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

26 NOVEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 220On the eve of the 2004 Thanksgiving holiday weekend—mission week#32—the GP-B spacecraft is in good health, with all subsystemsperforming well. We have now been collecting data for three months.Data collection is proceeding smoothly, and the quality of the data isexcellent. The spacecraft continues to fly drag-free around gyro #3,maintaining a constant roll rate of 0.7742 rpm (77.5 seconds perrevolution.) The temperature inside the dewar remains steady at justunder 1.82 Kelvin.3 DECEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 227The GP-B spacecraft continues to be in good health, with allsubsystems performing well. As of tomorrow, we will have beencollecting relativity data for 100 days. The data collection process isproceeding smoothly, and the quality of the data remains excellent. Allfour gyros are digitally suspended in science mode, and the spacecraftis flying drag-free around gyro #3, maintaining a constant roll rate of0.7742 rpm (77.5 seconds per revolution.) The temperature inside thedewar is holding steady at just under 1.82 kelvin.For much of the month of December, our Mission Planning staff hasscheduled minimal spacecraft activity, outside routine monitoring ofspacecraft functionality and downloading of science data. However,lest we become complacent, Mother Nature seems to have a way ofstirring up the pot—as was the case last Saturday afternoon.The Spacecraft’s TaleLast Saturday began as a rather “ho-hum” California winter’s day.Orbiting the Earth every 97.5 minutes, the GP-B spacecraft passeddirectly over California around 6:30AM PST, but the Sun was alreadyup, and the sky was too bright to see the satellite. In the GP-B MissionOperations Center (MOC), a skeleton crew consisting of the on-dutyMission and Flight directors and one or two resident engineersmonitored several telemetry passes (communications sessions) duringthe morning hours. Most were 25-minute satellite passes, duringwhich the spacecraft relays status information to the MOC throughthe NASA TDRS (Tracking and Data Relay Satellite) communicationssatellite system. And, during a 12-minute ground pass at 1:15PM PST,the spacecraft’s solid-state recorder relayed relativity data to the GP-Bscience database through a high-speed telemetry connection with theSvalbard ground tracking station, on the island of Spitsbergen inNorway. All in all, it was a normal Saturday, and the atmosphere in theMOC was quite relaxed.It has been relatively quiet here in the GP-B Mission OperationsCenter, since the strong solar flares and geomagnetic storm threeweeks ago. Our team continues to adjust the flow rate of the excesshelium from the dewar during the present a 6-week “hot” season,where the spacecraft is continually in sunlight throughout each orbit.(See last week’s highlights for a discussion of the spacecraft’s seasons.)Furthermore, seasonal warming of the spacecraft has resulted in theAttitude Reference Platform (ARP) on the outside of the spacecraftproducing data that slightly diminishes the pointing accuracy of theAttitude and Translation Control system (ATC). To address this issue,the ATC team installed a modified data filter, and tests performed thispast week indicate that spacecraft pointing accuracy has nowimproved.Finally, the team has been adjusting controls on the navigationalgyroscopes that are used by the ATC to keep the telescope pointedtowards the guide star, IM Pegasi (HR 8703), during periods when thespacecraft moves behind the Earth, eclipsing the telescope’s view of theguide star. These adjustments have reduced the time required for thetelescope to re-lock onto the guide star—when the spacecraft emergesover the North Pole—to less than one minute.10 DECEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 234Now, almost 34 weeks in orbit, the GP-B spacecraft is in fine health,with all subsystems continuing to perform well. The spacecraft, whichis in the middle of a 6-week un-eclipsed (full sun) period, is flyingdrag-free around gyro #3, maintaining a constant roll rate of 0.7742rpm (77.5 seconds per revolution.) The temperature inside the dewaris holding steady at just under 1.82 kelvin. All four gyros are digitallysuspended in science mode. We have been collecting relativity data for15 weeks. The data collection process is continuing to proceedsmoothly, and the quality of the data continues to be excellent.Following the successful ground pass with Svalbard, the spacecraftcontinued on its southward route. At around 1:30 PM PST, Pacifictime, the spacecraft was flying over South America—heading towardsthe South Pole—when it entered the South Atlantic Anomaly (SAA).This is a region above the Earth where the fluxes of trapped protonsand other particles, emitted by the Sun, are much greater thananywhere else on Earth, due to the asymmetry of the Earth’s protectiveVan Allen Radiation Belts. Thus, spacecraft are more vulnerable tobeing struck by protons when flying through this region.At 1:48PM, Pacific Time, an odd event silently occurred on-board thespacecraft, triggering four safemodes (pre-programmed commandsequences designed to automatically place the spacecraft, its gyros,telescope, and other systems and instruments, in a stable and safeconfiguration in response to anomalous or out-of-limits feedbackfrom various on-board sensors).Back in the MOC, the next telemetry pass was not scheduled until 3:16PM, so the operations staff was completely unaware of this change inthe spacecraft’s condition—for the time being. At 3:15PM, the MOCstaff settled into their seats for the upcoming satellite status telemetrypass. As the spacecraft’s antenna locked into the TDRSS satellite andGravity Probe B — Post Flight Analysis • Final Report March 2007 493