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

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Because there are only two SQUID brackets inside the Probe (each one holding two gyros) there are also onlytwo boards outside the Probe where the control circuits for the SQUID bracket temperatures are. Each of theseboards has a primary and backup heater and sensor. A PI algorithm controls the heaters and attempts to keepthe temperature of the SRE bracket at its set temperature. There are two modes: Coarse Mode (750 ohms/volt)and Fine Mode (10 ohms/Volt).The specification for temperature control of the SRE is given only in terms of the FLL. It states that, “Thethermal-induced bias drift shall be less than 20 PPM of full scale per K for FLL ranges 1 and 2.” In other words,a 1 K temperature change of the FLL will result in less than a 0.4 mV change in the SQUID readout. This isroughly equivalent to 0.54 marcs, quite large by GPB’s standards. However, the average orbital temperaturevariation of the FLLs under temperature control is only 1mK so the thermal induced bias drift of the SQUIDsignal at orbital frequency is only 0.54 μarc. An astoundingly small effect.8.2.2.3 Verifying QBS Temperature Control SpecificationsAnalyzing a typical 24 hr. period of telemetry data from the monitor TE_QBS_a_GT10P (the QBS temperature)shows that 1mK temperature control is achieved a majority of the time. More specifically: 85% of the time theQBS temperature was within +/- 0.5 mK of the average temperature over the 24 hr period.65% of the time the temperature was within +/-.25 mK of the average temperature over the 24 hr period. 96% ofthe readings were within +/- 1 mK of the average temperature. 99.3% of the readings were within +/-1.5 mK ofthe average temperature.The QBS temperature control loop must overcome temperature fluctuations caused by orbital, annual and rollcycles. The most rapid and drastic change in temperature is caused by an orbital cycle and this prevents the QBSfrom maintaining its specified thermal stability of 1 mK 100% of the time.When the telescope shutter is open while the earth is between the guide star and GPB, light from the sun canreflect off the earth and travel down the telescope, heating the Probe. Areas of the earth covered in ice, snow orclouds reflect more light from the sun than other parts of the earth, so when the telescopes viewing field is overone of these areas, there is a larger increase in temperature of the QBS.The PI control loop cannot respond (change the heater voltage) quickly enough to keep the QBS temperaturewithin the control range when light reflecting off the earth travels down the telescope. Closing the telescopeshutter during Guide Star Invalid periods would solve this problem. However, this was deemed an unacceptablesolution as constantly opening and closing the shutter would cause vibrations in the Probe that could disturbthe gyros. Another possible solution would be to use a PID (Proportional Band, Integral, Derivative) controlloop that would be able to react fast enough to keep the QBS temperature within the desired range. However,this solution is not feasible as it is not possible to change the control loop after launch.8.2.2.4 Verifying SQUID Bracket and SRE Temperature Control SpecificationsThrough telemetry, it was easy to verify that the thermal stability of the SQUID bracket met its specificationsexcept during the time period when sunlight reflected off the earth traveled down the telescope, heating up theQBS. The chart in Figure 8-16 below shows the temperature from of the SQUID Bracket (top), the QBS(middle) and the Cryo Pump, which is not under temperature control but was included to contrast temperaturecontrol against no temperature control (bottom).220 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses
Figure 8-16. The temperature from of the SQUID Bracket (top), the QBS (middle) and the Cryo PumpThe thermal stability specification for the SRE was tested by purposely and individually modulating thetemperature of all the SRE boards and observing the effect this had on the readout of the SQUID associated withthat board. The temperature controller setpoint on the board being tested was alternated between 0 C and 75 Cwith a 90 second dwell at each value.The results were given as Bias Temperature Coefficients in mV/K (in other words, the amount that changing thetemperature of each board or bracket 1 K affected the readout of the SQUID associated with that board orbracket). The results are shown in Table 8-1 below.Table 8-1. SRE Temperature Coefficients (in mV/K)SQUID 1SQUID 2SQUID 3SQUID 4FLL1 FLL2 FLL3 FLL4 DAC1 DAC 2 DAC 3 DAC 40.47 +/-?0.200.39 +/-0.140.51 +/-0.080.72 +/-0.110.28 +/-0.18?0.54 +/-0.11DAS A SQ Br El A DAS B SQ Br El BSQUID 1 1. 30 +/- 0.08 1.03 +/- 0.43SQUID 2 no data (SQ railing) 1.40 +/- 0.04SQUID 3 0.32 +/- 0.09 (no corr., bad SQ signal)SQUID 4 0.24 +/- 0.05 0.54 +/- 0.67Gravity Probe B — Post Flight Analysis • Final Report March 2007 221
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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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Page 200 and 201: 7.1.2 Vehicle ATC ModesThere are es
Page 202 and 203: The output of the pressure transduc
Page 204 and 205: Figure 7-3. Science Telescope Compo
Page 206 and 207: Figure 7-5. Pitch and Yaw Pointing
Page 208 and 209: Figure 7-7. Magnitude of the roll r
Page 210 and 211: Changing the method by which the gy
Page 212 and 213: 7.3.3 Drag-Free Control SystemFigur
Page 214 and 215: 7.3.3.1 DFS - PerformanceThe GP-B d
Page 216 and 217: Figure 7-15. Mass flow effects of a
Page 218 and 219: 7.5.2 Successful recovery from mult
Page 220 and 221: If the vehicle control system were
Page 222 and 223: accomplish this, the ARPs are mount
Page 224 and 225: 7.5.6.1 South Atlantic AnomalyProto
Page 226 and 227: Figure 7-23. Effects on telescope p
Page 228 and 229: 200 March 2007 Chapter 7 — Attitu
Page 230 and 231: 202 March 2007 Chapter 8 — Other
Page 232 and 233: Figure 8-2. Block Diagram of CDH co
Page 234 and 235: 8.1.1.4 WATCH DOG TIMERFigure 8-4.
Page 236 and 237: of eclipses each day (approximately
Page 238 and 239: Figure 8-8. GSS1 Temperature Trends
Page 240 and 241: Figure 8-10. Forward Dewar Vacuum S
Page 242 and 243: The Gravity Probe B spacecraft has
Page 244 and 245: Figure 8-13. Forward -X Thruster Te
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Page 250 and 251: Because the specifications for SRE
Page 252 and 253: 8.3.2 Critical Mission RequirementT
Page 254 and 255: Figure 8-19 is a plot of the power
Page 256 and 257: Battery Performance Summary:Figure
Page 258 and 259: Figure 8-23. Solar Array Temperatur
Page 260 and 261: 8.3.10 ConclusionThe electrical pow
Page 262 and 263: 8.4.1 TDRSS OperationsFigure 8-28.
Page 264 and 265: Figure 8-30. Xpndr-A STDN (green),
Page 266 and 267: Figure 8-32. Plot of TDRS AGC vs Te
Page 268 and 269: The FSW also met its subsystem leve
Page 270 and 271: Appendix E, Flight Software Applica
Page 272 and 273: Table 8-4. SCRs Addressed in On-orb
Page 274 and 275: The software and macro changes resu
Page 276 and 277: Figure 8-38 below is an excerpt fro
Page 278 and 279: Figure 8-40. A-side CCCA Single Bit
Page 280 and 281: Figure 8-43. A-side CCCA Single Bit
Page 282 and 283: When single MBEs did not result in
Page 284 and 285: 256 March 2007 Chapter 9 — Gyro S
Page 286 and 287: 9.1 GSS Hardware DescriptionFigure
Page 288 and 289: significantly reduces and thus pres
Page 290 and 291: 9.1.11 Clock synchronizationThe aft
Page 292 and 293: Figure 9-8. Block diagram of the LQ
Page 294 and 295: direction (transverse to the space
Page 296 and 297: 9.2.2.2 Spin-up SuspensionDuring sp
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Figure 9-16. Predicted and measured
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Figure 9-18. The GSS passes rotor c
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Figure 9-20. Representative drag-fr
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Table 9-1. GSS Science Mission Mode
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Table 9-3. GSW application source l
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The Design and Testing of the Gravi
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282 March 2007 Chapter 10 — SQUID
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ate uncertainty). Although we have
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All of the GP-B electronics have be
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10.3 Pre-Launch Ground-Based TestsW
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Figure 10-6. AC Magnetic Shielding
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Figure 10-8. Temperature Error of S
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Figure 10-10. Quiescent SQUID Noise
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Analog control loops on the electro
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The science support applications ru
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Table 10-7. SSW application source
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308 March 2007 Chapter 11 — Teles
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Figure 11-1. Block diagram of TRE a
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Additionally, the warm electronics
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Figure 11-3. CLL switch states duri
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Table 11-3. Dates and UTC times whe
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Figure 11-7. Low Gamma Angle Data S
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s N+w i= sign+w i++−+−w i−w i
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expression, which otherwise on a po
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Figure 11-14. Y axis, B side RMS po
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Figure 11-17. X axis, B side curren
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Figure 11-20. Pointing angle differ
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Figure 11-24. Scaled summed current
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Figure 11-26 through Figure 11-29 s
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334 March 2007 Chapter 12 — Cryog
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12.2 Temperature / pressure control
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control the space vehicle without t
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Figure 12-3. Helium flow rate predi
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helium at 1.8 K. It does not appear
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Figure 12-6. Flow meter and ATC flo
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were generally within a day or so o
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shields) and subsequent instability
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350 March 2007 Chapter 12 — Cryog
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352 March 2007 Chapter 13 — Other
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Figure 13-2. ECU-operated heaters i
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13.1.3.1 Vatterfly Valve Operations
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13.1.3.7 Payload MagnetometersThe E
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Figure 13-11. ECU performance durin
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As we entered the second month of o
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During the last month of IOC, prepa
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Figure 13-19. ECU heater activity d
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Table 13-1. Proton monitor channel
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Figure 13-21. Proton Monitor data i
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In November, 2004 there was a large
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Figure 13-28 shows a correlation th
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13.3.1 About the MagnetometersFigur
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13.3.2 Other Applications for Paylo
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Figure 13-35. GPS Antennae Field of
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Figure 13-37. Master Antenna Switch
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Figure 13-39. Time difference betwe
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13.4.6 On-Orbit ResultsThe GPS comp
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and velocity values erroneously cal
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E. G. Lightsey, C. E. Cohen, B. W.
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Figure 13-45. A Bottom View of the
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Figure 13-48. The GMA configuration
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Table 13-10. Summary for Helium Gas
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398 March 2007 Chapter 14 — Data
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a relatively slow data rate, so we
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Figure 14-4. NASA ground stations a
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electronic components to recover fr
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By a process of elimination, the GP
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A special room in the GP-B Mission
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Starting on 3 December 1725, Bradle
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seemed that aberration of starlight
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14.1.6 Telescope Dither—Correlati
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14.1.6.3 The Telescope Dither Patte
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amplifications. Thus, in addition t
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14.2.2 Independent Data Analysis Te
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monthly time scales. Though only sh
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424 March 2007 Chapter 15 — Preli
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Figure 15-2. A diagram of the GP-B
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15.4 The Two Surprises and Their Im
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Near Zeroes.) The exception that we
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Figure 15-8. A slide from the GP-B
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Figure 15-9. A poster on the effect
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electrostatic patches, located at o
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Last summer (2006), Mac Keiser devi
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440 March 2007 Chapter 15 — Preli
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442 March 2007 Chapter 16 — Lesso
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16.1.1.3 Flight science downlink da
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Description of the GP-B experience:
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Lessons:1. Perform all tests with a
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Figure 16-1. Six interacting transl
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16.1.3.2 Training and certification
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Below is an edited summary of six a
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460 March 2007 Chapter 16 — Lesso
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462 March 2007 Appendix A — Gravi
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Apogee altitude659.1 km (409.6 mile
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466 March 2007 Chapter B — Spacec
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Gravity Probe B — Post Flight Ana
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470 March 2007 Appendix C — Weekl
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16 April 2004—Vehicle is Prepared
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The electrical power system is full
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4 JUNE 2004—MISSION UPDATE: DAY 4
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SQUIDs to detect their rotation spe
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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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ReqIDLevel 1CSCSRM Solid StateRecor
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ReqIDLevel 1CSCSRP SafemodeResponse
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ReqIDLevel 1CSCGUP GSS Processing g
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570 March 2007 Appendix F — Acron
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AcronymDefinitionAcronymDefinitionB
Page 602 and 603:
AcronymDefinitionAcronymDefinitionD
Page 604 and 605:
AcronymDefinitionAcronymDefinitionF
Page 606 and 607:
AcronymDefinitionIRUInertial Refere
Page 608 and 609:
AcronymDefinitionAcronymDefinitionM
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AcronymPMPMAPMCPMEPMETPMSPMSUPNPoPO
Page 612 and 613:
AcronymSCMOSCMTSCNSCPASCPMSCRSCSASC
Page 614 and 615:
AcronymDefinitionAcronymDefinitionT
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588 March 2007 Appendix F — Acron
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