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

28 JANUARY 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 283 days (40 weeks/9.25 months)Current Orbit #: 4,178 as of 4:00 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): Multi-bit errors (MBE): 0, Singlebiterrors (SBE): 8.5 (daily avg.)Last Thursday morning (20 January 2005), GP-B entered Safemode 2A(gyro hold), due to the loss of the guide star. Analysis of this safemodeindicated that the telescope detectors were saturated by a large solarflare (X-7 class) that occurred Wednesday night.The solar flare had fluxes spiking several orders of magnitude abovenominal in the > 100MeV proton energy range. Less than a day later(16:27 PST), GP-B successfully re-locked onto the guide star and theAttitude and Translation Control (ATC) system returned to itsnominal configuration. During this event, the attitude error of thespacecraft and telescope remained within 800 arcseconds of the guidestar. A preliminary assessment suggests that this event had minimalimpact on the GP-B data or experiment.One issue currently being investigated is the spacecraft’s protonmonitor, which had been yielding spurious values before last week’ssolar storm. The proton monitor was power cycled this Tuesday night(25 January). After reboot, it provided two hours of meaningless datafollowed by zeros. The proton monitor team is examining the data foradditional clues, and a review of this issue will be forthcoming.4 FEBRUARY 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 290 days (41 weeks/9.5 months)Science Data Collection: 161 days (23 weeks/5.25 months)Current Orbit #: 4,281 as of 4:00 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): Multi-bit errors (MBE): 1 (See MDSummary below), Single-bit errors (SBE): See MD Summary below)Last weekend, on both Saturday and Sunday, the spacecraft’s maincomputer (CCCA) erroneously displayed a dramatic increase insingle-bit errors (SBE). We observed similar SBE increases in July 2004and one other time since then. In both previous cases, the SBE countquickly returned to normal, and we were unable to determine the rootcause of the erroneous increase. SBEs are self-correcting, so noparticular action is required, but we are monitoring this situation.Also, this past Tuesday, a new multi-bit error (MBE) was discovered ina memory location of the CCCA computer. A command wassubsequently sent to reload this memory location, clearing the MBE.The health of the spacecraft’s Proton Monitor is still underinvestigation. Telemetry indicates that it is powered on, but the dataappears to be corrupt. Note that the science mission is NOT affectedby the loss of the Proton Monitor. The Proton Monitor Engineeringunit is continuing to investigate this failure.We have been notified that the sun spot which caused the GP-Bscience telescope to lose track of the guide star two weeks ago, will becoming around to the front side of the sun on 5 February 2005.Geomagnetic activity around the Earth tends to increase after theappearance of sun spots. Thus, we have added extra telemetry passesand increased sensitivity to this issue during the next two weeks.11 FEBRUARY 2005—GRAVITY PROBE B MISSIONUPDATELast week’s intense solar storm has died down, and solar protonactivity, as reported by the National Oceanic & AtmosphericAdministration’s (NOAA) Space Environment Center (SEC), hasreturned to normal levels. (The left solar photo and graph are from lastweek; the solar photo and graph on the right are from today.)Likewise, the GP-B spacecraft has returned to normal functioning,with the ATC system locking normally on the guide star each orbitand the telescope detectors returning normal values. GP-B was not theonly satellite affected by last week’s solar storm; other satellites faredworse from the ordeal. In fact, given that last week’s solar storm wasthe worst one since October, 2003, the GP-B spacecraft performedremarkably well, with a very swift recovery and no significant impactto the experiment.Mission Elapsed Time: 297 days (42 weeks/9.75 months)Science Data Collection: 168 days (24 weeks/5.5 months)Current Orbit #: 4,384 as of 4:00 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): Multi-bit errors (MBE): 0, Singlebiterrors (SBE): 1,731 (daily average)This past week was a quiet one for GP-B, with very few activitiesscheduled. The solar flares that caused us to lose the guide star threeweeks ago came around to the front of the Sun, facing towards Earth,again this past week. We took extra precautions and scheduled extratelemetry passes to deal with possible effects of these solar flares on thespacecraft. However, this time, the solar flares produced no substantialgeomagnetic activity around the Earth. This may be a sign that the Sunhas moved into a quieter part of the solar cycle.496 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission