@phdthesis{,
    author = {Buinhas, Luisa dos Santos},
    title = {Spacecraft Formation Flying Dynamics at L2 and Operational Maneuvers Design : The case of the IRASSI Interferometer},
    editor = {},
    booktitle = {},
    series = {},
    journal = {},
    address = {},
    publisher = {},
    edition = {},
    year = {2021},
    isbn = {},
    volume = {},
    number =  {},
    pages = {},
    url = {},
    doi = {},
    keywords = {Satellite; Interferometry; Optimization; Maneuver; Formation Dynamics; Formation Geometry},
    abstract = {Formation flying and autonomy are two concepts that have increasingly become integral to space missions. Employing multiple spacecraft for achieving a common goal instead of a monolithic probe has the benefit of increasing mission robustness and flexibility. Onboard autonomy, on the other hand, confers a degree of intelligence and decision-making ability to the spacecraft. In turn, this allows overcoming communication delays and outages to ground stations, reduces ground segment availability demands and costs, and allows spacecraft to react in an unpredictable environment, maximizing the scientific potential of a mission. One such mission to whom the concept of autonomy is relevant is the IRASSI mission, a free-flying interferometer designed for the observation of far-infrared phenomena in deep space. Built on precursor mission studies, IRASSI distinguishes itself by exploiting a unique operational concept that involves a continuous change of the physical separation of the spacecraft during observations. Furthermore, during its lifetime, IRASSI has to conduct a set of activities that support the science program. That includes, for example, the reconfiguration of the formation. These maneuvers need to both be safe and fuel-economical and respect all science requirements, without the intervention of ground personnel. As a planned state-of-the-art facility, IRASSI brings simultaneously to the foreground unique challenges. As such, this thesis investigates the feasibility of the IRASSI mission by aggregating mission requirements into the formation-flying design and through the development of algorithms that simulate the operation of the formation as an independent entity. A sequential approach underpins this research work. First, the IRASSI mission is introduced and the major high-level requirements are described. In connection to the wavelengths observed in the far-infrared, the spacecraft are to separate up to 850 m in a direction perpendicular to the target. Sensitive payload motivates the selection of a quasi-Halo orbit around the second Lagrange point of the Sun-Earth/Moon system, L2. The ranging system is one of the payload instruments that warrants referral. It must operate at cold temperatures and is mounted opposite to the sunshield on each spacecraft. This ranging system permits the spacecraft to measure their relative separation during the science observations. The mission profile and its main phases are characterized, whereby an optimized transfer trajectory to L2 with a duration of up to four months is succeeded by a five-year operational phase. Each science task can have a duration ranging between a couple of hours and over 8 days. ΔV budgets and a bespoke mechanical configuration set a preliminary individual spacecraft wet mass at 2.3 tons, of which nearly 5% is dedicated to fuel. Thereafter, a bottom-up investigation of formation geometry requirements is pursued. Of particular interest, is the intervisibility requirement that stipulates that during the science phase the lasers of the ranging system must be within sight of one another. It is shown that the mechanical mounting of the ranging system severely limits where spacecraft can be positioned relative to each other. Other requirements include the relative positioning performance, sky access and science performance. The requirements are evaluated in an ephemeris around L2, without the use of any thrust control. Even with passive drift magnitudes of up to 450 m and drift rates of 18 mm/s for large formations, compliance with the requirements was observed at the end of the 8 days. The first violations were detected at the two-week mark, keeping in mind that this is well outside the range of science-task durations. During the operational phase, a behavioral-type coordination control was elected as that which is most suitable for supporting autonomous operations for IRASSI. Furthermore, simple control feedback loops are suggested for implementation in face of the slow dynamics of perturbations acting in the vicinity of L2. The design and implementation of a task- and maneuver-planning tool compose the last part of this research. The tool, called iSCOUT, is supported by four modules with the joint aim of optimally planning the sequence of science tasks, computing the associated reconfiguration maneuvers and the paths to be followed by individual spacecraft during the science observations, while simultaneously delivering collision-free trajectories which ensure the intervisibility of the spacecraft. The modules are the Task-Planner Module, the Reconfiguration Module, the Collision- and Invisibility-Avoidance Module and the Baseline Pattern Module. Maneuvers are optimized, among others, on the basis of maneuver-time minimization and fuel management (including fuel consumption and fuel-balancing across the fleet). As such, iSCOUT aggregates the complete set of mission, formation geometry and functional requirements. iSCOUT is ultimately intended to be executed onboard the spacecraft and support autonomous operations of the IRASSI interferometer. In its current standing, it serves well as a feasibility analysis tool that answers the goals of this research. As such, a comprehensive end-to-end mission simulation was carried out, where it was shown that 79% of observation tasks currently contained in the IRASSI catalog can be fulfilled, at a cost of 5.1~kg of fuel per spacecraft, after five years. Reconfigurations motivate nearly half of all fuel consumption. Over three-quarters of the mission can be dedicated to actual science observation, and the remaining time is allocated to maneuvers and activities, such as calibration of sensitive instruments. The conclusion follows, therefore, that even with the inclusion of the technical challenges of IRASSI, the mission can be successfully completed in the specified time frame. Nonetheless, the preliminary nature of the tool leaves room for future improvements, namely concerning the completeness of motion models and of operational scenarios that can emulate off-nominal conditions, such as hardware malfunctions. The plasticity of the tool is ensured, for example, by the implementation of common functions and features that are used across different modules. Advancing iSCOUT can refine the obtained results and close the gap between a feasibility study and a high-fidelity mission simulation tool, agnostic to the type of application. Overall, the present work showcases the complexities of operating a free-flying interferometry formation in space, whereby requirements of diverse origins (quality of science, the geometry of the spacecraft, visibility of metrology systems, target occultation, collision avoidance, etc.) must be taken into account in concert to deliver a unified optimal operational solution which fulfills all the mission's science goals. The understanding by mission planners of the impact of individual constraints and requirements on performance is particularly imperative nowadays, with the advent of mega-constellation satellite programs. The research and implementation work carried out in this thesis contributes to the field of autonomous spacecraft formation flying on several levels. First, it specifically addresses and analyzes the influence of spacecraft design on day-to-day mission operations. Secondly, the detailed requirements analysis behind the Task-Planner Module and the Baseline Pattern Module provides special insight into the complexity of task-scheduling and observation-planning for astronomy missions. For both modules, optimization methods aimed at autonomously maximizing science performance are presented. The Baseline Pattern Module is of special importance as it reflects the first steps in understanding how formation configuration in physical space affects the quality of the science data obtained. Lastly, optimized maneuver-planning strategies that manage onboard resources are devised for producing safe trajectories which abide by mission-specific constraints, such as intervisibility.},
    note = {},
    
    school = {Universität der Bundeswehr München},
    
}