Precision laser beam pointing: The Role of Fine Steering Mirrors in the GRACE-FO and Beyond.

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, launched in 2018, is the first mission to carry a Laser Ranging Interferometer (LRI) to accurately measure distance variations between the twin satellites, enabling a more accurate understanding of Earth's gravitational field. One of the critical components of the LRI is the Fast Steering Mirror (FSM) with an electro-mechanical tip/tilt mirror, responsible for directing the laser beams accurately from one spacecraft to the other.

In this presentation, the general working principle of the FSM in LRI with its two nested control loops is discussed. Initially, during the acquisition phase, the FSM performs a fast spatial scan and, during the science phase, compensates for the much slower attitude variations of the satellite platform using a so-called  Differential Wavefront Sensing feedback[Mahrdt et al, Koch et al]. For future missions like NGGM and GRACE-C, it is crucial to address potential failure points associated with the FSM operation. Two failure points have been identified and are addressed along with possible strategies to mitigate them. One is the failure of the position sensor system(PSS) and the other is the failure of the FSM actuator coils. We established an FSM testbed in our laboratory and will show results from the characterisation of an FSM similar to the flight model. The PSS failure can be mitigated by disabling the PSS and inner control loop, which is primarily used to suppress the resonance oscillation of the FSM arising from the internal low-friction suspensions. Such an open loop operation involves commanding the FSM current but may require changes to the laser link acquisition strategies. Since the FSM utilizes two coils per axis, which are usually operated in parallel, e.g., hot-redundant, a coil failure can be mitigated by operating the FSM with individual coils per axis.