1,720,957 research outputs found
Nonlinear orbit control for earth satellites using low-thrust propulsion
No full textThis research is focused on the definition, analysis, and numerical testing an effective orbit control strategy tailored to compensating orbit perturbations, as well as possible errors at orbit injection of low-Earth-orbit microsatellites. A general, systematic approach to real-time orbit control is presented, under the assumption that the satellite of interest is equipped with a low-thrust propulsion system. Two different operational orbits are considered: (a) very-low-altitude Earth orbit and (b) sunsynchronous orbit. A feedback control law based on Lyapunov stability theory is proposed and tested. A steerable, throttleable low-thrust propulsion system with an upper bound on the thrust magnitude is considered. The stability properties and the overall performance over 5 years are investigated for cases (a) and (b). For case (a), the effect of satellite eclipsing on available electrical power is considered as well. Suitable tolerances on the desired (nominal) conditions allow substantial savings in terms of propellant requirements
Low-thrust nonlinear orbit control using nonsingular equinoctial elements
No full textDuring their lifetime, orbiting satellites often perform corrective maneuvers, for the purpose of avoiding excessive performance degradation, related to perturbations inherent to the space environment. The definition and implementation of an effective orbit control strategy thus represents a crucial issue, in order to compensate these perturbation actions, as well as possible errors at orbit injection. This research proposes a general, systematic approach to real-time, feedback orbit control, under the assumption that the satellite of interest is equipped with a low-thrust ion propulsion system. Lyapunov stability theory, in conjunction with the LaSalle's invariance principle, supply the theoretical foundation for the definition of a feedback control law that includes saturation and is capable of driving the dynamical system at hand toward the desired operational conditions. These are expressed in a rather general form and are associated with an invariant set that belongs to the attracting set of the controlled system. Two different operational Earth orbits are considered: (a) a very-low-altitude, circular orbit and (b) the medium-altitude Molniya orbit. For each case, the relevant orbit perturbations are modeled. The stability properties and the overall performance (in terms of propellant expenditure) are investigated for scenarios (a) and (b). In each case, the attracting set is identified, and is proven to contain the invariant set associated with the operational conditions. Suitable tolerances on the desired operational conditions allow substantial propellant savings, because propulsion is switched on only when some prescribed flight conditions are encountered. As a further effort to model real scenarios with enhanced fidelity, satellite eclipsing is also considered for scenario (a). In this case, the numerical simulations demonstrate that the tolerances on operational conditions are occasionally violated in some limited time intervals (where the ion propulsion is unavailable), while the overall propellant consumption exhibits a moderate - yet still completely acceptable - increase
Low-thrust lunar capture leveraging nonlinear orbit control
Full text linkNonlinear orbit control with the use of low-thrust propulsion is proposed as an effective strategy for autonomous guidance of a space vehicle directed toward the Moon. Orbital motion is described in an ephemeris model, with the inclusion of the most relevant perturbations. Unfavorable initial conditions, associated with weak, temporary lunar capture, are considered, as representative conditions that may be encountered in real mission scenarios. These may occur when the spacecraft is released in nonnominal flight conditions, which would naturally lead it to impact the Moon or escape the lunar gravitational attraction. To avoid this, low-thrust propulsion, in conjunction with nonlinear orbit control, is employed, to drive the space vehicle toward two different, prescribed, lowaltitude lunar orbits. Nonlinear orbit control leads to identifying a saturated feedback law (for the low-thrust magnitude and direction) that is proven to enjoy global stability properties. The guidance strategy at hand is successfully tested on three different mission scenarios. Then, the capture region is identified, and includes a large set of initial conditions for which nonlinear orbit control with low-thrust propulsion is effective to achieve lunar capture and final orbit acquisition
Nonlinear Earth orbit control using low-thrust propulsion
Full text linkThis research is focused on the definition, analysis, and numerical testing of an effective nonlinear orbit control technique tailored to compensating orbit perturbations, as well as possible errors at orbit injection of low- and medium-altitude Earth-orbit satellites. A general, systematic approach to real-time orbit control is presented, under the assumption that the satellite of interest is equipped with a steerable and throttleable low-thrust propulsion system. Two different operational orbits are considered: (a) very-low-altitude Earth orbit and (b) medium-altitude Earth orbit. A feedback control law based on Lyapunov stability theory is proposed and tested. Some remarkable stability properties are established analytically. Then, the overall performance of the nonlinear control at hand is investigated for cases (a) and (b), over 5 years. The effect of satellite eclipsing on available electrical power is considered as well. For mission scenario (a), suitable tolerances on the desired (nominal) conditions allow substantial savings in terms of propellant requirements
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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