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

14.1.6.3 The Telescope Dither PatternA telescope scale factor translated the values on this normalized scale into a positive or negative angulardisplacement from the center or 0 point, measured in milliarcseconds (1/3,600,000 or 0.0000003 degrees).Likewise, a gyro scale factor translated converted digital voltage signals representing the X-axis and Y-axisorientations of the four science gyros into angular displacements, also measured in milliarcseconds. We thenhad both the telescope and gyro scale factors defined in comparable units of angular displacement. Thetelescope dither motion was then used to correlate these two sets of X-axis and Y-axis scale factors with eachother.The telescope dither caused the telescope (and the entire spacecraft) to oscillate back and forth in both the X andY directions around the center of the guide star by a known amount.Figure 14-17. The telescope dither pattern around Guide Star, IM PegasiBecause this motion moved the entire spacecraft, including the gyro housings that contain the SQUID pickuploops, the SQUIDs detected this oscillation as a spirograph-like (multi-pointed star) pattern that defined a smallcircle around the center of the guide star. This known dither pattern enabled us to directly correlate the gyrospin axis orientation with the telescope orientation. The result was a pair of X-axis and Y-axis scale factors thatwere calculated for the guide-star-valid period of each orbit. At the end of the data collection period on August15, 2005, we had stored approximately 7,000 sets of these scale factors, representing the motion of the gyro spinaxes for the entire experiment.At the beginning of May 2005, we turned off the dither motion for a day to determine what effect, if any, thedither itself was contributing to our telescope pointing noise and accuracy. The results of this test indicated thatthe navigational rate gyros, which were used to maintain the attitude of the spacecraft and telescope duringguide-star-invalid periods (when the spacecraft was behind the Earth), were the dominant source of noise in theATC system, whereas the science gyro signals were stronger than the noise in the SQUID Readout Electronics(SRE) system.14.1.7 Extracting the Gyro Signals from the NoiseConceptually the GP-B experimental procedure is simple: At the beginning of the experiment, we initiallypointed the science telescope on-board the spacecraft at the guide star, IM Pegasi, and we electrically nudged thespin axes of the four gyroscopes into the same alignment. Then, over the course of a just under a year, as the416 March 2007 Chapter 14 — Data Collection, Processing & Analysis