A compact pointing assembly for a CubeSat to ground station free-space optical communication based on a Risley prism scanner coarse and a tip/tilt mirror fine pointing

The usage of CubeSat platforms has seen a significant increase over the last decade. CubeSats are compact and cost-efficient. With it the need for free-space optical communication (FSOC) between satellites and optical ground stations did increase as well. Achieving a good ratio between the size and the performance of the optical payload is still a challenge. We propose a FSOC terminal to be implemented on a 16U CubeSat platform. A combination of a coarse pointing assembly (CPA) based on a dual Risley prism scanner in front of an all-metal freeform telescope, and a fine pointing assembly (FPA) with a fast-steering mirror (FSM) were developed. The Risley prisms have a smaller mechanical envelope compared to classical gimbal-based mirror mounts or periscopes but can still provide a suitable range of beam deflection. The use of such a Risley prism scanner has been a research topic in aerospace for quite some time. Especially the challenging driving and controlling of the nonlinear beam pointing behavior presents a challenge. To keep the payload on the CubeSat feasibly small, only a microcontroller with limited calculation power is used. Therefore, we propose a combined control scheme for the CPA and FPA based on simplified calculations and the use of classical digital control theory. Coarse and fine pointing are controlled in a closed loop pointing simultaneously.