1,720,980 research outputs found
Spacecraft break-up models for the atmospheric re-entry from highly eccentric and interplanetary trajectories
No full textDuring the past decades space agencies, and more recent space industry and academia as well, have developed software models to evaluate the break-up and demise of spacecraft during re-entries. Due to limited observational data for validation of these models and the operational needs of the agencies, these models are focused on re-entries from nearly circular orbits or controlled re-entries. This in turn implies that their usability for atmospheric re-entry from highly eccentric orbit and interplanetary trajectories is based on extrapolation, in some cases ignoring the physical differences. This work reviews the limited available observational data on uncontrolled spacecraft re-entries with high flight path angle. It establishes a first order approximation for the predicted break-up altitude during the re-entry from given trajectory conditions postulated on an equivalence with circular re-entry cases via the aero-thermal environment along the trajectory. The equivalence is analysed by means of simulation and inter-tool comparison
Evaluation of re-entry conditions from orbits at the libration points
No full textEven if no protected regions are currently defined around the Sun-Earth libration points, missions in these orbits still have to be compliant with Space Debris Mitigation guidelines, for example for what concerns the casualty risk in case of re-entry. Past studies on the re-entries from libration orbits were carried out by performing a limited sensitivity analysis of the casualty area to the variation ofre-entry parameters. In the current work, a Monte Carlo approach is used to study the evolution of trajectories in case of a fragmentation or a failure of a spacecraft at the libration points. A large number of possible re-entry trajectories are generated and the corresponding re-entry conditions and casualty area are obtained by simulating the last phase of the evolution with ESA DRAMA software. The goal is to find a practical procedure for casualty risk requirements verification for current and future missions
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
Automated Operations for the Maintenance of a Space Object Database
No full textTo cope with the increasing number of objects in space around Earth, the processing of observations of these objects shall be automated as much as possible. In this work, the automation of three parts of the maintenance of a space object orbit database, commonly called catalogue, is analysed. The first one is the association of new measurements with stored orbits in the database based on a comparison in a common coordinate frame using the Mahalanobis distance under the assumption of normally distributed errors. It is shown for radar observations, that making this comparison in either the observation or orbit coordinates is only feasible if the error in the transformed system is small enough to remain normal after the transformation. Additional work is dedicated to derive a quality measure from the orbit state and covariance to give feedback, which also considers new measurements, on the current status or quality of an object in the database. Different concepts from information theory, namely the entropy and the surprisal, are tested and found to be useful quantities for these tasks. Future work will need to extend these parameters to consider further information and test their robustness with large data sets
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
A stochastic approach to detect fragmentation epoch from a single fragment orbit determination
Full text linkIn the last decades, the growing in-orbit population of resident space objects has become one of the main concerns for space agencies and institutions worldwide. In this context, fragmentations further contribute to increase the number of space debris and, operationally, it is fundamental to identify the event epoch as soon as possible, even when just a single fragment orbital state, resulting from an Initial Orbit Determination (IOD) process, is available. This work illustrates the Fragmentation Epoch Detector (FRED) algorithm, which deals with the problem through a stochastic approach, starting from a single fragment IOD result (expressed through mean state and covariance) and parent ephemeris (assumed as deterministic). The process populates the fragment ephemeris with a multivariate normal distribution and, for each couple sample-parent, the epochs of parent transit through the Minimum Orbital Intersection Distance (MOID) are first computed on a time window and then clustered in time. For each cluster, both the three-dimensional MOID and the three-dimensional relative distance distributions are derived, and their similarity is statistically assessed. Given that, at the actual fragmentation epoch, MOID and relative distance were equal, the cluster featuring the best matching between the two distributions is considered as the optimal candidate, and the related fragmentation epoch is returned from the time of parent transit through the MOID, in terms of mean and standard deviation. FRED algorithm performance is assessed through a numerical analysis. The algorithm robustness decreases when parent and fragment orbits share a similar geometry, and results get deteriorated if the perturbations and, moreover, the IOD errors are included in the process, but the correct fragmentation epoch is always present among candidates. Overall, FRED algorithm turns out to be a valid choice in operational scenarios, and a sensitivity analysis tests the algorithm out of the nominal conditions
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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