A digital twin of an air-bearing platform for tethered satellite systems: from tether deployment to post-deployment control

Tethered Satellite Systems represent a promising technology for a wide range of space applications, including orbital maneuvering, de-orbiting operations, and in-orbit servicing. These systems enable novel mission concepts that can reduce propellant consumption and mission costs, while also supporting sustainable space operations. Despite their potential, TSS are characterized by complex dynamics and critical challenges, both during the deployment phase and once the tether has already been deployed: these aspects make both their modeling and experimental validation particularly demanding. Given the high costs associated with space missions, ground testing has always represented a crucial aspect of every project. Nowadays, however, a new trend is the development of digital twins, namely digital models of experimental setups, which make it possible to further reduce costs and to carry out an unlimited number of tests. To address this challenge, this thesis presents the development of a digital twin of an air-bearing platform for tethered satellite systems, implemented in Unity™. Of primary importance was, first and foremost, the analysis of the existing literature on tethered satellite systems, in order to identify—also with the support of tools such as AHP analysis—the optimal architecture capable of ensuring safe deployment and maximizing the expected lifetime of the tether in the hostile environment of Low Earth Orbit (LEO). With the aim of developing a digital twin that would also enable a 3D visualization of the implemented system, three graphical modeling software were examined, and the most suitable one for our requirements was selected. After the architectural design was defined, attention was devoted to the problem of scaling the real system to the dimensions of the experimental setup. Subsequently, the implementation of the air-bearing platform in Unity™ was undertaken. Two scenarios were developed: one in which the tether is already deployed, and one concerning the deployment phase. In the first case, two position control algorithms (PID and LQR) and a PID controller for tether tension were designed and tested, while in the second case a control strategy was implemented to halt the deployment once the desired tether length had been reached, along with a PID tension control algorithm that was likewise developed and validated. It’s important to note that the scenario with the tether already deployed was addressed first. This allowed for the immediate testing of the modeling approaches used in Unity™. Only after verifying their correctness was the deployment case developed. This case required more complex modeling due to the helical winding of the tether around the spool. Finally, based on the results obtained, it can be stated that the objective of developing a digital twin of an air-bearing platform—capable of simulating tether deployment, analyzing post-deployment dynamics, and validating control strategies—has been successfully achieved. Potential future developments may include the integration of sensors, the inclusion of force exchange and friction in the tether–spool interaction, and a more accurate modeling of the spool and floater geometries.