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

Figure 3-9. Schematic diagram of starlight image division in the GP-B telescopeAs shown in Figure 3-9, GP-B accomplishes this task by focusing the mirror-reflected starlight in the ImageDivider Assembly (IDA) at the telescope's front end, and passing it through a beam-splitter (a half-silveredmirror). The beam-splitter forms two separate images, each of which falls on a roof-prism (a prism shaped like apeaked rooftop). The prism slices the star's image into two half-disks, which are focused onto two silicon photodiodes at opposite ends of the Detector Mount Assemblies (DMA), shown in Figure 3-9.On the DMA sensors, the light signals of each half-disk are converted to electrical signals and then compared. Ifthe signals are not precisely equal, this means that the roof-prism is not splitting the image precisely in half. Theposition of the entire spacecraft is then adjusted until the signals are equal and the image is split right down themiddle. When this is accomplished in both sensors for each (x and y) axis, then the telescope is locked on theexact center of the guide star. The flight telescope on-board the GP-B spacecraft was capable of pointing to thecenter of the guide star with an accuracy of 0.5 milliarcseconds (1.4x10 -7 degrees) per orbit.3.2.2 Redundancy in GP-B Systems and TechnologyUnderlying the entire GP-B experiment is a philosophy of redundancy that has been built into the GP-Bspacecraft and all of its subsystems. Following is a description of this redundancy.3.2.2.1 Gyroscope RedundancyAt the highest level of the GP-B experiment, redundancy begins with the four gyroscopes. We actually need onlyone gyroscope to perform the GP-B experiment, but since it is unlikely that this experiment will ever berepeated, it seemed prudent to build redundancy into the experiment itself. Having four gyros obviouslyprovides backup in case one, two, or even three gyros should fail during the mission. More important, however,having multiple gyroscopes provides built-in crosschecks on the relativity data. In other words, a high degree ofcorrelation in data collected from four independent gyros provides greater confidence in the results than datacollected from a single gyro, or even two gyros—especially if the results should differ from the predictions ofgeneral relativity.78 March 2007 Chapter 3 — Accomplishments & Technology Innovations