Comparing LRI Steering Mirror and Star Camera Data at Low Frequencies

Abstract

GRACE Follow On measures the Earth's gravitational field by recording distance variations between two satellites flying behind each other around the planet. The attitude and orbit  control system ensures that nominal attitude variations do not exceed a magnitude of a few 100 urad in yaw and pitch. Star cameras and fibre optic gyroscopes measure the inter-satellite pointing angles and indicate when an attitude correction needs to be performed.The Laser Ranging Interferometer (LRI), which was designed as a technology demonstrator but will be implemented as the main science instrument in future missions, measures the distance changes in parallel to the microwave system. However, to maximize the interferometric contrast in the LRI, even when the satellite attitude jitters, a precise pointing between transmitted and received laser beam is necessary and achieved by a two-axes Fast Steering Mirror (FSM).  The FSM measurements provide essentially information about the satellite pointing angles pitch and yaw w.r.t. the line-of-sight connecting both satellites. That allows to compare the FSM to the different attitude sensors like star cameras.We are particularly interested in the characterization of FSM at low-frequencies, i.e. long-term stability of the FSM readout under temperature changes induced for instance by the varying geometry of orbital plane and Sun. We will present recent results on how to model the differences between the star camera and the FSM pointing angles and show analysis results,  which of the instruments might cause the variations in the residuals.Our results are of relevance for future missions, as these missions might employ the LRI FSM as an attitude sensors in order to control the satellite attitude.