Vela, Claudio (2024) Visual-based navigation, guidance and control solutions for autonomous multi-satellite operations in near-Earth and cislunar environments. [Tesi di dottorato]

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Abstract

The recent years have seen a progressive commercialization of space activities and an increase in the use of small platforms. In fact, standardization of spacecraft components and design schemes (such as the CubeSat standard) have resulted in the reduction of satellite weight and size and have reduced their production costs. In turn, the availability of smaller, readily deployable platforms has fostered the employment of distributed space systems (such as formations) compared to monolithic platforms to achieve mission goals, given their higher flexibility and reliability. However, the possibility for many to access small and standardized platforms has increased the number of satellites in the near-Earth environment, fostering the necessity of services such as on-orbit maintenance and refueling and active debris removal to guarantee a sustainable use of space. A similar trend can be expected in the cislunar environment as well, as a result of the increasing interest in this region has a proving ground and a bridgehead for the future exploration of the Solar system. In this context, this thesis presents novel methodologies for guidance, navigation, and control (GNC) methods to enable autonomous formation flying missions and close-proximity operations (CPOs). The first contribution of the thesis focuses on proposing pose estimation methods for visual-based relative navigation systems, focusing on CPOs towards semi-cooperative spacecraft (i.e., platforms equipped with fiducial markers to help rendezvous and docking or berthing operations). Specifically, the proposed methodologies consider the use of various types of fiducial markers, including retroreflective and code-based ones, to be employed both in the visible and near-infrared (NIR) bandwidth of the electromagnetic spectrum. The cases of interest include semi-collaborative spacecraft, assisting the CPOs by coarsely controlling their attitude, as well as freely tumbling targets. Elements of innovation include the proposal of segmentation techniques based on the use of range-adaptive processing parameters as well as the use of color information for a more robust detection of the features in the image plane. While the first two proposed solutions are investigated using numerical simulations with synthetically produced images to assess their performance, the latter involves the design of a relative positioning module dedicated to CubeSat platforms, using a combination of a monocular camera and a laser range finder (LRF). The associated pose estimation pipeline is designed as well: notably, the system employs the LRF as both a range finder unit and a means of illuminating the target to allow a more robust detection of the markers. The performance of the module and its pose estimation pipeline are then assessed through hardware-in-the-loop tests, conducted in an appropriately designed experimental testbed. The second contribution of this thesis focuses on the one hand on the design and control of a cluster of CubeSats for Earth Observation flying in formation with an illuminator of opportunity. As the latter performs its own drag make-up maneuvers independently, the cluster is required to maintain a desired baseline in the along-track direction with respect to the illuminator as well as the geometry between the CubeSats. Together with methods for the maintenance of the formation, a deployment strategy that solely relies on differential drag effects to reach the desired orbit is developed, using an analytical model of the drifting motion of the deployed CubeSats in the along-track direction. On the other hand, a trajectory tracking state-feedback controller based on a proportional-derivative scheme is proposed for the improvement of the distributed N-Satellites Formation Flying Controller. The controller is tuned and tested in a reference scenario and the effectiveness of the solution is verified. The last contribution of the thesis introduces methodologies for the execution of CPOs in the cislunar environment. First, the equations of relative motion are introduced in a velocity reference frame, featuring the velocity vector of the target spacecraft as one of the primary axes of the frame, in the framework of the circular restricted three-body problem. Then, a modal decomposition approach is applied to the equations through the Lyapunov-Floquet transformation to express the solution to the relative motion problem as a weighted sum of the fundamental modes of motion. The coefficients of this weighted sum are constants of the motion: as such, they can be effectively employed as relative orbital elements for motion description and can be related to relevant geometrical quantities of the motion for relative trajectory design. A variational equation for the constants is finally defined and employed in GNC methods, proposing a reformulation of existing methods for impulsive and finite-burn guidance and control of the relative motion, as well as for the development of an innovation-based adaptive Kalman filter relying on position-only measurements. Finally, the methodologies are combined in a GNC system and tested numerically in a scenario of interest.

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