Pointing, Acquisition, and Tracking (PAT) for Optical Satellite Communications: Recent Trends and the ALIGN UK Mission’s PAT Mechanism

In this work, a novel pointing, acquisition, and tracking (PAT) model is proposed and developed based on a new custom-designed 1.5U Free-space Optical Communications Unit for Satellites (FOCUS) payload. FOCUS is expected to be demonstrated as part of the Autonomous Laser Inter-Satellite Gigabit Network (ALIGN) UK space mission, which aims to achieve a full-duplex optical inter-satellite link between two 6U CubeSats in the same LEO plane at data rates > 1 Gbps with the FOCUS performance tests conducted across link distances ranging up to 1000 km. The key original contributions of this paper are three-fold; (i) a review of the state-of-the-art PAT models implemented in optical satellite communication missions to date; (ii) the ALIGN and FOCUS concept-of-operations in context of PAT; and (iii) algorithm development and numerical analysis of the FOCUS PAT sequence. The proposed PAT model, leading from coarse to fine line-of-sight pointing, utilizes a combination of open and closed loop schemes based on CubeSat’s attitude determination and control system and mirror-based steering mechanisms, where the optical transmitter and receiver follow the scan-stare strategy to precisely locate each other within seconds in a field-of-uncertainty defined by both orbital and optical misalignment errors. ALIGN is funded by the UK Space Agency (UKSA) via the National Space Innovation Programme (NSIP) and it is anticipated to be launched in late 2027 or later.