1,721,011 research outputs found
Evolution of Fragmentation Cloud in Highly Eccentric Orbit Using Representative Objects
No full textMany historical on-orbit satellite fragmentations occurred in Highly Eccentric Orbits (HEOs) such as the Geostationary Transfer Orbit (GTO). Such fragmentations produce fragment clouds that interfere with the Low Earth Orbit (LEO) environment and pose a threat to operational satellites. Objects in HEO undergo complex dynamics due to the influence of perturbations varying as a function mainly of their altitude and area-to-mass ratio. The evolution of such a cloud, including small objects down to 1 mm, is not well understood. This paper describes a method to model the evolution of a fragmentation cloud in HEO under the influence of atmospheric drag and Earth's oblateness. Semi-analytical techniques are applied to propagate represen- tative objects constituting the cloud; rather than following the evolution of many distinct fragments. The proposed method is applied on a GTO upper stage using the standard NASA break-up model to nd the distribution right after the fragmentation. The evolution of the fragment cloud is analysed statistically and time of closures are calculated for the formation of the torus along the parent orbit and the band around Earth. Assumptions on the evolution of the cloud that are valid in LEO are shown to be invalid for clouds in HEO
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
Drag and Solar Sail Deorbiting: Re-Entry Time Versus Cumulative Collision Probability
Full text linkSolar and drag sailing have been proposed as passive end-of-life deorbiting methods, and technological demonstrators are under development. For orbit above 800 km altitude solar radiation pressure can be exploited for increasing the orbit eccentricity until the perigee enters the drag region until final re-entry. The performance of the sailing strategy is determined by four parameters: the required effective area-to-mass ratio to deorbit the spacecraft, which determine the sail size given the satellite’s mass, the time to deorbit and the augmented collision probability caused on and by the sail through its passage in the Low Earth Orbit protected region densely populated by space debris. In this paper we assess the sail dimension with respect to the augmented collision risk depending on the sail area and the deorbiting time
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Analysis of the consequences of fragmentations in low and geostationary orbits
No full textThe present distribution of intact objects is a good proxy to quantify the catastrophic collision risk and consequences in the coming decades. The results of a large number of long term simulations of the LEO environment perturbed by the collisional fragmentation of massive objects are used to identify the main driving parameters of the long term collisional evolution of the debris population and measure the danger represented by “typical” classes of space objects. An evaluation norm, able to highlight the differences between comparative long term evolution scenarios and to give a quantitative measure of the effects of specific parameters affecting the evolution, is devised. It is shown how, for collisional fragmentations in LEO, due to the highly stochastic evolution of the LEO environment, even the fragmentation of a massive spacecraft might not be able to alter the long term evolution of the LEO population beyond the intrinsic statistical variability associated with the Monte Carlo procedure. Among the parameters determining the long term effects of a collisional fragmentation in LEO, a combination of mass and altitude of the event appears to be the driving factor. In GEO, the situation is different, and the addition of a massive fragmentation lives a signature on the environment that is detectable throughout the investigated time span, with the mass being the only factor important to asses the long term consequences of a collisional fragmentatio
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