1,720,988 research outputs found
Calibration procedures of a vision-based system for relative motion estimation between satellites flying in proximity
No full textRelative Measurements systems represent a key technology for next generation space missions that require proximity operations between satellites. Before on-orbit validation, the sensors and algorithms need to be validated in laboratory employing a good fiducial reference of the relative motion and specific calibration procedures concerning the estimation of roto-translation matrices between different reference frames. This paper presents a set of calibration procedures that allow to assess the accuracy in estimating the relative pose between a Target spacecraft, equipped with a set of square markers, and an Inspector satellite moving in proximity and hosting a monocular camera. An external Motion Capture system is used to track the motion of a set of spherical markers attached to both the Target and the Inspector, providing a reliable fiducial reference for the relative pose between the two spacecraft. The proposed calibration procedures were tested using the SPARTANS hardware facility of the University of Padova
Energy and orbital stability in a partially-deployed earth space elevator
No full textThe Earth Space Elevator is an ingenious concept aimed at providing easy access to space, eliminating the need for rockets and potentially reducing drastically the launch costs. Technical advancements that could make the Space Elevator possible, among others, would be the availability of long carbon nanotubes or other super-strong and light material. This paper aims at addressing issues related with the stability of the system when the elevator is partially deployed starting from a nucleus in geostationary orbit (GEO). As noted by various authors, the energy increase needed to keep the orbital centre (where the gravity and centrifugal forces balance out) at GEO altitude during deployment while the tether is lengthened leads to an orbit with a positive orbital energy, i.e., the circular orbit has an energy level pertaining to a hyperbolic orbit. This situation leads to the instability of the unanchored system that would tend to escape from the geostationary orbit. The paper illustrates the change in orbital energy during assembly for a system made of a cylindrical tether and then investigate the stability of the coupled orbital and attitude motion in the orbital plane before anchoring. The analysis leads to the conclusion that the orbital motion instability ensues well before the orbital energy of the unperturbed system becomes positive
ROS 2 Integrated Continuous Terrain Mapping Comparison for Autonomous Navigation in Unstructured Environments
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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
Stereoscopic vision-based relative navigation for spacecraft proximity operations
No full textThe capability of a spacecraft of determining its dynamic state with respect to a target object is a key technology for performing spaceborne autonomous proximity operations. This core component is crucial in developing robust and reliable Guidance, Navigation and Control techniques in the field of spacecraft formation flight, on-orbit servicing, rendezvous and docking and active debris removal. This paper describes the development of a relative navigation strategy relying on the sole information provided by a stereoscopic vision system. The performances of the estimation approach are assessed by means of both orbital numerical simulations and experimental tests on the ground. The experimental activities are conducted in a representative dynamic environment within the context of the SPARTANS ground-based spacecraft simulator facility under development at the University of Padova
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