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

Table 2-5. Weekly highlights of the 7-week final instrument calibration phaseWeekly Highlights of the GP-B Final Calibration PhaseWeek Activities & EventsWeek ending 8/19 Final calibration phase begins on Monday, August 14th when drag-free control on gyro #1was turned off, and gyro preloads were set to IOC levels of 10 volts.The telescope & spacecraft were maneuvered to point a the star HD216235, 1 degree awayfrom IM Pegasi.The spacecraft returned to its guide star orientation, and drag-free control was restored for aday.The calibration excursions to HD216235 were repeated four more times over the course ofthis week.Week ending 8/26 On Monday, the telescope & spacecraft were maneuvered to the star Zeta Pegasi (HR 8634),located 7 degrees away from IM Pegasi. This pointing orientation was maintained for ~30 hrs.On Wednesday, the spacecraft’s orientation was returned to IM Pegasi.On Thursday and Friday, DC mis-centering tests were performed on gyros #1, #2, and #4.At the end of the week, GSS torque calibrations were performed on gyros #1 and #4.Week ending 9/2 Although heat pulse meter tests had suggested that the helium would be depleted by thisday, some amount of liquid helium still remained in the dewar, and thus calibration testscontinued this week.On Wednesday, the telescope & spacecraft were maneuvered to point to the star HR Pegasi, 4degrees away from IM Pegasi in the east-west plane (all previous slewing tests had been inthe north-south plane).On Thursday & Friday, several slewing tests were made to “virtual stars” less than one degreeaway from IM Pegasi in various directions. (Virtual stars don’t actually exist; they are simplyposition markers at pre-determined distances from the guide star, IM Pegasi.)Tony Lyons, GP-B Program Manager from MSFC, joined the team at Stanford for this week ofcalibration tests.Various other slewing tests, in which the telescope/spacecraft was pointed at locations in thevicinity of IM Pegasi continued throughout the weekend.Week ending 9/9 Heat pulse meter tests had predicted that the helium in the dewar would be depleted by thebeginning of this week, but like the Energizer Bunny that keeps on running, the helium in thedewar kept on flowing. Thus, planned calibration tests continued.Over the weekend, we visited virtual stars located 0.1 degrees away from star HD216635, aneighbor of IM Pegasi, remaining pointed at the virtual star for 24 hours and then returningto IM Pegasi for 16 hours. We then repeated this procedure with a virtual star located 0.1degrees in the opposite direction.On Tuesday, we visited a virtual star located halfway to the star HR Pegasi (HD216672),located to the West of IM Pegasi and remained pointed there for 24 hours. We then slewedthe telescope/spacecraft back to IM Pegasi.On Thursday, we visited a virtual star 0./3 degrees towards HD216635 and remained there for24 hours.We continued these “slewing” tests during the weekend, visiting virtual star locations withina 4 degree radius of IM Pegasi.Gravity Probe B — Post Flight Analysis • Final Report March 2007 63