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

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Figure 15-8. A slide from the GP-B plenary talk that Francis Everitt will deliver at the APS meeting on April 14,2007. This slide shows how dither and aberration of starlight are used as calibrating signals in determining thegyro-telescope scale factor, C g .Fortunately, the GP-B experimental data includes natural calibrating signals in the form of the aberration ofstarlight—both orbital and annual. (For a discussion of how the aberration of starlight is used as calibratingsignal in the GP-B data, see Section 14.1.5, Aberration of Starlight—Nature’s Calibrating Signal.) Our goal is toperform this calibration to a precision of 1 part in 100,000 (0.001 percent), but our measurements are limited bythe noise in our SQUID readout system. If there were no variation in the polhode period of the gyros, we couldsimply string together the data taken during the Guide Star Valid (GSV) portion of each orbit (when thespacecraft has a direct line of sight to the Guide Star) from a sequence of orbits until we have reached the desired0.001 percent precision. (We must string together data from ~15 orbits to reach this level of precision.) However,the polhode periods for the gyros did vary over the data collection period, and this makes it much more difficultto connect the data from successive orbits.432 March 2007 Chapter 15 — Preliminary Results
The complication is due to the presence of a residual magnetic flux that becomes trapped on the gyro rotorswhen they are cooled to superconductive temperatures. Whereas a constant polhode period can easily beremoved, a varying polhode period must be precisely modeled for each orbit before it can be removed. Considerthree cases:1. London moment with varying polhode rates and no trapped magnetic flux—In this case, there is noproblem; the data from successive orbits can easily be strung together to yield the desired precision sincethe London Moment is independent of rotor polhode dynamics to the levels we hope to measure.2. London moment with 1% trapped magnetic flux, but constant polhode period (no variation)—Again, inthis case there is no problem; the data from successive orbits can still be strung together because theconstant polhode period clearly stands out from other signals as the data analysis proceeds. Thus, it canbe precisely separated and removed.3. London moment with both trapped magnetic flux and time-varying polhode rates—This situation,which we have determined is the case for the GP-B gyros, requires very precise modeling of the polhodemotion for each gyro. Initial modeling that was based on information from the primary science readoutgave reasonable results, but it lacked a detailed dynamic model of the polhode motion. Now, some GP-Bgraduate students are working on a method that uses high-rate SQUID data “snapshots” to map out thetrapped flux patterns on each gyro. Using this map, together with a rotor dynamic model, we can moreaccurately predict the effect of rotor trapped flux on the gyro readout calibrations. Moreover, withaccurate trapped flux maps, we can string together relativity data from successive orbits to achieve thedesired experimental precision. Once we have the trapped flux modeling, the fitting process becomesmuch easier.15.4.3 Classical Misalignment Torques on the GyrosAfter months of research and analysis, our GP-B science team has determined that electrostatic patches are alsothe cause of the small torques we have observed on the gyroscopes when the spacecraft's axis of symmetry is notaligned with the gyroscope spin axes. Torques cause the spin axis of a gyroscope to change orientation, and incertain circumstances, this effect can look like the relativity signal GP-B measures.Fortunately, the change in gyro spin axis orientation due to these torques has a precise geometrical relationshipto the misalignment of the gyro spin/vehicle symmetry axis, and thus effects of these so-called misalignmenttorques can be removed from the data without directly affecting the relativity measurement.Important data that has enabled us to understand and model these torques was collected during a series ofplanned instrument calibration tests that we conducted during the final six weeks of the flight mission, frommid-August through the end of September 2005, when the helium in the dewar was finally exhausted. Duringthese calibration tests, we purposely slewed the spacecraft and telescope from the guide star, IM Pegasi, toobserve other stars nearby for 12 to 24 hours, and then returned to IM Pegasi. These exaggerated mis-pointingmaneuvers produced measurable torques on the gyros. From the results of these tests, the team has been able todetermine the nature and limits of these torques and has devised methods for modeling and removing themduring the data analysis phase. To learn more about these instrument calibration tests, see Section C.5,Instrument Calibration Phase: 8/19/05 – 9/30/05.Gravity Probe B — Post Flight Analysis • Final Report March 2007 433
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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
Page 410 and 411: Figure 13-37. Master Antenna Switch
Page 412 and 413: Figure 13-39. Time difference betwe
Page 414 and 415: 13.4.6 On-Orbit ResultsThe GPS comp
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Page 418 and 419: E. G. Lightsey, C. E. Cohen, B. W.
Page 420 and 421: Figure 13-45. A Bottom View of the
Page 422 and 423: Figure 13-48. The GMA configuration
Page 424 and 425: Table 13-10. Summary for Helium Gas
Page 426 and 427: 398 March 2007 Chapter 14 — Data
Page 428 and 429: a relatively slow data rate, so we
Page 430 and 431: Figure 14-4. NASA ground stations a
Page 432 and 433: electronic components to recover fr
Page 434 and 435: By a process of elimination, the GP
Page 436 and 437: A special room in the GP-B Mission
Page 438 and 439: Starting on 3 December 1725, Bradle
Page 440 and 441: seemed that aberration of starlight
Page 442 and 443: 14.1.6 Telescope Dither—Correlati
Page 444 and 445: 14.1.6.3 The Telescope Dither Patte
Page 446 and 447: amplifications. Thus, in addition t
Page 448 and 449: 14.2.2 Independent Data Analysis Te
Page 450 and 451: monthly time scales. Though only sh
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Page 454 and 455: Figure 15-2. A diagram of the GP-B
Page 456 and 457: 15.4 The Two Surprises and Their Im
Page 458 and 459: Near Zeroes.) The exception that we
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
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indicate that we have reduced the t
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C.4 Science Mission Phase: 8/27/04
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• GP-B also has an independent da
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free suspension parameters to de-tu
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We have received inquiries about a
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One of the effects of geomagnetic s
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egan transmitting, one-by-one, stat
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28 JANUARY 2005—GRAVITY PROBE B M
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magnetic pole. This event triggered
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8 APRIL 2005—GRAVITY PROBE B MISS
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On Tuesday afternoon (19-April), GP
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Norway or through the NASA space ne
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Dewar Temperature: 1.82 kelvin, hol
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visitors on a tour of the GP-B faci
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test, we are planning on running fi
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contractor at the Goddard Space Fli
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On Wednesday, we visited the star H
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Gyro Suspension System (GSS): All 4
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The helium in the dewar has now sur
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the all the spacecraft status data.
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522 March 2007 Appendix D — Summa
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Figure D-2 below shows the distribu
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DateSeveritySubtypeTitle Descriptio
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DateSeverity24 26-Apr-04 Obs F Nois
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DateSeverity40 11-May-04 Obs T Dwel
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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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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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Page 586 and 587:
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ReqIDLevel 1CSCSRM Solid StateRecor
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Page 592 and 593:
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ReqIDLevel 1CSCSRP SafemodeResponse
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ReqIDLevel 1CSCGUP GSS Processing g
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570 March 2007 Appendix F — Acron
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AcronymDefinitionAcronymDefinitionB
Page 602 and 603:
AcronymDefinitionAcronymDefinitionD
Page 604 and 605:
AcronymDefinitionAcronymDefinitionF
Page 606 and 607:
AcronymDefinitionIRUInertial Refere
Page 608 and 609:
AcronymDefinitionAcronymDefinitionM
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AcronymPMPMAPMCPMEPMETPMSPMSUPNPoPO
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AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
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AcronymDefinitionAcronymDefinitionT
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588 March 2007 Appendix F — Acron
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