GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Late last Thursday, Gyro #2 transitioned from its normal digitalsuspension to analog backup suspension mode. This gyro was resuspendedin digital mode last Saturday, 28 May 2005, and it hasremained digitally suspended since that time. The root cause of thistransition is still under investigation.gyros are located in slightly different positions on the quartz block,we must apply small voltages to their suspension systems to flythem drag-free around Gyro #3's position. The feed-forward termis an adjustment factor for anticipating the position of the othergyros in order to prevent error build-up. Our initial calculation ofthe feed-forward term prior to launch was fairly accurate, and we'vebeen using it thus far in the mission. However, based on 13 monthsof flight data, we are able to update this term to be even moreaccurate, which we did this past week. We will now use the updatedfeed-forward term for the duration of the science (data collection)phase.• Increasing the preload voltage on science gyro #3 to 1.5V for a brieftime. The Gyro Suspension System (GSS) for each gyro has a“preload voltage” and a “control voltage.” You can think of thepreload voltage as coarse positioning and the control voltage as finepositioning. Generally, we like to keep these voltages about equal.However, right now as part of our calibration tests, we arepurposely changing the preload voltage for a brief time to see theeffect of this asymmetry. Note that we can perform this calibrationtest while remaining in drag-free flight.Last Sunday, 20 May 2005, the SQUID Readout Electronics (SRE)computer experienced two multi-bit errors (MBEs). One of theseerrors was located in a data buffer location that cleared itself, thusremoving the error. The second error was located in a section ofprogram code that is not planned for use. In response, the MissionOperations team has since removed this code module from thecomputer's database, so that it cannot be accessed inadvertently.Various planned preliminary calibration tests are continuing,including some tests of Gyro #3 and the drag-free system. Such testsare standard operating procedure on experimental missions. Duringthese tests, we purposely vary certain parameters that would not beexpected to change greatly during the experimental portion of themission for the purpose of characterizing instrument componentperformance. In order to maximize the length of the science datacollection phase of the GP-B mission, we are currently running somepreliminary calibrations that can safely be performed while we are stillcollecting science data. Our final calibration tests, which must becompleted before the helium in the dewar is depleted, involve placingtorques (forces) on the gyros, so we can only perform these tests afterdata collection is finished—early in August.10 JUNE 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 416 days (59 weeks/13.64 months)Science Data Collection: 287 days (41 weeks/9.41 months)Current Orbit #: 6,139 as of 4:30 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Dewar Temperature: 1.82 kelvin, holding steadyCommand & Data Handling (CDH): B-side (backup) computer incontrol, Multi-bit errors (MBE): 1 (in SRE computer on 6/6), Single-biterrors (SBE): 8 (daily average)Over the past two weeks, we have performed several routineadjustments to various systems on the spacecraft and continued ourpreliminary calibration tests. These included:• Updating the feed-forward term of the backup drag-free controller.The spacecraft is flying drag-free around Gyro #3, which meansthat this gyro is treated as its center of mass. Since the other three• Starting a new readout calibration cycling plan where we operatethe calibration signal on either SQUID's #1 and #2, or SQUIDs #3and #4, with the other pair off. (We toggle the paris every 24 hours.)This is another of the preliminary calibration tests that we arecurrently conducting.On Monday 6 June 2005, a multi-bit error (MBE) occurred in theSQUID Readout Electronics (SRE) computer. A preliminary analysisof this error suggests that the memory location in which the erroroccurred is not being used, and most likely, this MBE is benign.However, if after further investigation, this is not the case, we willreboot the SRE computer to clear the error.Finally, another Heat Pulse meter test to re-check the level of thehelium remaining in the dewar is scheduled for Monday, 13 June 2005.If the results of this test correlate well with previous results, this will bethe last heat pulse test of the mission.17 JUNE 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 423 days (60 weeks/13.87 months)Science Data Collection: 294 days (42 weeks/9.64 months)Current Orbit #: 6,242 as of 4:30 PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Gravity Probe B — Post Flight Analysis • Final Report March 2007 505