Dans la quasi-totalité des missions d'observation de la Terre requérant une grande précision de pointage, les micro-vibrations sont le principal élément dégradant les performances de pointage. Les principales sources de micro-perturbations sont les roues et, lorsqu'il y en a, les refroidisseurs cryogéniques. D’autres sources de perturbations sont les propulseurs chimiques, les moteurs pas à pas de l'antenne solaire, les mécanismes d'entraînement,… L'objectif de cette thèse (NPI) est de concevoir et de valider un système de pointage actif de haute précision à base d’actionneurs piézoélectriques capable de rejeter les micro-vibrations au niveau d’un miroir, avec des pénalités de masse et de puissance contrôlées. Les caractéristiques attendues de ce système sont : • une grande bande passante en boucle fermée : typiquement jusqu'à 100 Hz • une faible erreur résiduelle: typiquement inférieure à 50-100 rad (ordre de grandeur approximatif) • un encombrement et une masse faibles (à quantifier au cours de la thèse) • une puissance requise minimale (à optimiser au cours de la thèse) • la modularité • une possible évolution Ce sujet est fortement pluridisciplinaire (mécanique, automatique, optique et instrumentation). Les défis scientifiques de la thèse sont : • la conception d’un système de pointage actif à bande passante élevée avec impact de masse et de volume faible et une puissance requise à minimiser • la commande robuste du système de pointage actif permettant de rejeter des micro-perturbations dont le spectre varie en fonction des phases de la mission • la tenue des performances en précision • la définition d'une méthodologie générique de conception intégrée applicable à d'autres systèmes de pointage (plusieurs degrés de liberté, ...) / On almost all high accuracy pointing Science and Earth observation missions, micro-vibrations are the major contributor to pointing performances degradations (RPE). The main sources of micro-disturbances being the wheels and, when present, the cry-coolers. Other disturbance sources may originate from chemical thrusters, antenna stepper motors, Solar Array Drive Mechanisms (SADM), antenna trimming mechanisms, or payload mechanisms set either inside the sensitive payload, or inside another payload of the same spacecraft. The objective of this NPI is to investigate and validate a high accuracy active pointing system able to reject micro-vibrations at instrument level: • large control bandwidth : typically up to 100Hz • low residual error : typically lower than 50-100nrad (rough order magnitude to be further defined in the frame of this NPI) • low mass and volume impacts • scalable • modular This subject is strongly multidisciplinary (mechanics, control theory, optics and instrumentation). The scientific challenges of the thesis are: • the design of an active pointing system with high bandwidth, low impact of mass and volume and minimized power • the robust control of the active pointing system allowing to reject micro-disturbances whose spectrum varies according to the phases of the mission • obtaining high accuracy performances • the definition of a generic methodology of integrated design applicable to other pointing systems (several degrees of freedom e.g.)

The problem of system identification for dynamic effects on spacecraft has become increasingly relevant with the surge of agile spacecraft, which must perform large amplitude maneuvers at high rates. Precise knowledge of the state of the spacecraft, as well as of the parameters characterizing its motion, is vital for the design of control algorithms enabling stabilization and pointing accuracy. Traditional rigid body models and estimation methods are no longer sufficient to provide this knowledge. This thesis focuses on estimation of flexibility effects and spacecraft parameters through methods based on the unscented Kalman filter, an estimator for nonlinear dynamic systems. A spacecraft model consisting of a rigid central body and a flexible appendage described as an Euler-Bernoulli beam in pure bending is built, and equations for its translational and rotational motion, as well as the deflection of the beam, are derived in the Newton-Euler framework considering the first bending mode of the flexible deformation. Observability tests are successfully conducted to ensure that estimation of the relevant states and parameters can be performed exclusively from linear and angular velocity measurements. A total of eight filters, estimating the spacecraft’s state along with different combinations of parameters, are developed, implemented, and tested on simulated data. Grouped under the common denomination “UFFE” (Unscented Filter for Flexibility Effects), they are made available as Simulink library blocks. State estimation is performed for the linear and angular velocities of the spacecraft and the modal coordinate and velocity of the appendage, with estimates following closely the truth model of the state variables and estimation errors at least an order of magnitude lower than true state values. Simultaneous state and parameter estimation is implemented from two approaches, joint estimation and dual estimation, whose performance and applications are compared. Estimated parameters include the moments of inertia of the system and natural frequency, damping ratio, and modal participation factors of the flexible appendage. Convergence to true parameter values is reached in the first 100s of the estimation for inertia terms and natural frequency, while the estimation for modal participation factors is conditioned to precise tuning of the filter. Estimates of the damping ratio are biased, most likely due to the control input not being optimal for observation of this parameter. The dual approach to parameter estimation is found to be advantageous when proper filter tuning is possible, as it enables the continuous operation of a state filter combined with short runs of the parameter filter activated at will; this configuration could be employed to track the variation of spacecraft parameters along space missions.  The causes of estimation error are identified and methods for automatic tuning of the process noise and process noise covariance are researched. Five such tuning techniques are implemented and tested, with promising results found for online sampling of the process noise covariance through Monte Carlo methods. A discussion on the limitations of the chosen dynamic model and estimator, along with recommendations for extensions and future applications, concludes this work.