# Steering Mirror System with Closed-Loop Feedback for Free-Space Optical Communication Terminals
## 1 Introduction  
Free-Space Optical (FSO) communication links from Low-Earth Orbit (LEO) to the ground, as well as inter-satellite links (LEO to LEO) or LEO to Geostationary Earth Orbit (GEO), have been globally developed by major space agencies. LEO satellites, known for their decreased latency and capacity for improved bandwidth, present a promising option for delivering global internet access, especially in remote and underserved areas when compared to GEO satellites. This has resulted in increased interest from academia and industry [1]. However, implementing FSO links within LEO satellite constellations poses unique challenges not encountered by their counterparts. The challenges of implementing FSO links within LEO satellite constellations stem from their unique orbit characteristics. LEO satellites, moving at faster speeds (7-11 km/s) than GEO satellites, face shorter communication windows, necessitating frequent handovers and increasing the risk of disruptions. Unlike GEO satellites, LEO satellites experience drag from the Earth's atmosphere, requiring frequent orbital adjustments that can disrupt communication links and add complexity. Pointing, Acquisition, and Tracking (PAT) are critical aspects of FSO systems, ensuring the precise targeting of the optical signal from the transmitter to the receiver. PAT systems usually include a steering and tracking capability [2] to establish and maintain the link, as well as to compensate for the satellite's motion, atmospheric turbulence, vibration, and jitters. Recent flight demonstrations have primarily focused on LEO-to-ground communication. There is an increasing demand for higher data rates for near real-time downloading of earth observation data. The first successful reported demonstration occurred in 2010 with a pioneer LEO-to-ground bidirectional link between the NFIRE satellite and a commercial laser communication terminal jointly developed by the US and Germany [3]. Subsequent achievements include the Small Optical TrAnsponder (SOTA) onboard the SOCRATES satellite in 2014 [4], showcasing stable links over an extended period. In both NFIRE and SOTA cases, the optical head was mounted on a gimbal with four transmitter channels and a single receiver. A significant step toward miniaturization was demonstrated with the OC4 instrument on board the PIXL-1 satellite in 2021 [5]. This compact system successfully demonstrated a downlink, incorporating a laser communication terminal with a Fine Pointing Assembly (FPA) consisting of a 4-Quadrant Diode and a Fast Steering Mirror (FSM) to compensate for satellite pointing variation. NASA's AeroCube/B and 7C [6], both part of the CubeSat Optical Communications and Sensors Demonstration (OCSD) program [7], followed by the Laser Infrared CrosslinK Mission Click-A system [8] in 2017 and 2022, have also achieved LEO-to-ground links. While the AeroCube/ optical head did not include any beam steering capability or gimbal mechanism, relying on a high-performance star tracker for attitude control, and larger beam divergence for link stability, the more recent Click-A incorporated an FPA similar to the OC4 instrument for closed-loop control of the pointing system. Another notable miniaturized system for LEO-to-ground communication is CubeCAT, featuring the optical head developed by TNO [9]. CubeCAT also incorporates an FPA similar to both CLICK-A and OC4. Most of these groups are also developing Inter-Satellite Link (ISL) terminals for CubeSats. While for LEO-to-ground links, the telescope's aperture on the ground can be as large as needed (typically ranging from 100 mm to 600 mm) to maximize signal to noise for restricted laser power onboard the satellite, ISL telescope apertures are limited by the satellite platform dimensions. DLR is working on CubeISL, an ISL version of the OC4 [5]. With Click-B and -C, NASA plans to establish bidirectional ISL between two 6U CubeSats in the same orbit [10].