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

Figure 7-15. Mass flow effects of a Heat Pulse Meter Test and the Magnetic Torquing System7.3.5 Calibration PhaseAfter the science phase of the mission ended, there was a period of data collection called the calibration phase ofthe mission. Several ATC operations were done during the calibration phase. The most intense operation wasmaneuvering the spacecraft to point at neighbor stars, other visible stars varying from 0.4 degrees away from IMPegasi to 7 degrees away. Over the course of a few weeks, three different stars were visited multiple times.Each maneuver consisted of a vehicle reconfiguration, a vehicle reorientation, and another vehiclereconfiguration that includes finding the new telescope target star. During the maneuver and neighbor starlocation, the star tracker was used for attitude reference. The planned neighbor star location procedure was tomaneuver to the expected location of the star, then dwell around in a spiral if the star was not seen in thetelescope field of view. However, during every neighbor star visit, the star was found and locked onto using onlythe predicted position. Thus, no costly time was used dwelling for the stars, and a more aggressive calibrationschedule was achieved.The other ATC calibration phase operations consisted of drag-free system testing. Drag-free biases were putinto the drag-free system in order to measure the torque effects on the gyroscope pointing.188 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis