1,720,978 research outputs found
Range-only positioning of a deep-diving Autonomous Underwater Vehicle from a surface ship
No full textThis paper describes a method for the precise postprocessed
positioning of a deep-diving autonomous underwater vehicle
(AUV) using only a set of acoustic ranges from a surface ship
while the AUV executes a closed path under the ship. This approach
avoids the use of either a precisely calibrated (and consequently
expensive) ultrashort baseline (USBL), or a long baseline
(LBL) system (which is expensive in ship time to deploy). Results of
Monte Carlo simulation and field results from the first trials of the
Autosub6000 AUV are presented to support the hypothesis that an
AUV at 6000-m depth can be positioned to an accuracy commensurate
with global positioning system (GPS) quality or better, within
a period of 1 h
A New Collision Avoidance System for the Autosub6000 Autonomous Underwater Vehicle
No full textThe recently developed Autosub6000 Autonomous Underwater Vehicle (AUV) is the latest in the Autosub series of AUVs that
have been built at the National Oceanography Centre, Southampton. The previous AUV have been deployed on numerous science missions over the last decade. Autosub6000, with its 6000m depth capability, is currently in demand for deep ocean bathymetric surveys using its Simrad EM2000 multibeam sonar system. However, the terrain that can currently be surveyed is limited due to the lack of an obstacle avoidance system. In order to survey rugged and challenging regions a new obstacle avoidance system is being developed. This system uses a Tritech Seaking mechanically scanned sonar system oriented to scan vertically in front of the AUV to provide details of obstacles as well as changes in terrain. This paper describes the obstacle avoidance approach adopted and outlines its implemenation within Autosub6000. Simulation outputs showing the performance of the system are presented and results from trials of the Tritech Seaking sonar are also given
Terrain-Aided Navigation With Coarse Maps—Toward an Arctic Crossing With an AUV
No full textThe desire to conduct research in the Arctic on an ever-larger spatiotemporal scale has led to the development of long-range autonomous underwater vehicles (AUVs), such as the Autosub Long-Range 1500 (ALR1500). While these platforms open up a world of new applications, their actual use is limited in GPS-denied environments since self-contained navigation remains yet unavailable. In response, this study evaluates whether terrain-aided navigation (TAN) can enable multimonth deployments using basic navigation sensors and sparse bathymetric maps. To evaluate the potential, ALR1500 undertakes a hypothetical science-driven mission from Svalbard (Norway) to Point Barrow (Alaska, USA) under the sea ice (a mission over 3200 km). Therefore, a simulated environment is developed, which integrates a state-of-the-art model of water circulation, error models for heading estimation at high latitudes, and an Arctic bathymetric map. Recognizing that this map is constructed based on sparse depth measurements and interpolation techniques, a bathymetric uncertainty model is developed. The performance of the TAN algorithm is examined with respect to the type of the heading sensor utilized and a range of vertical map distortions, calculated using the developed bathymetric uncertainty model. Simulations show that unaided navigation experiences an error of hundreds of kilometers, whereas TAN provides acceptable accuracy given a moderate map distortion. By degrading the quality of the map further, it appears that the navigation filter may diverge when traversing large regions subject to interpolation. Therefore, a rapidly-exploring random tree star algorithm is used to design a new path such that the AUV traverses reliable and rich in topographic information areas
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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