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

perceptible, but still relatively small effect on the experimental results. Furthermore, Dr. Duhamel's researchindicates that in most cases, the effects of gyro drift and data loss tend to be larger if they occur at the beginningor end of the science phase, rather than in the middle.Based in part on Dr. Duhamel's prescient research over 20 years ago, the GP-B science team has developed andperfected a comprehensive, state-of-the-art data analysis methodology for the GP-B experiment that, amongmany other things, takes into account data lapses and the possibility of accompanying non-relativistic torqueson the gyros. Also, many improvements have been made in the GP-B technology since Dr. Duhamel did hisresearch. These improvements provide us with more extensive data and extra calibrations that enable us tofurther refine the original error analysis predictions.Dr. Thierry Duhamel now lives in France (his native country), where he works for EADS Astrium, a leadingEuropean aerospace company.14.1.5 Aberration of Starlight—Nature’s Calibrating SignalGP-B Principal Investigator, Francis Everitt, once said: “Nature is very kind and injects a calibrating signal, dueto the aberration of starlight, into the GP-B data for us.” This phenomenon actually provides two naturalcalibration signals in the relativity data that are absolutely essential for determining the precise spin axisorientation of the gyros over the life of the experiment. However, the word “aberration” typically refers tobehavior that departs or deviates from what is normal, customary, or expected—and usually, such behavior isnot welcome. So, what does aberration have to do with starlight? And, why would we want to use somethingaberrant as a calibration signal? To answer the first of these questions, we must first travel back in time to the18th century.14.1.5.1 18 th Century British Astronomer James Bradley Discovers Stellar AberrationAt the beginning of the 18th century, astronomers were still seeking some form of direct proof of theCopernican theory that all the planets in our solar system orbit around the Sun. One such person was BritishAstronomer James Bradley, who in 1718 was recommended by the Astronomer Royal Edmund Halley tobecome a Fellow of the Royal Society, and who eventually succeeded Halley as British Astronomer Royal in1742.Figure 14-10. British Astronomer Royal, James BradleyGravity Probe B — Post Flight Analysis • Final Report March 2007 409