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

7.5.5 ARP Motion ChallengesFigure 7-17. Large Scale Dither Performance (2004/197)The Attitude Reference Platforms (ARPs) are the mounting location of the control rate gyroscopes, as well asthe roll star trackers. At one time, one set of gyroscopes and one star tracker are used in the control loop thatmaintains the attitude of the vehicle. While thermal effects were expected, the motion of the ARP and rategyroscope housing have introduced disturbances into the rate signal that have presented challenges throughoutmost of the mission. The ARP moves with components of motion at annual, orbit, and roll rates, and all three ofthese components have provided unique challenges to overcome to arrive at the pointing performance thevehicle now achieves.7.5.5.1 Mode 1A vs 1BOne of the most significant operational decisions resulting from ARP motion analysis is that Mode 1B (directmode) was chosen as the science configuration, instead of Mode 1A (observer mode) while pointing at the guidestar. The dependence on the gyroscope signal of the observer would lead to the telescope tracking the ARPmotion rather than keeping the guide star centered in the field of view.As the vehicle rolls around the telescope boresight (the z axis in the body frame), the ARP is heated and cooledin an almost sinusoidal fashion at precisely roll rate. As the ARP deforms, the orientation of the gyroscopehousing changes with respect to the z axis, causing the roll rate of the vehicle to project onto the pitch and yawaxes. This effect shows up in the gyroscope output and yields a fictitious sinusoidal rate signal in both the pitchand yaw axes.Gravity Probe B — Post Flight Analysis • Final Report March 2007 191