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    Joint Optimization of Server and Service Selection in Satellite-Terrestrial Integrated Edge Computing Networks

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    Game-Based Computation Offloading and Power Allocation for LEO Constellation Networks in Distributed and Dynamic Environment

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    Emergence of collective effects in complex plasmas (Entstehung von kollektiven Effekten in komplexen Plasmen)

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    This work is dedicated to studying the emergence of collective effects in ionised gases with micrometre sized particles immersed in them, also known as complex plasmas. These are often used as model systems to study a variety of emergent phenomena since the particles are large enough to be imaged directly. I present a theoretical model of the self-formation of droplets based on recent experiments per- formed in weightless complex plasmas. The model is based on balancing the ion drag force with electrostatic repulsion to explain the formation of stable droplets. It produces quantitative results that predict the size of a droplet as a function of the plasma parameters in agreement with the experimental observations. For the first time, this model causally connects the size of the droplet to plasma parameters (such as the electron temperature). Not only can the model predict what size and shape of droplets may form in the experiment given the parameters, but it can also determine the parameters from the size of the observed droplet. This allows the observation of droplets to be used as a diagnostic tool to determine plasma parameters in complex plasma experiments. Beyond this, I investigate additional collective fluid effects such as the formation of shocks and the onset of turbulence by simulating a flow of microparticles past a spherical obstacle in a complex plasma. This work is one of the first systematic particle-resolved investigations of turbulence, in which I demonstrate that the formation of shocks is important for the onset of turbulence in damped systems. By simulating a supersonic flow, I can reliably generate Mach cones and bow shocks both up- and downstream of the obstacle. I report the observation of double bow shocks in these simulations for the first time in complex plasmas, showing a similar structure as observed in astrophysical plasmas. In regions where particles flow directly into a shock, the increased microparticle density - and hence, strength of interactions - triggers the onset of turbulence. This link between increased microparticle density and the onset of turbulence in damped fluids is in agreement with previous complex plasma experiments. I report that the onset of turbulence in the simulations depends on parameters such as particle charge and flow speed. A non-turbulent simulation can be made turbulent by changing one of these two parameters. Both of these parameters can be controlled in experiments, allowing the simulations to predict and control the onset of turbulence in complex plasma experiments even under the influence of damping, opening the pathway towards detailed studies of the onset and control of turbulence at the level of individual particles. Finally, I study the onset of electrorheological effects through the formation of string-like clusters (SLCs) based on microgravity experiments. This process is led by the deformation of the ion shielding cloud around the particles due to an alternating ion flow. I mimic this in the simulations by placing a positive wake charge in-front of and behind the particle to modify the interparticle potential. By doing so, I can reproduce the formation, destruction, and recrystallisation of SLCs as seen in the experiments with qualitatively similar results. I test whether an effective long-range interparticle attraction is required to produce SLCs, and report that this is not the case. The excellent qualitative agreement between experiment and simulation is definitive proof that effective long-range interparticle attraction is not a necessity for electrorheological effects in complex plasmas. Overall, this thesis advances the knowledge of emergent phenomena in complex plasmas in a variety of conditions. As complex plasma experiments can resolve individual particle dynamics, this work can be used to inform future investigations of collective effects at the particle-resolved level

    Learning Variational Models with Unrolling and Bilevel Optimization

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    In this paper we consider the problem of learning variational models in the context of supervised learning via risk minimization. Our goal is to provide a deeper understanding of the two approaches of learning of variational models via bilevel optimization and via algorithm unrolling. The former considers the variational model as a lower level optimization problem below the risk minimization problem, while the latter replaces the lower level optimization problem by an algorithm that solves said problem approximately. Both approaches are used in practice, but unrolling is much simpler from a computational point of view. To analyze and compare the two approaches, we consider a simple toy model, and compute all risks and the respective estimators explicitly. We show that unrolling can be better than the bilevel optimization approach, but also that the performance of unrolling can depend significantly on further parameters, sometimes in unexpected ways: While the stepsize of the unrolled algorithm matters a lot (and learning the stepsize gives a significant improvement), the number of unrolled iterations plays a minor role

    Spaceborne Multiple-Swath SAR Imaging with Frequency Scanning

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    This paper presents innovative multiple-swath SAR imaging modes making use of the frequency scanning technique (F-Scan). A novel technique is proposed for high-resolution wide-swath imaging based on analogue beamforming, a less complex and inexpensive option compared to conventional digital beamforming systems. The separation of the swaths - both in the time and in the frequency domain - is taken into account. The performance is assessed for different scenarios, confirming the high performance and high flexibility that can be achieved with the proposed imaging modes. For instance, it is shown the possibility to image a contiguous 290 km swath with a resolution below 9 m2

    Moderne abbildende Radartechnologie zur Weltraumlageerfassung der Zukunft

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    Angesichts der extrem zunehmenden Zahl von Weltraumobjekten wird eine umfassende und qualitativ hochwertige Weltraumüberwachung immer wichtiger. Radar ist eine leistungsfähige und dafür bestens geeignet Technologie, die neben der Erkennung und Verfolgung von Objekten auch räumlich hochauflösende Bilder unabhängig von Tageslicht und den meisten Wetterbedingungen ermöglicht. In Verbindung mit der invers angewandten Technik des Radars mit synthetischer Apertur (ISAR – Inverse Synthetic Aperture Radar) können heute schon sehr hochauflösende und entfernungsunabhängige zweidimensionale Bilder gewonnen werden. Moderne und damit deutlich weiterentwickelte Radarabbildung von Weltraumobjekten ist jedoch eine komplexe und anspruchsvolle Aufgabe, die viele technologische und signalverarbeitende Fragen berührt. Daher hat das Institut für Hochfrequenztechnik und Radartechnik des Deutschen Zentrums für Luft- und Raumfahrt (DLR) neben theoretischen Arbeiten ein experimentelles Radarsystem mit der Bezeichnung IoSiS (Imaging of Satellites in Space) für die Grundlagenforschung zu neuen Konzepten zur Erfassung von hochauflösenden Radarbildprodukten von Objekten in einer niedrigen Erdumlaufbahn (LEO) entwickelt und aufgebaut. Auf der Grundlage der Pulsradartechnologie, die eine präzise Kalibrierung und Fehlerkorrektur ermöglicht, hat IoSiS bereits Weltraumobjekte mit einer räumlichen Auflösung im Zentimeterbereich abgebildet, was in der öffentlichen Wahrnehmung und in der zugänglichen Literatur neuartig ist. Zudem hat sich das DLR dem Forschungsgebiet „Responsive Space“ verschrieben, was sich intensiv mit Fragen der kurzfristigen Reaktion auf den Ausfall von Weltraumsystemen auseinandersetzt. Eine essentielle Komponente dabei ist eine leistungsstarke Radarsensorik, die z.B. vom Erdboden aus eine hochgenaue Charakterisierung des mechanischen Zustands von Weltraumobjekten ermöglicht, sowie Informationen über Art und Verhalten von unbekannten und ggf. bedrohlichen Objekten im Orbit liefern kann. Das DLR-Radarsystem IoSiS ist ein erster Meilenstein zur Erfüllung dieser Aufgabe, mittels dem die Erforschung und Entwicklung moderner und zukunftsweisender Monitoring-Konzepte auch experimentell unterstützt wird. Ein in der jüngeren Vergangenheit dazu ausgearbeitetes Konzept erlaubt z.B. die echt dreidimensionale als auch multi-statische Abbildung von Objekten, neben einer Vielzahl weiterer Vorzüge. Das System IoSiS sowie eindrucksvolle Messergebnisse und Simulationen zu zukünftigen Fähigkeiten werden präsentiert und diskutiert

    Forest Change Analysis by means of Pol-InSAR Measurements at L- and P-band

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    This work focuses on the detection and interpretation of forest structural changes by exploiting Pol-InSAR acquisitions. Following a two-layer model assumption, the response from the forest canopy can be decomposed into ground and volume layers. A polarimetric change analysis can be applied over the separated ground and volume scattering components acquired at different times. The analysis of the forest (structural) changes is carried out by exploiting L- and P-band data acquired by DLR’s F-SAR sensor over the Traunstein forest during the TMPSAR campaign. Results corroborate that the decomposition of the forest into simpler layers eases the interpretation of the changes

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