Grasso, Marco (2021) DISTRIBUTED SAR SYSTEMS BASED ON CLUSTER OF COMPACT MODULAR SATELLITES FOR NEW EARTH OBSERVATION MISSIONS. [Tesi di dottorato]

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A space program targeted to rapid development and launch high-performance small satellites for Earth Observation (EO) represents a big challenge for the present and the next generation of satellite designers. Indeed, many EO scientific missions have very tight requirements which usually can be met only by traditional large satellites. Moreover, platform design is typically complex and time-consuming, with a strong relation to the specific mission scenario. The ability to start and complete an affordable space program based on small satellites in short timeframe, with performance capabilities suitable for high-demanding scientific missions, is linked to the exploitation of some promising paradigms, introduced in last decades. These paradigms are distributed mission configurations enabling advanced operational modes, fleet of compact satellites flying in formation, and modular design for new satellite development techniques. This PhD thesis aims to analyze the paradigms in question by focusing on different concepts for the design of distributed Synthetic Aperture Radar (DSAR) systems based on modular small satellites flying in close formation. In a first step, a literature review on distributed SAR systems, small satellites, and spacecraft modularity has been conducted. This is intended to define the state-of-the-art and the current technological capabilities. Subsequently, different applications enabled by the distribution of SAR payload are collected and described. Procedures to select system parameters (e.g., the number of platforms and suitable formation configurations) are also given. To demonstrate distributed concepts, several scenarios have been designed, simulated, and tested. Each scenario involves a specific space architecture with different mission configurations to evaluate the performance enhancements achievable by exploiting DSAR techniques. Afterwards, a DSAR simulator is introduced to evaluate the effects of different error sources on system performances. These error sources are modelled and applied during the processing phase, enabling the estimation of the effects in terms of bistatic SAR focusing and high level DSAR processing. Critical design aspects related to spacecraft subsystems dictated by formation-flying and SAR operations are addressed to confirm the technical feasibility of the spaceborne distributed system based on compact satellites. Additionally, technological aspects and space components related to satellite budgets are outlined, in order to match mission requirements with either space-qualified or in-development hardware, feasible for small platforms. Finally, methodologies and concepts for the development of a modular satellite architecture have been investigated in this thesis. These concepts were analyzed to identify the problems that arise by designing a traditional satellite subsystem in a modular fashion. The focus of this work is specifically on the Attitude Determination and Control Subsystem (ADCS) considered as a reference point for the design of the entire modular platform since it is strongly coupled with other subsystems. In this context, a solution for abstracting the attitude estimation algorithms from the actual hardware is discussed. The proposed algorithm is a generic version of the Murrell’s Multiplicative Extended Kalman Filter (MMEKF) used to perform attitude estimation for different test cases with different pointing modalities and accuracies, different spacecraft and hardware configurations, without any modifications from case to case. In summary, this thesis analyses distributed SAR systems in several respects evaluating the performances of different mission scenarios, achievable by exploiting new operational modes, and the feasibility of using small satellites as the reference platforms for the missions. In addition, a preliminary investigation about spacecraft modularity is performed to develop a generic and reusable method for attitude estimation.

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