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

amplifications. Thus, in addition to the desired signals, the GP-B science data include a certain amount ofrandom noise, as well as various sources of interference. The random noise averages out over time and will notbe an issue. Some of what appears to be regular, periodic interference in the data is actually importantcalibrating signals that enable us to determine the size of the scale factors that accompany the science data. InSection 14.1.5, “Aberration of Starlight—Nature’s Calibrating Signal” above, we described how the orbital andannual aberration of the starlight from IM Pegasi is used as a means of calibrating the gyro readout signals.Likewise, in Section 14.1.6, “Telescope Dither—Correlating the Gyro & Telescope Scale Factors” above, wedescribed how the telescope dither motion is used to calibrate the telescope signals. In addition, over the courseof the mission we have discovered a few unexpected sources of interference that must be appropriately modeledand removed from the data. The error analysis is well in hand, but careful and painstaking cross checks arerequired by our science team to ensure the validity of the final experimental results.14.1.7.4 Proper Motion of IM Pegasi—The Final Data ElementFinally, there is one more very important factor that must be addressed in calculating the final results of theGP-B experiment. We selected IM Pegasi, a star in our galaxy, as the guide star because it is both a radio sourceand it is visually bright enough to be tracked by the science telescope on-board the spacecraft. Like all stars inour galaxy, the position of IM Pegasi, as viewed from Earth and our science telescope changed over the course ofthe experimental year. Thus, the GP-B science telescope was tracking a moving star, but the gyros wereunaffected by the star's so-called “proper motion;” their pointing reference was IM Pegasi's position at thebeginning of the experiment. Thus, for each orbit, we must subtract out the telescope's angle of displacementfrom its original guide star orientation so that the angular displacements of the gyros can be related to thetelescope's initial pointing direction, rather than its actual pointing direction each orbit. The annual motion ofIM Pegasi with respect to a distant quasar has been measured with extreme precision over a number of yearsusing a technique called Very Long Baseline Interferometry (VLBI) by a team at the Harvard-SmithsonianCenter for Astrophysics (CfA) led by Irwin Shapiro, in collaboration with astrophysicist Norbert Bartel andothers from York University in Canada and French astronomer Jean-Francois Lestrade. However, to ensure theintegrity of the GP-B experiment, we added a “blind” component to the data analysis by requesting that the CfAwithhold the proper motion data that will enable us to pinpoint the orbit-by-orbit position of IM Pegasi untilthe rest of our data analysis is complete. Therefore, the actual drift angles of the GP-B gyros will not be knownuntil the end of the data analysis process.14.2 The GP-B Science Data Analysis ProcessIn this section we first provide a brief overview of what is involved in analyzing the GP-B science data. Dataanalysis actually began before launch. The science data analysis team built prototype computer codes to processsimulated science data in preparation for the actual data from the spacecraft. Following on-orbit checkout, theteam used real data to test and refine these routines and to take into account the anomalies the mission hasencountered (i.e. the time varying polhode period) that were not part of the initial simulations. These activitiesall preceded the formal data analysis effort.14.2.1 Formal Data Analysis PhasesFollowing acquisition and archiving of the science data from the satellite, the formal analysis of these data isbroken into three phases; each subsequent phase builds upon the prior toward the final science results.418 March 2007 Chapter 14 — Data Collection, Processing & Analysis