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

egan transmitting, one-by-one, status monitors around the MOCbegan turning red, signaling the spacecraft had triggered itssafemodes. There is a problem on-board.During the next 20 minutes, phones rang, pagers beeped, and soon,the MOC was teeming with activity. An assessment of the safemodesthat were triggered indicated that an error—never seen before—hadoccurred in a module of the Attitude and Translation Control (ATC)computer system. The spacecraft’s GPS had registered an off-the-scalevelocity spike, which if correct, indicated that, for one brief moment,the spacecraft had traveled faster than the speed of light—or to useStar Trek terminology, it had “warped into hyperspace.” In fact, theGPS system had reported a single data point with an erroneously highvelocity, which when squared, caused a computer overflow. The ATCcomputer module took exception to this data overflow and triggered asafemode test, which in turn activated a chain reaction responsesequence.17 DECEMBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 241As we near the end of 2004, the GP-B spacecraft has been in orbit forjust under eight months and collecting science data for almost fourmonths. It remains in fine health, and all subsystems are continuing toperform well. The spacecraft is beginning to experience brief periodsof being eclipsed from sunlight by the Earth during each orbit,signaling that its recent full-sun season is waning. It is flying drag-freearound gyro #3, maintaining a constant roll rate of 0.7742 rpm (77.5seconds per revolution.) The temperature inside the dewar is holdingsteady at just under 1.82 Kelvin. Recent measurements indicate thatapproximately half of the superfluid has now been expended from thedewar. All four gyros are digitally suspended in science mode. We arenow approximately 40% of the way through the science phase of themission. The data collection process is continuing to proceedsmoothly, and the quality of the data remains excellent.A GP-B RETROSPECTIVE ON 2004Looking back, 2004 has been a year of monumental triumph for GP-B.The year began with a re-work of the Experiment Control Unit (ECU),and its subsequent reinstallation into the space vehicle.At the beginning of April, as the space vehicle was being readied forlaunch, the official GP-B pre-launch press conference was held atNASA headquarters in Washington DC. (You can view a streamingvideo of this press conference, which includes many technical detailsabout the GP-B science experiment and technology, on NASA'sKennedy Space Center ELV Web page.)Three weeks following the press conference, at 9:57 am PST on 20April 2004, a Boeing Delta II rocket carried the GP-B spacecraft,embodying over 40 years of dogged persistence in science andengineering, into a perfect orbit. That emotionally overwhelming day,culminating with the extraordinary live video of the spacecraftseparating from the second stage booster meant, as GP-B ProgramManager Gaylord Green put it, “that 10,000 things went right.” TheGP-B launch will long be remembered—not only by GP-B PrincipalInvestigator, Francis Everitt, and the GP-B team, but also bythousands of people who have been associated with GP-B in one wayor another over the years and countless others who have beenfollowing it.The MOC staff immediately scheduled several extra satellitecommunication passes so they could communicate with the spacecraftmore frequently. Then, over the ensuing 24 hours, they methodicallyworked through a series of tests and command sequences to return thespacecraft to its normal science operation mode. We initially assumedthat the GPS receiver had suffered a proton hit in the SAA region, butfurther analysis suggests that this was not the case. Rather, thisanomaly was apparently caused by one of the four accessible GPSsatellites being in the wrong position for proper GPS triangulation.The ATC system usually catches situations of this kind and disallowsthe data; but, this one slipped through the filter.The spacecraft has returned to normal operations. This incident wasnot detrimental to the GP-B experimental data. And, once again, thefact that an anomalous event occurred while the spacecraft was flyingthrough the SAA region appears to be a coincidence—or is it?The launch was the “end of the beginning” for GP-B. The ensuing 4-month IOC period demonstrated the exceptional preparedness anddedication of the GP-B team, from dealing with anomalies in orbit tospinning up the four gyros last August, which following the launch,was the second greatest milestone in the program.Now, four months into the 10-month science phase of the mission,our whole GP-B team can reflect back on the past year with anenormous sense of pride and accomplishment. And, as we lookforward to the coming year, we are keenly aware of our continuedresponsibility to see this once-in-a-lifetime experiment through to itsfinal conclusion.7 JANUARY 2005—GRAVITY PROBE B WEEKLYUPDATE: Day 262Throughout the holiday season, the GP-B spacecraft has remained inexcellent health, with all subsystems performing well and noanomalous events. The GP-B spacecraft has now completed more than3,870 orbits over 8.5 months, and on average, the four gyros have eachmade approximately one billion revolutions.494 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission