A low-thrust finite state machine based controller for N-satellites formations in distributed synthetic aperture radar applications

Spacecraft formation flying has attracted notable interest in recent years, thanks to the properties of flexibility to different mission needs and enhanced robustness and capabilities compared to monolithic space systems. A key element for the full exploitation of this technology consists in the automation of formation management and control, allowing higher independence from ground stations as well as prompter reactions and higher autonomy in task-assignment and decision making. In this context, this work proposes a distributed architecture to autonomously coordinate and control formations containing any number of members applying low-thrust propulsion and employing finite state machines to make decisions and assign tasks to the elements of the formation based on their current state. The nominal relative trajectories (called slots) are represented within the architecture through a set of geometric parameters characterizing their stability, size, and shape. The architecture can autonomously maintain the spacecraft on a set of nominal slots, as well as reconfigure the formation to a newly assigned geometry and deal with contingency situations, performing collision avoidance manoeuvres and keeping track of the slots of the formation that remain empty to autonomously define a course of action allowing the evading spacecraft to re-join the formation safely. At the same time, a strategy for the identification of the geometrical conditions to be violated to efficiently perform formation maintenance is proposed. The architecture is tested by means of numerical simulations within a GMAT-MATLAB integrated environment considering a quasi-linear safe ellipse formation aimed at distributed synthetic aperture radar applications as reference test case. Simulation results demonstrate that the proposed architecture allows effective autonomous coordination within tight formations, with cm-level ΔV expenses, while being fully scalable with the number of satellites.