Optimized Pointing, Acquisition and Tracking Control for Over-Actuated CubeSat Laser Communication Terminals

Kurzfassung

The rapid expansion of small satellite constellations requires high-bandwidth, secure, and reliable optical links. However, strict size, weight, and power constraints severely limit the ability to achieve this objective with CubeSats. One critical bottleneck is the pointing, acquisition, and tracking (PAT) system. The limited accuracy of the satellite’s attitude determination and control system (ADCS), as well as the dynamic performance of miniaturized actuators, makes it challenging to avoid prolonged acquisition times and reduced link availability. Current solutions mostly rely on body-pointing isolated from the laser communication terminal (LCT), which limits utilization of the full potential of those over-actuated systems. This work is based on the principles of mechatronic system design, optimization, and control theory for over-actuated systems in the context of space-based free-space optical communication (FSOC). The considerations made explicitly account for the coupled dynamics of the satellite, optical payload, and environment. This includes range, bandwidth, and accuracy limitations, launch loads, micro-vibrations, and atmospheric disturbances. The research employed a multidisciplinary approach that combined simulation, hardware testing, and on-orbit validation. A Monte Carlo simulation framework was developed to optimize acquisition patterns by analyzing how actuator bandwidth, sensor dynamics, and attitude knowledge uncertainty affect mean acquisition time and success rate. The proof of concept was validated through an inter-island link campaign of 143 km and a ground-to-ground campaign of 334 m that emphasized automated test setups. Furthermore, a hardware-in-the-loop setup demonstrated how to propagate and feed-forward attitude knowledge from the ADCS to the LCT using dedicated gyroscopes. The structural dynamics of an LCT were analyzed using the finite element method (FEM), leading to design modifications, such as optimized PCB material and custom dampers, which were validated via vibration testing. A comparative study of two high-performance hybrid variable reluctance fast steering mirrors (FSMs) was conducted using identical test benches to evaluate their performance. Furthermore, a controller for a new coarse pointing assembly (CPA) design was implemented and could demonstrate pointing performance that allows operation without a dedicated fine pointing assembly in defined scenarios. A novel master attitude controller (MAC) was designed and manufactured, integrating the satellite’s ADCS with the LCT’s control system. The research yielded several significant results. First, implementing a dual-scan acquisition scheme that incorporated spiral and rose patterns significantly reduced the mean acquisition time to 1.66 seconds, while achieving a 99.9% success rate. This improvement is a substantial increase over conventional baseline methods. Second, implementing feed-forward compensation using an onboard gyroscope substantially reduced the field of uncertainty from an initial range of ±1 degree to a final value of 0.1 degrees. Third, FEM-based design optimization effectively isolated the FSM from launch vibrations. This optimization achieved a resonance separation factor of 1.28 and a 3σ tracking error of 72 µrad under worst-case micro-vibrations. Fourth, the hybrid-variable-reluctance actuation technology for FSMs was found to have superior steady-state jitter of 3.2 µrad and a broad operational range of ±1.5 deg while achieving high bandwidths up to 1.5 kHz. This makes it an ideal candidate for space-based FSOC, and with further miniaturization, a superior choice for CubeSat based applications. The developed CPA was demonstrated to maintain tracking error within 2 µrad accuracy for profiles within typical downlink dynamics. Regarding future generation LCTs, the MAC’s control allocation algorithm reduced peak-to-peak power consumption by 12.1% while maintaining acquisition and tracking performance of traditional approaches. Finally, an in-orbit demonstration of the full end-to-end chain—enabled by reliable PAT implementation—substantiated the viability of optical downlinks from a CubeSat. This work provides a comprehensive, high-performance PAT solution for CubeSats and small satellites, at the subsystem and system levels. This advancement is significant because it sets new standards for the state-of-the-art of acquisition optimization; demonstrates a practical method for improving attitude knowledge without additional optical feedback during acquisition, while taking third-party satellite buses into account; and introduces a new subsystem called MAC and its tailored control allocation algorithm, which fosters a paradigm shift, enabling the development of modular, maintainable, and highly efficient, multi-link satellite PAT systems.