Guidance and Control in Autonomous Debris Removal Space Missions via Adaptive Nonlinear Model Predictive Control

Space debris orbiting around the Earth are becoming a major problem that could impair the future of space exploration. Among the different approaches to this problem that have been proposed in recent years, this work focuses on a possible innovative solution, consisting in an autonomous spacecraft that performs a rendezvous maneuver, collects a debris of unknown mass and then moves to a parking orbit. When the spacecraft collects a debris of unknown mass, the dynamics of the system may change substantially, and this may affect the control stability and performance of the spacecraft. In this paper, a control system is designed, capable of handling situations with time-varying and uncertain parameters, as it occurs in space debris removal missions. A control strategy based on an Adaptive Nonlinear Model Predictive Control (ANMPC) is considered. The unknown mass of the debris is treated as an uncertain parameter and is estimated by means of two different methods (Recursive Average and Extended Kalman Filter (EKF)). Then, the estimated mass is used to update the internal model of the ANMPC, which later solves an on-line optimization problem, providing an optimal trajectory and control action for reaching the debris and then the parking orbit. The simulations carried out show that the proposed control system is able to effectively accomplish the requested task

A Pontryagin-based NMPC approach for autonomous rendez-vous proximity operations

This paper proposes a Pontryagin-based approach to Nonlinear Model Predictive Control for autonomous guidance and control in spacecraft maneuvering. The proposed approach guarantees, under suitable conditions, an explicit control law also in presence of nonlinearities. Taking advantage of the Pontryagin Minimum (or Maximum) Principle, the optimization problem solution turns into a two-points boundary value problem, whose differential equations and boundary conditions are the Karush-Kuhn-Tucker necessary conditions of optimality. Conversely to the numerical methods for nonlinear/non-convex optimization, the proposed methodology returns an explicit control law without any a-priori assumption about the input signal parametrization, achieving high performances without increasing the computational complexity of the algorithm. The proposed control algorithm is designed for the proximity operations of a rendez-vous problem which dynamics is described by the so-called Clohessy-Wiltshire equations. A modified NMPC cost function is employed in order to promote the bang-bang behavior of the input signal. This latter yields an improvement of the performances in terms of propellant consumption with respect to the classic quadratic cost indexes

A Sparse Nonlinear Model Predictive Control for Autonomous Space Missions

Propellant consumption minimization is a key factor in space missions, as it strongly affects the duration of any mission. Nowadays, delta-V guidance strategies are obtained by means of classical ground based open loop methods, while academic research has mainly focused on autonomous low-thrust strategies. However, classical methods return instantaneous impulsive thrust actions that are not always feasible in practice, due to the technical limitations of real propulsion systems. In this paper, a novel Nonlinear Model Predictive Control framework for autonomous guidance and control with high-thrust quasi-impulsive maneuvers is presented. The internal prediction model is based on the so-called Modified Equinoctial Orbital Elements, which allow us to overcome relevant singularities given by the standard Keplerian elements. Different NMPC cost functions are compared in order to have a sparse thrust profile, minimize at the same time the propellant consumption and the tracking error with respect to the target orbit. In particular, it is shown how non-quadratic norms could achieve better performances. Finally, an Earth Observation mission, employing different NMPC functionals, is used as a benchmark and the results are compared with the ones coming from the classical astrodynamics solutions

Raman imaging and electronic properties of graphene

Graphite is a well-studied material with known electronic and optical properties. Graphene, on the other hand, which is just one layer of carbon atoms arranged in a hexagonal lattice, has been studied theoretically for quite some time but has only recently become accessible for experiments. Here we demonstrate how single- and multi-layer graphene can be unambiguously identified using Raman scattering. Furthermore, we use a scanning Raman set-up to image few-layer graphene flakes of various heights. In transport experiments we measure weak localization and conductance fluctuations in a graphene flake of about 7 monolayer thickness. We obtain a phase-coherence length of about 2 $\mu$m at a temperature of 2 K. Furthermore we investigate the conductivity through single-layer graphene flakes and the tuning of electron and hole densities via a back gate

role of winter host plants in vineyard colonization and phenology of zygina rhamni hemiptera cicadellidae typhlocybinae

Abstract Zygina rhamni Ferrari (Hemiptera: Cicadellidae: Typhlocybinae) is a mesophyll-feeding leafhopper that infests grapevine, Vitis vinifera L. This leafhopper's overwintering strategy and the dynamics of its vineyard colonization, relative to the distance between the grapevines and the winter host plants and different grapevine vegetative densities, are described herein. This species shows a facultative heteroecious life cycle. In autumn, it migrates from vineyards to bramble, Rubus gr. fruticosus, and rose, Rosa chinensis Jacquin. On these host plants, females lay eggs that start to hatch at the end of March. Adults that developed on bramble colonize grapevines beginning in May; the leaf fall of rose, which takes place during the first half of April, probably causes the death of unhatched eggs and nymphs. We found that overwintering females can move back to vineyards and lay eggs until early June. The dynamics of vineyard colonization by the leafhopper involved initial concentration in areas close t..

The LISA DFACS: effects of micrometeoroid impacts in the drag-free mode

LISA is a space-based gravitational wave observatory under study and prototyping by the European Space Agency and other institutions/companies. It consists in a triangular constellation of three spacecraft traveling in heliocentric orbits, connected through bi-directional laser links in order to detect/measure gravitational waves by means of interferometry. In the science mode (also called drag-free mode), each spacecraft compensates for the disturbances and noises affecting the control loops by performing tiny adjustments around a suitable working point. Micrometeoroid streams may collide with the spacecraft surface and generate impulsive forces and torques on the spacecraft body. Such impulsive disturbances determine attitude perturbations, which in some cases cause the incoming laser beams to move outside the optical sensor ranges. In these cases, the laser links are lost and interferometry cannot be performed anymore. Link recovery may be accomplished by performing the maneuver conceived for the initial constellation acquisition by this maneuver may take a long time (several hours), implying a significant reduction of the science time. In this paper, the effects of micrometeoroid impacts on the LISA spacecraft in the drag-free mode are evaluated and a fast recovery control strategy is proposed to quickly return in science/drag-free mode

Ion Beam Modification of PVDC and Pe Polymers

Electronic and nuclear stopping effects produced by MeV ion bombardment in polyvinylidine chloride and polyethylene are separated by stacking thin films of the polymers. Resulting multi-layer laminates of each polymer were bombarded with 3.5-MeV alpha particles. Energy of the incident ions was selected using the TRIM code so that the first layers experienced most of the effects of the electronic energy deposited and the last layers received most of the effects of the nuclear stopping power. Changes in conductance and chemical structure of each layer were measured by direct resistivity measurements and Raman microprobe analysis

Raman Topography and Strain Uniformity of Large-Area Epitaxial Graphene

We report results from two-dimensional Raman spectroscopy studies of large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large variation in Raman peak position across the sample resulting from inhomogeneity in the strain of the graphene film, which we show to be correlated with physical topography by coupling Raman spectroscopy with atomic force microscopy. We report that essentially strain free graphene is possible even for epitaxial graphene.Comment: 10 pages, 3 figure

Genetic differentiation among host-associated Alebra leafhoppers (Hemiptera: Cicadellidae)

The limited importance ascribed to sympatric speciation pro cesses via host race formation is partially due to the few cases of host races that have been reported among host populations. This work sheds light on the taxonomy of Alebra leafhoppers and examines the possible existence of host races among host-associated populations. The species of this genus show varying degrees of host association with deciduous trees and shrubs and, frequently, host popu lations of uncertain taxonomic status coexist and occasion ally become pests. Allozyme electrophoresis of 21 Greek populations including sympatric, local and geographically distant samples collected on 13 different plant species, show that they represent at least five species: A. albostriella Falle´n, A. viridis (Rey) (sensu Gillham), A. wahlbergi Bo Keywords: host races; leafhoppers; sympatric speciation; sibling species; allozymes; Alebra Introduction Sympatric speciation is a controversial subject in evol utionary biology (see Mayr, 1963; Futuyma and Mayer, 1980; Paterson, 1981; Via, 2001). One of the reasons for this controversy is that sympatric speciation seems to be an extremely rare phenomenon occurring only in very few groups of taxa, represented chiefly by phytophagous insects (Tauber and Tauber, 1977; Menken, 1981; Wood, 1993; Emelianov et al, 1995; Via, 1999; Finchak et al, 2000; Craig et al, 2001). The limited number of reported cases among organisms with sexual reproduction can be at least partially attributed to the fact that taxa undergoing sympatric speciation events must fulfill very restrictive biological and ecological requirements. Most sympatric speciation models demand that there is intraspecific genetic variation in traits that differentially affect the fitness of individuals that colonise new habitats or hosts (Dieckman and Doebeli, 1999; Hawthorne and Via, 2001 but see Higashi et al, 1999 and Takimoto et al, 2000). They assume that selection acting on these traits can prevent genetic exchange between populations (Bush, 1975; Tauber and Tauber, 1977; Diehl and Bush, 1989). In phytophagous insects, this means that host pref erences must be genetically determined and mating should occur on the host (Bush, 1975; Diehl and Bush, Correspondence: D Aguin-Pombo, Department of Biology, University of Madeira, Campus Universitario da Penteada, 9000 Funchal, Madeira, Portugal. E-mail: aguin uma.pt Received 12 December 2000; accepted 13 December 2001 heman and two new species. Of these, one is associated to Quercus frainetto and other is specific to Crataegus spp. Significant genetic differences among sympatric and local host populations were found only in A. albostriella, between populations on Turkey oak, beech and common alder. It is suggested that the last two of these host populations may represent different host races. The results show that both the host plant and geographical distance affect the patterns of differentiation in the genus. The formation of some spec ies seems to have been the result of allopatric speciation events while, for others, their origin can be equally explained either by sympatric or allopatric speciation.info:eu-repo/semantics/publishedVersio