Comparing GRACE Follow-On Inter-Satellite Pointing Angles from Star Camera and LRI Fast Steering Mirror

Abstract

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is observing the time variable gravity field by measuring distance variations between two satellites. The first Laser Ranging Interferometer (LRI) between distant spacecraft uses a so-called Fast Steering Mirror (FSM) to satisfy the narrow LRI pointing requirements despite the larger spacecraft pointing variations. The position readout of these mirrors allow to compute the inter-satellite pointing angles yaw and pitch w.r.t. the line-of-sight connecting both satellites. The nominal pointing variations have a magnitude of a few 100 µrad. Typically, the attitude information of roll, pitch and yaw angles is derived from three star cameras and fibre optic gyroscopes.

Since we are particularly interested in the characterization of the FSM at low-frequencies, we compare the FSM readout with star camera measurements for the time-span of December 2018 until December 2022. The residuals show temperature induced changes which are related to the varying orientation of the orbital plane and the sun, expressed through the angle β’. We will present our analysis that attempts to attribute and model the residuals.

This analysis is of interest for future gravity missions, where the FSM could be implemented as an additional attitude sensor to control the satellite orientation.