L’objectif de la thèse est de développer un système de type jonction tunnel tout oxyde à base de La2/3Sr1/3MnO3 (LSMO) où il serait possible de contrôler l’aimantation des électrodes magnétiques par des processus à faible consommation d’énergie. Des jonctions tunnel épitaxiées de LSMO/SrTiO3/LSMO ont été obtenues montrant un double renversement de l’aimantation à température ambiante et un taux de magnétorésistance de 71 % à 10 K. En exerçant une contrainte sur le LSMO par le substrat il a été possible de moduler l’anisotropie des couches magnétiques. Des anisotropies perpendiculaire ou dans le plan ont pu être obtenues. Afin de contrôler le renversement des moments magnétiques dans une des électrodes ferromagnétiques trois options ont été envisagées : l’utilisation de l’injection de spin à partir d’un métal à fort couplage spin-orbite (Pt, Ir) ou d’un oxyde contenant de tels ions (ici Ru dans SrRuO3), et l’utilisation du Bi2FeCrO6, un oxyde multiféroïque pouvant présenter un couplage magnétoélectrique. Malgré des résultats prometteurs, aucune solution n’a permis des tests sur des jonctions afin d’estimer leur efficacité. L’objectif final n’est pas encore atteint mais des avancées intéressantes ont été faites afin d’envisager des dispositifs permettant le stockage et la manipulation de l’information. / The objective of this work is to develop La2/3Sr1/3MnO3 (LSMO) based all-oxide magnetic tunnel junction systems where it would be possible to control the magnetization of magnetic electrodes by low energy consumption processes. Epitaxial tunnel junctions of LSMO/SrTiO3/LSMO were obtained showing a double magnetization switching at room temperature and a magnetoresistance ratio of 71 % at 10 K. Using strain engineering, it was possible to modulate the anisotropy of the LSMO magnetic layers. Perpendicular or in plane anisotropies could be thus obtained. In order to control the reversal of the magnetic moments in one of the ferromagnetic electrodes three options were considered : the use of spin injection from a metal with a strong spin-orbit coupling (Pt, Ir) or an oxide containing this type of ions (here Ru in SrRuO3), and the use of Bi2FeCrO6 multiferroic oxide that may exhibit a magnetoelectric coupling. Despite promising results, no solution has allowed tests on junctions to be carried out to estimate their effectiveness. Although the final objective is not yet achieved, interesting progress has been made on the way to information storage and manipulation devices.

<div>Over the past twenty years, ridesharing opportunities for smallsats, i.e., secondary payloads, has increased with the introduction of Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA) rings. However, the orbits available for these secondary payloads is limited to Low Earth Orbits (LEO) or Geostationary Orbits (GEO). By incorporating a propulsion system, propulsive ESPA rings offer the capability to transport a secondary payload, or a collection of payloads, to regions beyond GEO. In this investigation, the ridesharing scenario includes a secondary payload in a dropped-off Geosynchronous Transfer Orbit (GTO) and the region of interest is the vicinity near the Sun-Earth Lagrange points. However, mission design for secondary payloads faces certain challenges. A significant mission constraint for a secondary payload is the drop-off orbit orientation, as it is dependent on the primary mission. To address this mission constraint, strategies leveraging dynamical structures within the Circular Restricted Three-Body Problem (CRTBP) are implemented to construct efficient and flexible transfers from GTO to orbits near Sun-Earth Lagrange points. First, single-maneuver ballistic transfers are constructed from a range of GTO departure orientations. The ballistic transfer utilize trajectories within the stable manifold structure associated with periodic and quasi-periodic orbits near the Sun-Earth L1 and L2 points. Numerical differential corrections and continuation methods are leveraged to create families of ballistic transfers. A collection of direct ballistic transfers are generated that correspond to a region of GTO departure locations. Additional communications constraints, based on the Solar Exclusion Zone and the Earth’s penumbra shadow region, are included in the catalog of ballistic transfers. An integral-type path condition is derived and included throughout the differential corrections process to maintain transfers outside the required communications restrictions. The ballistic transfers computed in the CRTBP are easily transitioned to the higher-fidelity ephemeris model and validated, i.e., their geometries persist in the ephemeris model. To construct transfers to specific orbits near Sun-Earth L1 or L2, families of two-maneuver transfers are generated over a range of GTO departure locations. The two-maneuver transfers consist of a maneuver at the GTO departure location and a Deep Space Maneuver (DSM) along the trajectory. Families of two-maneuver transfers are created via a multiple- shooting differential corrections method and a continuation process. The generated families of transfers aid in the rapid generation of initial guesses for optimized transfer solutions over a range of GTO departure locations. Optimized multiple-maneuver transfers into halo and Lissajous orbits near Sun-Earth L1 and L2 are included in this analysis in both the CRTBP model and the higher-fidelity ephemeris model. Furthermore, the two-maneuver transfer strategy employed in this analysis are easily extended to other Three-Body systems. </div>

<div>In order to answer the most pressing scientific questions about the two Martian moons, Phobos and Deimos, new remote sensing observations are required. The best way to obtain global high resolution observations of Phobos and Deimos is through dedicated missions to each body that utilize close-proximity orbits, however much of the orbital tradespace is too unstable to realistically or safely operate a mission.</div><div><br></div><div>This thesis explores the dynamics and stability characteristics of trajectories near Deimos. The family of distant retrograde orbits that are inclined out of the Deimos equatorial plane, known as quasi-satellite orbits, are explored extensively. To inform future mission design and CONOPS, the sensitivities and stability of distant retrograde and quasi-satellite orbits are examined in the vicinity of Deimos, and strategies for transferring between DROs are demonstrated. Finally, a method for designing quasi-satellite science orbits is demonstrated for a set of notional instruments and science requirements for a Deimos remote sensing mission.<br></div>

<pre>Because of the increasing population of space objects, there is an increasing necessity to monitor and predict the status of the near-Earth space environment, especially of critical regions like geosynchronous Earth orbit (GEO) and low Earth orbit (LEO) regions, for a sustainable future. Space Situational Awareness (SSA), however, is a challenging task because of the requirement for dynamically insightful fast orbit propagation models, presence of dynamical uncertainties, and limitations in sensor resources. Since initial parameters are often not known exactly and since many SSA applications require long-term orbit propagation, long-term effects of the initial uncertainties on orbital evolution are examined in this work. To get a long-term perspective in a fast and efficient manner, this work uses analytical propagation techniques. Existing analytical theories for orbital perturbations are investigated, and modifications are made to them to improve accuracy. While conservative perturbation forces are often studied, of particular interest here is the orbital perturbation due to non-conservative forces. Using the previous findings and the developments in this thesis, two SSA applications are investigated in this work. In the first SSA application, a sensor tasking algorithm is designed for the detection of new classes of GEO space objects. In the second application, the categorization of near-GEO objects is carried out by combining knowledge of orbit dynamics with machine learning techniques.</pre>

<p>A dynamical understanding of orbits in the Earth-Moon neighborhood that can sustain long-term activities and trajectories that link locations of interest forms a critical foundation for the creation of infrastructure to support a lasting presence in this region of space. In response, this investigation aims to identify and exploit fundamental dynamical motion in the vicinity of a candidate ‘hub’ orbit, the L2 southern 9:2 lunar synodic resonant near rectilinear halo orbit (NRHO), while incorporating realistic mission constraints. The strategies developed in this investigation are, however, not restricted to this particular orbit but are, in fact, applicable to a wide variety of stable and nearly-stable cislunar orbits. Since stable and nearly-stable orbits that may lack useful manifold structures are of interest for long-term activities in cislunar space due to low orbit maintenance costs, strategies to alternatively initiate transfer design into and out of these orbits are necessary. Additionally, it is crucial to understand the complex behaviors in the neighborhood of any candidate hub orbit. In this investigation, a bifurcation analysis is used to identify periodic orbit families in close proximity to the hub orbit that may possess members with favorable stability properties, i.e., unstable orbits. Stability properties are quantified using a metric defined as the stability index. Broucke stability diagrams, a tool in which the eigenvalues of the monodromy matrix are recast into two simple parameters, are used to identify bifurcations along orbit families. Continuation algorithms, in combination with a differential corrections scheme, are used to compute new families of periodic orbits originating at bifurcations. These families possess unstable members with associated invariant manifolds that are indeed useful for trajectory design. Members of the families nearby the L2 NRHOs are demonstrated to persist in a higher-fidelity ephemeris model. </p><p><br></p> <p>Transfers based on the identified nearby dynamical structures and their associated manifolds are designed. To formulate initial guesses for transfer trajectories, a Poincaré mapping technique is used. Various sample trajectory designs are produced in this investigation to demonstrate the wide applicability of the design methodology. Initially, designs are based in the circular restricted three-body problem, however, geometries are demonstrated to persist in a higher-fidelity ephemeris model, as well. A strategy to avoid Earth and Moon eclipse conditions along many-revolution quasi-periodic ephemeris orbits and transfer trajectories is proposed in response to upcoming mission needs. Lunar synodic resonance, in combination with careful epoch selection, produces a simple eclipse-avoidance technique. Additionally, an integral-type eclipse avoidance path constraint is derived and incorporated into a differential corrections scheme as well. Finally, transfer trajectories in the circular restricted three-body problem and higher-fidelity ephemeris model are optimized and the geometry is shown to persist.</p>

Network connectivity is a fundamental property affecting network performance. Given the reliability of each link, network connectivity determines the probability that a message can be delivered from the source to the destination. In this thesis, we study the directed network connectivity where the message will be forwarded toward the destination hop by hop, so long as the neighbor(s) is (are) closer to the destination. Directed connectivity, closely related to directed percolation, is very complicated to calculate. The existing state-of-the-art can only calculate directed connectivity for a lattice network up-to-the size of 10 × 10. In this thesis, we devise a new approach that is simpler and more scalable and can handle general network topology and heterogeneous links. The proposed approach uses an unambiguous hop count to divide the networks into hops and gives two steps of pre-process to transform hop-count ambiguous networks into unambiguous ones, and derive the end-to-end connectivity. Then, using the Markov property to obtain the state transition probability hop by hop. Second, with tens of thousands of Low Earth Orbit (LEO) satellites covering the Earth, LEO satellite networks can provide coverage and services that are otherwise not possible using terrestrial communication systems. The regular and dense LEO satellite constellation also provides new opportunities and challenges for network protocol design. In this thesis, we apply the directed connectivity analytical model on LEO satellite backbone networks to ensure ultra-reliable and low-latency (URLL) services using LEO networks, and propose a directed percolation routing (DPR) algorithm to lower the cost of transmission without sacrificing speed. Using Starlink constellation (with 1,584 satellites) as an example, the proposed DPR can achieve a few to tens of milliseconds latency reduction for inter-continental transmissions compared to the Internet backbone, while maintaining high reliability without link-layer retransmissions. Finally, considering the link redundancy overhead and delay/reliability tradeoff, DPR can control the size of percolation. In other words, we can choose a part of links to be active links considering the reliability and cost tradeoff. / Graduate

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[pt] Espaços de moduli de polígonos em R(3) com comprimento dos lados fixados é um exemplo amplamente estudado de variedade simplética. Esses espaços podem ser descritos como quociente simplético de um número finito de órbitas coadjuntas pelo grupo SU(2). Nesta tese esses espaços de moduli são identificados como folhas simpléticas de uma variedade de Poisson que pode ser construída como quociente. Essa construção é a seguir generalizada ao caso de um produto de um número finito de órbitas coadjuntas pelo grupo SU(n), e o resultado principal desse trabalho de tese descreve como esses espaços de moduli de polígonos generalizados formam uma folheação em folhas simpléticas de uma variedade de Poisson. / [en] Moduli spaces of polygons in R(3)with fixed sides length are a widely studied example of symplectic manifold that can be described as the symplectic quotient of a finite number of SU(2)−coadjoint orbits by the diagonal action of the group SU(2). In this thesis these spaces are identified as the symplectic leaves of a Poisson manifold, that can itself be obtained by a quotient procedure. The construction is then generalized to the case of the quotient of a product of finitely many SU(n)−coadjoint orbits by the diagonal action of SU(n), and the main result of this thesis describes how these moduli spaces of generalized polygons fit together as the symplectic leaves of a quotient Poisson manifold.

Attitude Determination and Control System (ADCS) is often a complex system on-board any  satellite  which  needs  validation  and  testing  to  prove  its  operability  and  verify  its software  compatibility  with  hardware  and  other  subsystems.    One  failure  in  orbit  is extremely expensive in terms of cost and time due to payload preparation and launch. The ideal test bench would be the one that perfectly simulates the space environment and all its main factors such as weightlessness, Earth’s Magnetic Field (EMF), vacuum, neutral particles, plasma and radiation, among others.  The target in this case was the Earth’s Magnetic Field (EMF), solved with a Helmholtz Cage in a Merritt Configuration, and weightlessness, not implemented but analysed in detail where different alternatives are proposed, similar to market solutions.As  derived  from  literature  and  simulations  executed  along  this  M.  Sc.    Thesis,  the Merritt Cage seems beneficial against any other configuration in terms of magnetic field uniformity and effective volume.  After the design and assembly of the test bench, both properties were verified and successfully achieved, despite the lack of calibration, not executed because of time limitation, and tiny issues encountered along the full evolutionof the project. / Attitude Determination and Control System (ADCS) är ofta ett komplicerat system ombord på alla satelliter som behöver validering och testning för att bevisa dess användbarhet och  verifiera dess  programvarukompatibilitet med  hårdvara och  andra delsystem.   Ett fel  i  omloppsbana  är  extremt  dyrt  med  avseende  på  kostnader  och  tid  på  grund  av förberedelse  och  lansering  av  nyttolast  Den  ideala  testbänken  skulle  vara  den  som perfekt  simulerar  rymdmiljön  och  alla  dess  huvudfaktorer  såsom  viktlöshet,  Earth’s Magnetic  Field  (EMF),  vakuum,  neutrala  partiklar,  plasma  och  strålning,  bland  andra. Målet  i  detta  fall  var  EMF,  löst  med  en  Helmholtz-bur  i  en  Merritt-konfiguration,  och viktlöshet,  inte  implementerad  men  analyserad  i  detalj  där  olika  alternativ  föreslås, liknande marknadslösningar.Som härstammar från litteratur och simuleringar utförda längs denna M. Sc. Avhandling verkar Merritt Cage vara gynnsam mot annan konfiguration när det gäller magnetfältens enhetlighet och effektiv volym.  Efter konstruktionen och montering av testbänken, var båda egenskaperna verifierade och framgångsrikt uppnådda, trots bristen på kalibrering, inte  genomförda  på  grund  av  tidsbegränsning,  och  små  problem  som  uppstod  underprojektets fulla utveckling.