L'autonomie est l'une des préoccupations majeures lors du développement de missions spatiales que l'objectif soit scientifique (exploration interplanétaire, observations, etc) ou commercial (service en orbite). Pour le rendez-vous spatial, cette autonomie dépend de la capacité embarquée de contrôle du mouvement relatif entre deux véhicules spatiaux. Dans le contexte du service aux satellites (dépannage, remplissage additionnel d'ergols, correction d'orbite, désorbitation en fin de vie, etc), la faisabilité de telles missions est aussi fortement liée à la capacité des algorithmes de guidage et contrôle à prendre en compte l'ensemble des contraintes opérationnelles (par exemple, saturation des propulseurs ou restrictions sur le positionnement relatif entre les véhicules) tout en maximisant la durée de vie du véhicule (minimisation de la consommation d'ergols). La littérature montre que ce problème a été étudié intensément depuis le début des années 2000. Les algorithmes proposés ne sont pas tout à fait satisfaisants. Quelques approches, par exemple, dégradent les contraintes afin de pouvoir fonder l'algorithme de contrôle sur un problème d'optimisation efficace. D'autres méthodes, si elles prennent en compte l'ensemble du problème, se montrent trop lourdes pour être embarquées sur de véritables calculateurs existants dans les vaisseaux spatiaux. Le principal objectif de cette thèse est le développement de nouveaux algorithmes efficaces et validés pour le guidage et le contrôle impulsif des engins spatiaux dans le contexte des phases dites de "hovering" du rendez-vous orbital, i.e. les étapes dans lesquelles un vaisseau secondaire doit maintenir sa position à l'intérieur d'une zone délimitée de l'espace relativement à un autre vaisseau principal. La première contribution présentée dans ce manuscrit utilise une nouvelle formulation mathématique des contraintes d'espace pour le mouvement relatif entre vaisseaux spatiaux pour la conception d'algorithmes de contrôle ayant un traitement calculatoire plus efficace comparativement aux approches traditionnelles. La deuxième et principale contribution est une stratégie de contrôle prédictif qui assure la convergence des trajectoires relatives vers la zone de "hovering", même en présence de perturbations ou de saturation des actionneurs. [...] / Autonomy is one of the major concerns during the planning of a space mission, whether its objective is scientific (interplanetary exploration, observations, etc.) or commercial (service in orbit). For space rendezvous, this autonomy depends on the on-board capacity of controlling the relative movement between two spacecraft. In the context of satellite servicing (troubleshooting, propellant refueling, orbit correction, end-of-life deorbit, etc.), the feasibility of such missions is also strongly linked to the ability of the guidance and control algorithms to account for all operational constraints (for example, thruster saturation or restrictions on the relative positioning between the vehicles) while maximizing the life of the vehicle (minimizing propellant consumption). The literature shows that this problem has been intensively studied since the early 2000s. However, the proposed algorithms are not entirely satisfactory. Some approaches, for example, degrade the constraints in order to be able to base the control algorithm on an efficient optimization problem. Other methods accounting for the whole set of constraints of the problem are too cumbersome to be embedded on real computers existing in the spaceships. The main object of this thesis is the development of new efficient and validated algorithms for the impulsive guidance and control of spacecraft in the context of the so-called "hovering" phases of the orbital rendezvous, i.e. the stages in which a secondary vessel must maintain its position within a bounded area of space relatively to another main vessel. The first contribution presented in this manuscript uses a new mathematical formulation of the space constraints for the relative motion between spacecraft for the design of control algorithms with more efficient computational processing compared to traditional approaches. The second and main contribution is a predictive control strategy that has been formally demonstrated to ensure the convergence of relative trajectories towards the "hovering" zone, even in the presence of disturbances or saturation of the actuators.[...]

The Auroral Acceleration Region (AAR) is a key region in understanding the interactionbetween the Magnetosphere and Ionosphere. To understand the physical, spatial, and temporal features of the region, multi-point measurements are required. Distributed small-satellite missions such as constellations of multiple nano satellites (for example multi-unit CubeSats) would enable such type of measurements. The capabilities of such a mission will highly depend on the number of satellites - one reason that makes low-cost platforms like CubeSats a very promising choice. In a previous study, the state-of-the-art of miniaturized payloads for AAR measurements was analyzed and evaluated on the capabilities of different multi-CubeSat configurations equipped with such payloads in addressing different open questions in AAR. This thesis will provide the mission analysis of such a multi-CubeSat mission to the AAR and possible mission design. This includes defining the mission scenario and associated requirements, developing a mathematical description of AAR that allows for specific regions in space to be targeted, an optimisation process for designing orbits targeting these regions, conversion of a satellite formation to appropriate orbits, verifying the scientific performance of this formation and the various costs associated with entering, maintaining, and exiting these orbits.

No description available.

The Global Positioning System (GPS) satellite signals provide not only traditional radionavigation service but inexpensive and convenient radio beacons for signal propagation studies on ionosphere and atmosphere. This thesis describes the development and testing of a specialized GPS sensor which measures, plots and records real-time high-resolution L1 (1575.42MHz) GPS signal strength at a data rate of up to 10Hz. The instrument is based on an open architecture GPS receiver development kit that can be modified and rebuilt. The signal strength is defined as mean-square signal strength in the thesis. The coarse/acquisition code (C/A-code) correlation is applied and the raw correlation data from a GPS correlator chip is obtained to calculate the signal strength. The gain variation of the automatic gain control (AGC) in the GPS signal link is considered, and a model is designed and implemented in data post-processing to reduce the AGC distortion to GPS signal strength measurements. Speed limitation of 1,000 knots and height limitation of 60,000 feet are removed so that it can track spacecraft such as low earth orbit (LEO) satellite. Four testing plans are developed and conducted to test the GPS signal strength sensor. A GPS simulator is used and the testing results prove that the space-borne sensor is fully operational and the signal strength resolution can be smaller than 0.05dB. Additionally, a COM-port-to-TCP/IP GPS simulation remote control gateway is designed and implemented for the senor and the GPS simulator to conduct formation flying. A graphic user interface (GUI) program is also built to retrieve data from a commercial high-performance space-borne GPS receiver for comparison. A Red Hat Linux signal strength sensor based on the National Aeronautics and Space Administration (NASA) PiVoT GPS receiver is achieved by modifications. The NASA PiVoT sensor, working together with the former signal strength sensor and the commercial space-borne GPS receiver, will strengthen our academic research strength in the studies on the ionospheric and atmospheric effects and irregularities which cause GPS signal degradation and scintillations. / Master of Science

This report summarizes the work conducted during a six-month internship at Lawrence Berkeley National Laboratory. The main objective was to implement the interactions between particles and surfaces in the particle-in-cell code WarpX. This work specifically addressed the scenario where particles collide with embedded surfaces, which can potentially result in the emission of secondary particles. The implementation represents a significant advancement in the development of WarpX, enabling users to include interactions such as secondary particle emission and reflection in their simulations. Furthermore, this work also offered initial insights into the spacecraft charging problem and other scenarios involving particle-surface interactions. / Denna rapport sammanfattar det arbete som utförts under en sex månader lång praktikperiod vid Lawrence Berkeley National Laboratory. Huvudsyftet var att implementera interaktionen mellan partiklar och ytor i partikel-i-cell-koden WarpX. Detta arbete behandlade specifikt scenariot där partiklar kolliderar med inbäddade ytor, vilket potentiellt kan leda till utsläpp av sekundära partiklar. Implementeringen utgör ett betydande framsteg i utvecklingen av WarpX, vilket gör det möjligt för användare att inkludera interaktioner som emission och reflektion av sekundära partiklar i sina simuleringar. Dessutom gav detta arbete också inledande insikter i rymdfarkostens laddningsproblem och andra scenarier som involverar interaktioner mellan partiklar och ytor.

Wave propagation is a phenomenon resulting from high transient loadings where the duration of the load is in µ seconds range. In aerospace and space craft industries it is important to gain knowledge about the high frequency characteristics as it aids in structural health monitoring, wave transmission/attenuation for vibration and noise level reduction. The wave propagation problem can be approached by the conventional Finite Element Method(FEM); but at higher frequencies, the wavelengths being small, the size of the finite element is reduced to capture the response behavior accurately and thus increasing the number of equations to be solved, leading to high computational costs. On the other hand such problems are handled in the frequency domain using Fourier transforms and one such method is the Spectral Finite Element Method(SFEM). This method is introduced first by Doyle ,for isotropic case and later popularized in developing specific purpose elements for structural diagnostics for inhomogeneous materials, by Gopalakrishnan. The general approach in this method is that the partial differential wave equations are reduced to a set of ordinary differential equations(ODEs) by transforming these equations to another space(transformed domain, say Fourier domain). The reduced ODEs are usually solved exactly, the solution of which gives the dynamic shape functions. The interpolating functions used here are exact solution of the governing differential equations and hence, the exact elemental dynamic stiffness matrix is derived. Thus, in the absence of any discontinuities, one element is sufficient to model 1-D waveguide of any length. This elemental stiffness matrix can be assembled to obtain the global matrix as in FEM, but in the transformed space. Thus after obtaining the solution, the original domain responses are obtained using the inverse transform. Both the above mentioned manuscripts present the Fourier transform based spectral finite element (FSFE), which has the inherent aliasing problem that is persistent in the application of the Fourier series/Fourier transforms. This is alleviated by using an additional throw-off element and/or introducing slight damping in to the system. More recently wave let transform based spectral finite element(WSFE) has been formulated which alleviated the aliasing problem; but has a limitation in obtaining the frequency characteristics, like the group speeds are accurate only up-to certain fraction of the Nyquist(central frequency). Currently in this thesis Laplace transform based spectral finite elements(LSFE) are developed for sandwich members. The advantages and limitations of the use of different transforms in the spectral finite element framework is presented in detail in Chapter-1. Sandwich structures are used in the space craft industry due to higher stiffness to weight ratio. Many issues considered in the design and analysis of sandwich structures are discussed in the well known books(by Zenkert, Beitzer). Typically the main load bearing structures are modeled as beam sand plates. Plate structures with kh<1 is analysed based on the Kirch off plate theory/Classical Plate Theory(CPT) and when the bending wavelength is small compared to the plate thickness, the effect of shear deformation and rotary inertia needs to be included where, k is the wave number and h is the thickness of the plate. Many works regarding the wave propagation in sandwich structures has been published in the past literature for wave propagation in infinite sandwich structure and giving the complete description of dispersion relation with no restriction on frequency and wavelength. More recently exact analytical solution or simply supported sandwich plate has been derived. Also it is seen by comparison of dispersion curves obtained with exact (3D formulation of theory of elasticity) and simplified theories (2D formulation as generalization of Timoshenko theory) made on infinite domain and concluded that the simplified theory can be reliably used to assess the waveguide properties of sandwich plate in the frequency range of interest. In order to approach the problems with finite domain and their implementation in the use of general purpose code; finite degrees of freedom is enforced. The concept of displacement based theories provides the flexibility in assuming different kinematic deformations to approach these problems. Many of the displacement based theories incorporate the Equivalent Single Layer(ESL) approach and these can capture the global behavior with relative ease. Chapter-2 presents the Laplace spectral finite element for thick beams based on the First order Shear Deformation Theory (FSDT). Here the effect of different choices of the real part of the Laplace variable is demonstrated. It is shown that the real part of the Laplace variable acts as a numerical damping factor. The spectrum and dispersion relations are obtained and the use of these relations are demonstrated by an example. Here, for sandwich members based on FSDT, an appropriate choice of the correction factor ,which arises due to the inconsistency between the kinematic hypothesis and the desired accuracy is presented. Finally the response obtained by the use of the element is validated with experimental results. For high shock loading cases, the core flexibility induces local effects which are very predominant and this can lead to debonding of face sheets. The ESL theories mentioned above cannot capture these effects due to the computation of equivalent through the thickness section properties. Thus, higher order theories such as the layer-wise theories are required to capture the local behaviour. One such theory for sandwich panels is the Higher order Sandwich Plate theory (HSaPT). Here, the in-plane stress in the core has been neglected; but gives a good approximation for sandwich construction with soft cores. Including the axial inertial terms of the core will not yield constant shear stress distribution through the height of the core and hence more recently the Extended Higher order Sandwich Plate theory (EHSaPT) is proposed. The LSFE based on this theory has been formulated and is presented in Chapter-4. Detailed 3D orthotropic properties of typical sandwich construction is considered and the core compressibility effect of local behavior due to high shock loading is clearly brought out. As detailed local behavior is sought the degrees of freedom per element is high and the specific need for such theory as compared with the ESL theories is discussed. Chapter-4 presents the spectral finite element for plates based on FSDT. Here, multi-transform method is used to solve the partial differential equations of the plate. The effect of shear deformation is brought out in the spectrum and dispersion relations plots. Response results obtained by the formulated element is compared and validated with many different experimental results. Generally structures are built-up by connecting many different sub-structures. These connecting members, called joints play a very important role in the wave transmission/attenuation. Usually these joints are modeled as rigid joints; but in reality these are flexible and exhibits non-linear characteristics and offer high damping to the energy flow in the connected structures. Chapter-5 presents the attenuation and transmission of wave energy using the power flow approach for rigid joints for different configurations. Later, flexible spectral joint model is developed and the transmission/attenuation across the flexible joints is studied. The thesis ends with conclusion and highlighting futures cope based on the developments reported in this thesis.

In July 2031, a new chapter in the exploration of the Jovian system will begin with the arrival of the Jupiter Icy Moons Explorer (Juice) at Jupiter. Launched on April 14 2024 as part of ESA’s Cosmic Vision programme, the mission aims to study Jupiter and its icy Galilean moons Callisto, Europa, and Ganymede. Juice carries a whole suite of instruments for in-situ and remote ground observations, one of which is the Jovian plasma Dynamics and Composition analyser (JDC). As a part of the Particle Environment Package (PEP), the particle detector will measure the energy, mass, charge and arrival direction of ions and electrons in the Jovian magnetosphere. Spacecraft charging caused by interactions between the spacecraft and its surrounding plasma environment poses a significant problem for JDC because the electrostatic potential of the spacecraft accelerates/decelerates charged particles, resulting in distorted measurements, particularly for the lowest energy particles.  In this report, we show the results of spacecraft charging and instrument simulations performed in the Spacecraft Plasma Interaction System (SPIS) for the solar wind and Earth’s magnetosheath—two environments that Juice will encounter at the start of the cruise phase. We found that the conductive surfaces that cover the majority of the spacecraft become positively charged as a result of a large photoelectron current in both the solar wind and magnetosheath environments. We show that these surfaces are expected to reach potentials of 9 V in the solar wind and 4 V in the magnetosheath. The four radiators on Juice that are covered with dielectric paint and shaded by the sun shield become negatively charged in both simulated environments. The radiator potentials can be as low as -40 V in the solar wind and -100 V in the magnetosheath. We also conclude that due to blocking by the spacecraft main body, the ion population cannot be sampled in the solar wind unless a spacecraft roll is performed. Furthermore, due to the high ion f low energy, spacecraft charging will not influence JDC measurements in this environment.  In the magnetosheath, the ion population can be sampled by JDC, and we identified three distortion mechanisms: (1) repulsion by the main body, (2) attraction by two of the radiators, and (3) repulsion by the MAG boom. Of all the distortion modes, the one originating from a negatively charged (-67.8 V) radiator close to JDC is the strongest, affecting ions with energies above 80 eV. The least powerful but most prevalent mode is the repulsion of ions by the main body. Our results can be compared with future in-situ measurements to identify distortion mechanisms well ahead of the science phase in which the scientifically important measurements will be carried out.

For the transport of their spacecraft from the vehicle assembly building to the launch pads at Kennedy Space Centre, Florida, the National Aeronautics and Space Administration (NASA) is using two special crawler transporters since 1965. First developed for the Saturn V rocket the crawler transporters have been sufficient for all following generations of space ships so far. But for the new generation of Orionspacecraft which is under development now, a load capacity increase for the crawler transporter of plus 50% was necessary. For this task Hunger Hydraulik did develop new jacking, equalizing and levelling (JEL) cylinders with sufficient load capacity but also with some new features to improve the availability, reliability and safety of this system. After design approval and manufacture of the cylinders they have been tested in a special developed one-to-one scale dynamic test rig and after passing this the cylinders had to prove their performance in the crawler transporter itself. This article describes the general application and introduces the technical requirements of this project as well as the realized solution.

Attitude and position determination systems in satellites are absolutely necessary to keep the desired trajectory. A very accurate, reliable and most used sensor for attitude determination is the star tracker, which orient itself in space by observing and comparing star constellations with known star patterns. For on earth tests of movements and docking maneuvers of spacecrafts, the new Spacecraft Maneuvering and Docking (SMD) facility at the chair of Aerospace Information Technology at the University of Würzburg has been built. Air bearing systems on the space ve- hicles help to create micro gravity environment on a smooth surface and simulate an artificial space-like surrounding. A new star tracker based optical sensor for indoor application need to be developed in order to get the attitude and position of the vehicles. The main objective of this thesis is to research on feasible star tracking algorithms for the SMD facility first and later to implement a star detection software framework with new developed voting methods to give the star tracker system its fully autonomous function of attitude determination and position tracking. Furthermore, together with image processing techniques, the software framework is embedded into a controller board. This thesis proposes also a wireless network system for the facility, where all the devices on the vehicles can uniquely communicate within the same network and a devel- opment of a ground station to monitor the star tracker process has also been introduced. Multiple test results with different scenarios on position tracking and attitude determination, discussions and suggestions on improvements complete the entire thesis work.

La reconnexion magnétique est un processus omniprésent et fondamental dans la physique des plasmas spatiaux. La "Magnetospheric multiscale mission" (MMS) de la NASA, lancée le 12 mars 2015, a été conçue pour fournir des mesures in-situ permettant d'analyser le processus de reconnexion dans la magnétosphère terrestre. Dans ce but, quatre satellites identiquement instrumentés mesurent les champs électromagnétiques et les particules chargées dans les régions de reconnexion, avec une résolution temporelle cent fois meilleure que celle des missions précédentes. MMS permet, pour la première fois, d'étudier les structures microscopiques associées à la reconnexion magnétique et, en particulier, la région de diffusion électronique. Au niveau de la magnétopause terrestre, la reconnexion magnétique a un rôle chef dans le transport de l'énergie du vent solaire vers la magnétosphère terrestre, en convertissant l'énergie magnétique en énergie cinétique et thermique. Les événements à transfert de flux (FTEs) sont considérés comme l'un des produits principaux et les plus typiques de la reconnexion magnétique à la magnétopause terrestre. Cependant, des structures magnétiques 3D plus complexes, avec des signatures similaires à celles des FTEs, peuvent également exister à la magnétopause. On retrouve, par exemple, des tubes de flux entrelacés qui résultent de reconnexions magnétiques ayant eues lieu à des sites différents. La première partie de cette thèse étudie l'un de ces événements, qui a été observé dans des conditions de vent solaire inhabituelles, au voisinage de la magnétopause terrestre par MMS. Malgré des signatures qui, à première vue, semblaient cohérentes avec un FTE classique, cet événement a été interprété comme étant le résultat de l'interaction de deux tubes de flux avec des connectivités magnétiques différentes. La haute résolution temporelle des données MMS a permis d'étudier en détail une fine couche de courant observée à l'interface entre les deux tubes de flux. La couche de courant était associée à un jet d'ions, suggérant ainsi que la couche de courant était soumise à une compression qui a entraîné une reconnexion magnétique à l'origine du jet d'ions. La direction, la vitesse de propagation et la taille de différentes structures ont été déduites en utilisant des techniques d'analyse de données de plusieurs satellites. La deuxième partie de la thèse fournit une étude complémentaire à la précédente et s'intéresse aux ondes observées autour de la couche de courant. / Magnetic reconnection is a ubiquitous and fundamental process in space plasma physics. The NASA's Magnetospheric Multiscale mission (MMS) launched on 12 March 2015 was designed to provide in-situ measurements for analyzing the reconnection process at the Earth's magnetosphere. In this aim, four identically instrumented spacecraft measure fields and particles in the reconnection regions with a time resolution which is one hundred times faster than previous missions. MMS allows for the first time to study the microscopic structures associated with magnetic reconnection and, in particular, the thin electron diffusion region. At the Earth's magnetopause, magnetic reconnection governs the transport of energy and momentum from the solar wind plasma into the Earth's magnetosphere through conversion of magnetic energy into kinetic and thermal energies after a rearrangement of magnetic field lines. Flux Transfer Events (FTEs) are considered to be one of the main and most typical products of magnetic reconnection at the Earth's magnetopause. However, more complex 3D magnetic structures with signatures akin to those of FTEs might also occur at the magnetopause like interlaced flux tubes resulting from magnetic reconnection at multiple sites. The first part of the work presented in this thesis consisted of the investigation of one of these events that was observed, under unusual and extreme solar wind conditions, in the vicinity of the Earth's magnetopause by MMS. Despite signatures that, at first glance, appeared consistent with a classic FTE, this event was interpreted to be the result of the interaction of two separate sets of magnetic field lines with different connectivities. The high time resolution of MMS data allowed to resolve a thin current sheet that was observed at the interface between the two sets of field lines. The current sheet was associated with a large ion jet suggesting that the current sheet was submitted to a compression which drove magnetic reconnection and led to the formation of the ion jet. The direction, velocity and scale of different structures were inferred using multi-spacecraft data analysis techniques. This study was completed with a plasma wave analysis that focused on the reconnecting current sheet.