Automated Design and Validation of Acquisition Patterns for Optical Inter-CubeSat Links

Abstract

Commercially available CubeSats with volumes of up to six units cannot achieve the precision required for an instantaneous establishment of a low-divergence optical inter-satellite link employing solely their attitude-determination and control system. Those residual attitude errors are present due to vibrations and limited control precision caused by the commonly used reaction wheel actuators. Thus, search patterns are used to scan the remaining field of uncertainty in order to achieve an optical inter-satellite. This work focuses on the development of an automated procedure to optimize the interaction between each of the individual search patterns. The performance of the two combined patterns is measured by their mean acquisition time and probability of success based on a Monte-Carlo simulation. Four patterns – Spiral, Rose, Lissajous and Grid – are considered and modified according to the optical inter-satellite link scenario between two CubeISL laser communication terminals. They are distinguished by their respective tasks within the acquisition scheme. The terminal for pointing, acquisition and tracking (T-PAT) scans the field of uncertainty in order to establish the link. The terminal for detection, adjustment and tracking (T-DAT) scans for a hit on its optical detector with a matched pattern period. It then gradually compensates for the remaining error until both terminals can switch to active tracking mode. The proposed acquisition scheme and generated patterns were verified in a campaign over 334 m link distance. To achieve a controllable test environment, both CubeISL terminals and attitude manipulation actuators were automated. This approach offers the advantage of repeatable parameter variations and a higher number of tests that can be carried out. Each run takes approximately 10 minutes, which emulates the envisaged runtime in space, including the configuration of the terminals and supporting equipment. Additionally, all configurations are executed multiple times to evaluate the standard deviation of individual tests. The presented procedure demonstrates that simulations can exclude a significant number of design parameter combinations. The remaining pattern sets are implemented for final optimization during a field-test with the actual hardware of the optical terminals. As both CubeSats will operate in space without real-time supervision, the same validation process can be applied during commissioning. Therefore, the proposed automated design and validation procedure reduces the time required for supervised measurement campaigns while increasing confidence in the reliability of the overall system for remote on-orbit operation.