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

7.1 ATC Background and OverviewThe Attitude and Translation Control System (ATC) controls all proportional micro thrusters and magnetictorque rods that determine and maintain the spacecraft’s precise position relative to the science gyro serving asthe drag-free proof mass and the spacecraft’s pointing direction. The position, acceleration, attitude androtation rates of the space vehicle are measured by several redundant sensors. These sensors include both thecontrol and science gyroscopes, star sensors, magnetometers, coarse sun sensors, GPS receivers, and the sciencetelescope. The choice of which set of sensors are used at a given time depends on the configuration of the ATCsubsystem. Feedback control logic takes the measured position, acceleration, attitude and rotation rates andcomputes commands sent to the thrusters and magnetic torque rods in order to maintain the space vehicle inthe desired position and attitude.The requirements for the Attitude and Translation Control subsystem are twofold. First, the position of thespace vehicle must be accurately centered about one of the Science Gyroscopes so that the forces (controlefforts) and torques applied to the science gyroscopes are rigorously minimized during the Science Mission.Second, the GP-B space vehicle must remain accurately pointed at the Guide Star IM Pegasi, during gyroscopespin up and throughout the Science Mission phase to provide the distant inertial reference and to minimizetorques due to mis-pointing.7.1.1 ATC HardwareThe ATC subsystem consists of a network of sensors, actuators, and computer/data management hardware andsoftware, with the purpose of maintaining the spacecraft’s pointing and position within the orbit to very stricttolerances. Of all the metrics to measure the ATC’s performance, the ability to maintain pitch/yaw attitude onthe guide star, the ability to maintain the spacecraft’s position in roll phase and roll rate, and helium gas massflow usage are of primary importance. In the table below is a list of the control system equipment used on orbit.Table 7-1. Control System Equipment ListGravity Probe B — Post Flight Analysis • Final Report March 2007 171