1,721,032 research outputs found
An Acoustic Network Navigation System
No full textThis work describes a system for acoustic-based navigation that relies on the addition of localization services to underwater networks. The localization capability has been added on top of an existing network, without imposing constraints on its structure/operation. The approach is based on the inclusion of timing information within acoustic messages through which it is possible to know the time of an acoustic transmission in relation to its reception. Exploiting such information at the network application level makes it possible to create an interrogation scheme similar to that of a long baseline. The advantage is that the nodes/autonomous underwater vehicles (AUVs) themselves become the transponders of a network baseline, and hence there is no need for dedicated instrumentation. The paper reports at sea results obtained from the COLLAB–NGAS14 experimental campaign. During the sea trial, the approach was implemented within an operational network in different configurations to support the navigation of the two Centre for Maritime Research and Experimentation Ocean Explorer (CMRE OEX) vehicles. The obtained results demonstrate that it is possible to support AUV navigation without constraining the network design and with a minimum communication overhead. Alternative solutions (e.g., synchronized clocks or two-way-travel-time interrogations) might provide higher precision or accuracy, but they come at the cost of impacting on the network design and/or on the interrogation strategies. Results are discussed, and the performance achieved at sea demonstrates the viability to use the system in real, large-scale operations involving multiple AUVs. These results represent a step toward location-aware underwater networks that are able to provide node localization as a service
A Robust, Opportunistic Clock Synchronization Algorithm for Ad Hoc Underwater Acoustic Networks
No full textProliferation of deployed sea-going autonomous platforms, such as autonomous underwater vehicles (AUVs), unmanned surface vehicles (USV), and sensor nodes anchored to the seabed, make the deployment of true underwater acoustic networks more and more feasible. An important feature of any network is the ability to synchronize the clocks of the participants, for the purpose of, e.g., time-slotted media access control (MAC) and navigation. Terrestrial clock synchronization protocols, such as the well-established network time protocol (NTP), are not readily applicable to underwater acoustic networks, because of long propagation times, low packet delivery success rates, communication ranges that vary over time in an unpredictable manner, and, in the presence of mobile nodes, the ad hoc nature of the composition of the network. This paper proposes a continuous estimation of internode clock offset and drift, based on the continuous exchange of modem packets, possibly containing transmission and reception timestamps. The proposed solution takes explicitly into account the limitations of the acoustic communication channel and network node mobility. This robust, opportunistic clock synchronization (ROCS) is robust against modem reset, and will work even if packet delivery success rates are not optimal or if no communication is possible for extended periods of time. Experimental results are given from the COLLaborative Asw Behaviours-Next Generation Autonomous Systems (COLLAB-NGAS14) campaign, held October 19-31, 2014, off the west coast of Italy. During the sea trial, the proposed clock synchronization algorithm was deployed and successfully tested within an underwater acoustic network composed of mobile and fixed nodes
An Autonomous Underwater Vehicle Data-Driven Control Strategy for Target Tracking
No full textThis paper presents a data-driven approach to control the movement of autonomous underwater vehicles (AUVs) operating as receivers of a multistatic sonar surveillance network. The algorithm adopts a nonmyopic receding horizon policy to control the AUV heading, with the objective of minimizing the expected estimate error in the target location produced by the onboard tracker. Minimizing this error is of the utmost interest in target state estimation since it is one way of maintaining tracks. The proposed policy builds on an automated perception module which produces a target track and on an acoustic model to estimate the target measurement signal-to-noise ratio. Driven by the mission objectives, it explores the best decisions for the AUV by evaluating the evolution of the tactical situation over a future time window. Nonmyopic approaches can offer increased performance if compared to myopic ones. The downside is an increase of the computational burden. Methods are described to simplify and solve effectively the resulting decision tree to allow the execution of these kinds of computational intensive algorithms onboard autonomous vehicles. At the same time, they aim at producing AUV paths robust to possible misleading target measurements, which might cause meandering trajectories. These are crucial factors to use data-driven strategies for AUVs which must work in real applications. Results from the COLLAB13 sea trial are reported and discussed. They show both the feasibility of executing the algorithm in realtime on an onboard computer and the benefits of using the proposed approach over conventional predefined paths. These results are, to the best of our knowledge, the first successful demonstration at sea of a complex nonmyopic algorithm running in realtime onboard AUVs in a realistic multistatic littoral surveillance scenario
Continuous Active Sonars for Littoral Undersea Surveillance
No full textRecent advances in transducer and computing technology have pushed the concept of continuous active sonar (CAS) or high duty cycle sonar as an area of interest for application to antisubmarine warfare. Unlike conventional pulsed active sonars, CAS processing aims at detecting echoes while transmitting with a nearly 100% duty cycle. This paper describes the signal processing chain developed at the Centre for Maritime Research and Experimentation (CMRE), La Spezia, Italy, for real-time CAS processing within an autonomous networked multistatic sonar system. The algorithm uses subband processing, which can potentially provide a higher target update rate than the traditional pulsed active sonar, while maintaining the same search radius. The higher rate of contact information can improve target tracking performance. Performance results are given from the COLLAB-NGAS14 sea trial where the CAS processor was deployed on two autonomous underwater vehicles (AUVs) acting as receivers of the CMRE experimental multistatic demonstrator. Results show the feasibility of the CAS concept in littoral scenarios, using AUVs as real-time receivers
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Navigation of AUVs based on Ocean Fields Variability
No full textA key to enable long-endurance and large area missions for Autonomous Underwater Vehicles is the development of novel navigation methods. Typical solutions based on periodic surfacing or the deployment of static beacons suffer from a number of limitations ranging from interrupting the vehicle task to constrain the operational area. To overcome some of these limitations, this paper moves the first steps into a different direction and aims at using marine environmental information (e.g. temperature, salinity, etc.) as navigational aid for the robots. Towards this aim the paper presents a Particle Filter able to use temperature and salinity maps produced by state-of-the-art ocean models, and assesses the navigation performance over a week long simulated mission. The obtained numerical results show that the proposed approach is able to substantially bound the navigation error, and hence to support the navigation of underwater robots for long-range missions. Discussion on advantages, limitations and promising ways forward are also presented
Variations on the Author
Get PDF“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
Get PDFWe 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
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