1,721,021 research outputs found
A distributed space radar sounder using a cross-track flying tethered satellite system
Full text linkThe objective of this paper is to analyze the performance of two possible architectures of tethered satellite systems, used as a platform for a distributed radar sounder. The first architecture consists in a cross-track oriented tethered satellite system, controlled and stabilized by exploiting the aerodynamic forces generated by the interaction with the rarefied atmosphere in Low Earth Orbit. The second architecture involves a tethered satellite system controlled through gyroscopic stabilization, obtained by spinning the system about an axis contained in the orbital plane. After a brief survey of radar sounding techniques, the methodology is introduced for describing the geometry of the systems and their characteristics, the performance of the two architectures are then compared with each other and with the current state of the art. By analyzing the modeled nominal behavior, it is shown that the two proposed architectures can achieve continuous or multiple observations, respectively, at maximum cross-track resolution, during one orbit, minimizing clutter noise. This is a considerable improvement of performance versus a formation flight architecture which can typically achieve only up to four observations per orbit. Finally, the advantages and disadvantages of each architecture are studied, and their possible mission scenarios are discussed
Analysis of Alternative Energy Harvesting Methods to Power Atmospheric Robotic Explorers on Jupiter
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Hovering of an Electrically Actuated Spacecraft in a Small-Body Plasma Field
Full text linkThis paper presents simulation models and an analysis of the hovering capability of an electrostatic spacecraft around a small celestial body. The hovering capabilities of an electrically actuated spacecraft are evaluated by combining orbital dynamics analysis with 3-D fully kinetic particle-in-cell simulations of asteroid/spacecraft interactions with the solar-wind plasma. The zero-velocity curves obtained from the analysis allow the identification of the equilibrium points for different levels of charge. The analysis of the system equilibria indicates the presence of equilibrium points in the combined gravitational, electrostatic, and solar illumination fields, most of which can be obtained by charging the spacecraft negatively. The charge-to-mass ratio needed to hover is obtained for different orbital positions, and an analysis of the sensitivity of the equilibria with respect to the spacecraft equivalent radius and with respect to the sun-to-main-body distance provides additional insight into the system dynamics
Effective Sensitivity Analyses of Radar Systems in Formation Flying using Differential Algebra
No full textSpacecraft Formation Flying is a technology that has been increasingly used in recent years. Distributed formations of low-cost SmallSats, both deployable and free-flying, can provide mission ca pability comparable to or greater than large monolithic spacecraft, but with greatly improved flexibility (adaptability, scalability, evolvability and maintainability) and robustness (reliability, survivability and fault tolerance). These advantages are accompanied by an increase in complexity in maintaining and managing the system. Especially in the case of systems whose purpose is the creation of Synthetic Aper ture Radar (SAR), relative positions and relative velocities must be finely controlled so that the acquired data are not partially or totally corrupted. Several studies deal with the active and passive control of satellite formations with the aim of minimizing the fuel needed to carry out orbital correction maneuvers or to lengthen the life of the mission. Such works often take place in a deterministic context. Accurate simulations must take into account the uncertainties affecting the quantities involved. In reality, the initial state of the system may not be known precisely, so it should be treated as a random variable. In this context, uncertainty propagation helps to know how the uncertainty on the initial state influences the uncertainty on the final state when the system is subject to complex nonlinear dynamics. The Monte Carlo procedure (MC) is the best known and most intuitive method for uncertainty propagation. The problem is that in case of large state vectors (as in the case of control of satellite formations with 6-DoF dynamics control) or of propagation of the system for multiple orbits, MC is difficult to implement due to the high computational cost. In this paper, a different methodology based on Differential Algebra (DA) is presented. DA is able to obtain the same information as the classical MC method in a shorter amount of time, at the cost of an acceptable loss of accuracy. In particular, we present simulation studies of Uncertainty Propagation and Sensitivity Analysis applied to Formation Flying by using DA, and discuss the advantages and disadvantages with respect to the MC method
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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