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

seemed that aberration of starlight was going to be a source of experimental error bundled into the scale factor.But upon examining this issue more closely, it became clear that, quite to the contrary, the orbital and annualaberration of light from the guide star actually provided two built-in calibration signals that would enable thegyro scale factor to be calculated with great accuracy.14.1.5.3 The Orbital Aberration SignalTo see how this works, let's first take a closer look at how the orbital aberration of the starlight from the guidestar, IM Pegasi, is “seen” by GP-B spacecraft.Figure 14-12. GP-B's Orbital aberration of light from the Guide Star, IM PegasiAs mentioned earlier, the spacecraft orbits the Earth once every 97.5 minutes. During the mission, as thespacecraft emerged over the North Pole, the guide star came into the field of view of the science telescope, andthe telescope then locked onto the guide star. This began what was called the “Guide Star Valid (GSV)” phase ofthe orbit. At this point in its orbit, the orientation of the spacecraft's velocity was directly towards the guide star,and thus, there was no aberration of the star's light—it traveled straight down the center of the telescope.However, as the spacecraft moved down in front of the Earth, the orientation of its velocity shifted in the orbitaldirection until it became perpendicular to the direction of the light from the guide star, slightly above theequator. This is the point of maximum aberration since the telescope was then moving perpendicular to theguide star’s light. As the spacecraft moved on towards the South Pole and then behind the Earth and directlyaway from the guide star, the aberration receded back to zero. At this point, the telescope unlocked from theguide star, transitioning into what was called the “Guide Star Invalid (GSI)” phase of the orbit. The navigationalrate gyros on the outside of the spacecraft maintained the telescope's orientation towards the guide star whilethe spacecraft was behind the Earth, but we did not use the science gyro data collected during the GSI phase.412 March 2007 Chapter 14 — Data Collection, Processing & Analysis