amount of heat used in the test must be large enough tocause a measurable temperature change (approximately 10millikelvin) but not so large as to appreciably shorten the missionlifetime. The heat pulse test yielded a remaining superfluid heliummass of 216 kg, which translates into a remaining mission lifetime of9.9 months. This means that the science (data collection) phase of themission will continue for a little over 8 more months, and then we willspend the final month re-calibrating the science instrument.Finally, this past Tuesday, 28 September 2004, we observed a slightincrease in the helium required by the micro thrusters to maintaindrag-free flight around gyro #1. To be conservative, we decided to turnoff drag-free mode and evaluate the situation. Over a period of 4-6hours, the oscillations that were causing the ATC to require excesshelium died out and have not returned. After some analysis anddiscussion, we have determined these oscillations were caused by asympathetic resonance between the ATC drag-free control efforts anda sloshing wave on the surface of the now reduced superfluid heliumin the dewar. This situation is somewhat analogous to placing avibrating tuning fork on the body of a guitar, which then causes guitarstrings tuned to harmonics of the same frequency to start vibrating.This is a transient situation, and the team has adjusted the ATC drag-Gravity Probe B — Post Flight Analysis • Final Report March 2007 487
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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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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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Page 466 and 467: Last summer (2006), Mac Keiser devi
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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 510 and 511: indicate that we have reduced the t
Page 512 and 513: C.4 Science Mission Phase: 8/27/04
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
Page 560 and 561: DateSeveritySubtypeTitle Descriptio
Page 562 and 563: 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
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AcronymDefinitionIRUInertial Refere
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AcronymDefinitionAcronymDefinitionM
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AcronymPMPMAPMCPMEPMETPMSPMSUPNPoPO
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AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
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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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