Showing papers in "IEEE Journal of Oceanic Engineering in 2013"
TL;DR: In this paper, the authors proposed a statistical channel model which incorporates physical laws of acoustic propagation (frequency-dependent attenuation, bottom/surface reflections), as well as the effects of inevitable random local displacements.
Abstract: Underwater acoustic channel models provide a tool for predicting the performance of communication systems before deployment, and are thus essential for system design. In this paper, we offer a statistical channel model which incorporates physical laws of acoustic propagation (frequency-dependent attenuation, bottom/surface reflections), as well as the effects of inevitable random local displacements. Specifically, we focus on random displacements on two scales: those that involve distances on the order of a few wavelengths, to which we refer as small-scale effects, and those that involve many wavelengths, to which we refer as large-scale effects. Small-scale effects include scattering and motion-induced Doppler shifting, and are responsible for fast variations of the instantaneous channel response, while large-scale effects describe the location uncertainty and changing environmental conditions, and affect the locally averaged received power. We model each propagation path by a large-scale gain and micromultipath components that cumulatively result in a complex Gaussian distortion. Time- and frequency-correlation properties of the path coefficients are assessed analytically, leading to a computationally efficient model for numerical channel simulation. Random motion of the surface and transmitter/receiver displacements introduce additional variation whose temporal correlation is described by Bessel-type functions. The total energy, or the gain contained in the channel, averaged over small scale, is modeled as log-normally distributed. The models are validated using real data obtained from four experiments. Specifically, experimental data are used to assess the distribution and the autocorrelation functions of the large-scale transmission loss and the short-term path gains. While the former indicates a log-normal distribution with an exponentially decaying autocorrelation, the latter indicates a conditional Ricean distribution with Bessel-type autocorrelation.
436 citations
TL;DR: In this paper, a survey was conducted in northern Europe and covers the continental shelf, Norwegian fjords, a sheltered bay, a channel, and the Baltic Sea, and measurements were performed in various frequency bands between 2 and 32 kHz.
Abstract: Systematic measurements were performed to characterize shallow-water acoustic propagation channels for applications in the field of underwater communications. The survey was conducted in northern Europe and covers the continental shelf, Norwegian fjords, a sheltered bay, a channel, and the Baltic Sea. The measurements were performed in various frequency bands between 2 and 32 kHz. The outcome of the study is a variety of channels that differ in many ways, defying any attempt to define a typical acoustic communication channel. Miscellaneous forward propagation effects are presented, which are relevant to channel models for the design of modulation schemes, network protocols, and simulation environments.
127 citations
TL;DR: In this paper, the frequency response of the underwater optical channel in the forward direction for the purpose of wireless optical communications was measured out to 1 GHz over a wide range of water clarities (approximately 1-20 attenuation lengths).
Abstract: This paper describes a high-sensitivity, high-dynamic range experimental method for measuring the frequency response of the underwater optical channel in the forward direction for the purpose of wireless optical communications. Historically, there have been few experimental measurements of the frequency response of the underwater channel, particularly with regard to wireless communication systems. In this work, the frequency response is measured out to 1 GHz over a wide range of water clarities (approximately 1-20 attenuation lengths). Both spatial and temporal dispersions are measured as a function of pointing angle between the transmitter and the receiver. We also investigate the impact of scattering function and receiver field of view. The impact of these results to the link designer is also presented.
100 citations
TL;DR: A feature-based navigation (FBN) system using autonomous underwater vehicles equipped with low-cost sonar and navigation sensors and a novel simultaneous localization and mapping (SLAM) algorithm that detects and tracks features in the FLS images to renavigate to a previously mapped target.
Abstract: This paper describes a system for reacquiring features of interest in a shallow-water ocean environment, using autonomous underwater vehicles (AUVs) equipped with low-cost sonar and navigation sensors. In performing mine countermeasures, it is critical to enable AUVs to navigate accurately to previously mapped objects of interest in the water column or on the seabed, for further assessment or remediation. An important aspect of the overall system design is to keep the size and cost of the reacquisition vehicle as low as possible, as it may potentially be destroyed in the reacquisition mission. This low-cost requirement prevents the use of sophisticated AUV navigation sensors, such as a Doppler velocity log (DVL) or an inertial navigation system (INS). Our system instead uses the Proviewer 900-kHz imaging sonar from Blueview Technologies, which produces forward-looking sonar (FLS) images at ranges up to 40 mat approximately 4 Hz. In large volumes, it is hoped that this sensor can be manufactured at low cost. Our approach uses a novel simultaneous localization and mapping (SLAM) algorithm that detects and tracks features in the FLS images to renavigate to a previously mapped target. This feature-based navigation (FBN) system incorporates a number of recent advances in pose graph optimization algorithms for SLAM. The system has undergone extensive field testing over a period of more than four years, demonstrating the potential for the use of this new approach for feature reacquisition. In this report, we review the methodologies and components of the FBN system, describe the system's technological features, review the performance of the system in a series of extensive in-water field tests, and highlight issues for future research.
96 citations
TL;DR: This paper proposes a family of front-end receiver structures that utilize multiple-resampling (MR) branches, each matched to the Doppler scaling factor of a particular user and/or path, and proposes a gradient-descent approach to refine the channel estimates obtained by standard sparse channel estimators.
Abstract: In this paper, we focus on orthogonal frequency-division multiplexing (OFDM) receiver designs for underwater acoustic (UWA) channels with user- and/or path-specific Doppler scaling distortions. The scenario is motivated by the cooperative communications framework, where distributed transmitter/receiver pairs may experience significantly different Doppler distortions, as well as by the single-user scenarios, where distinct Doppler scaling factors may exist among different propagation paths. The conventional approach of front-end resampling that corrects for common Doppler scaling may not be appropriate in such scenarios, rendering a post-fast-Fourier-transform (FFT) signal that is contaminated by user- and/or path-specific intercarrier interference. To counteract this problem, we propose a family of front-end receiver structures that utilize multiple-resampling (MR) branches, each matched to the Doppler scaling factor of a particular user and/or path. Following resampling, FFT modules transform the Doppler-compensated signals into the frequency domain for further processing through linear or nonlinear detection schemes. As part of the overall receiver structure, a gradient-descent approach is also proposed to refine the channel estimates obtained by standard sparse channel estimators. The effectiveness and robustness of the proposed receivers are demonstrated via simulations, as well as emulations based on real data collected during the 2010 Mobile Acoustic Communications Experiment (MACE10, Martha's Vineyard, MA) and the 2008 Kauai Acomms MURI (KAM08, Kauai, HI) experiment.
93 citations
TL;DR: NEPTUNE Canada as mentioned in this paper is the world's first cabled ocean observatory, located in the northeast Pacific Ocean, off Canada's coast, which provides abundant power and high bandwidth communications into coastal to abyssal environments allows discrimination between short and long-term events, interactive experiments, real-time data and imagery, and multidisciplinary teams interrogating a vast database over 25 years.
Abstract: The first cabled ocean observatories demonstrate the challenges, benefits, and opportunities for ocean science and commercial applications. NEPTUNE Canada's 800-km subsea infrastructure and 130 diverse instruments established the world's first regional cabled ocean observatory, northeast Pacific Ocean, off Canada's coast. Introducing abundant power and high bandwidth communications into coastal to abyssal environments allows discrimination between short- and long-term events, interactive experiments, real-time data and imagery, and multidisciplinary teams interrogating a vast database over 25 years. The principal scientific themes addressed through the NEPTUNE Canada infrastructure are: plate tectonic processes and earthquake dynamics; dynamic processes of seabed fluid fluxes and gas hydrates; regional ocean/climate dynamics and effects on marine biota; deep-sea ecosystem dynamics; and engineering and computational research. Resulting data can be applied to important science issues such as ocean/climate change, ocean acidification, natural hazards, and nonrenewable and renewable natural resources. Socioeconomic benefits include many applications in sovereignty, security, transportation, data services, and public policy. The Data Management and Archive System has largely been developed internally. It controls the observatory network and gives transparent access using interoperability techniques within a Web 2.0 environment. The principal challenges encountered during design, installation, and operations involve technical innovations, enlarging the user base, management, securing funding, maximizing educational/outreach, and commercialization opportunities. Cabled ocean observatories are progressively wiring the oceans. Expandable in footprint, nodes, instruments, and scientific questions, they provide testing technology facilities and generate new research opportunities and socioeconomic benefits.
86 citations
TL;DR: In this paper, the Chinese Sea-wing underwater glider is introduced and the authors develop methods to increase its gliding range by optimizing the steady motion parameters to save energy and a sensor scheduling strategy accounts for the distributed features of vertical profiles so that the sampling resolution is adjusted to reduce energy consumption of sensing.
Abstract: Underwater gliders adjust buoyancy to generate gliding motion through water columns using a pair of wings. Various types of underwater gliders have been developed and have been tested as efficient long-distance, long-duration ocean sampling platforms. We introduce the Chinese Sea-Wing underwater glider and develop methods to increase its gliding range by optimizing the steady motion parameters to save energy. The methods are based on a model that relates gliding range to steady gliding motion parameters as well as energy consumption. A sensor scheduling strategy accounts for the distributed features of vertical profiles so that the sampling resolution is adjusted to reduce energy consumption of sensing. The effect of the proposed methods to increase gliding range is evaluated on the Sea-Wing glider. The proposed methods may be applicable to other types of underwater gliders.
80 citations
TL;DR: In this paper, a generic model of signal strength in underwater optical communications is presented, which includes light sources, detectors, amplifier and detector circuitry, optics, as well as a simple extinction model of the water channel.
Abstract: In this paper, we present a generic model of signal strength in underwater optical communications. The model includes light sources, detectors, amplifier and detector circuitry, optics, as well as a simple extinction model of the water channel. The end-to-end model provides insights into optimization approaches for underwater optical modems and enables relative pose estimation between underwater optical transmitters and receivers. We instantiate our model to the AquaOptical model by determining its parameters and verifying the model prediction in a suite of pool experiments.
76 citations
TL;DR: A new algorithmic framework for sparse channel identification is proposed that can be applied in any field where sequential noisy signal samples are obtained from a linear time-varying system and the clear superiority of the new algorithms over state-of-the-art sparse adaptive algorithms is shown.
Abstract: A new algorithmic framework for sparse channel identification is proposed. Although the focus of this paper is on sparse underwater acoustic channels, this framework can be applied in any field where sequential noisy signal samples are obtained from a linear time-varying system. A suit of new algorithms is derived by minimizing a differentiable cost function that utilizes the underlying Riemannian structure of the channel as well as the L0-norm of the complex-valued channel taps. The sparseness effect of the proposed algorithms is successfully demonstrated by estimating a mobile shallow-water acoustic channel. The clear superiority of the new algorithms over state-of-the-art sparse adaptive algorithms is shown. Moreover, the proposed algorithms are employed by a channel-estimate-based decision-feedback equalizer (CEB DFE). These CEB DFE structures are compared with a direct-adaptation DFE (DA DFE), which is based on sparse and nonsparse adaptation. Our results confirm the improved error-rate performance of the new CEB DFEs when the channel is sparse.
67 citations
TL;DR: This study presents a novel approach based on concurrent, bidirectional data packet exchange to improve the data transmission efficiency in underwater networks, and proposes an asynchronous handshaking-based MAC protocol, which is called biddirectional-concurrent MAC with packet bursting (BiC-MAC).
Abstract: The underwater acoustic channel is fundamentally different from the terrestrial wireless channel. Its unique characteristics, such as slow propagation speed and small bit rate distance product, present both challenges and opportunities for media access control (MAC) protocol design. In existing handshaking-based MAC protocols, each successful handshake only allows an initiating sender to transmit a single or multiple consecutive data packets to its intended receiver. In a long propagation delay environment, this unidirectional data transmission often results in extremely poor channel utilization due to the long waiting time for the handshake to complete. By exploiting the channel's unique characteristics, we present a novel approach based on concurrent, bidirectional data packet exchange to improve the data transmission efficiency. To further amortize the high latency overhead, we adopt a packet bursting idea, where a sender-receiver pair can exchange multiple rounds of bidirectional packet transmissions. Based on these strategies, we propose an asynchronous handshaking-based MAC protocol, which we call bidirectional-concurrent MAC with packet bursting (BiC-MAC). Via extensive simulations, we compare BiC-MAC against two representative unidirectional handshaking-based protocols, as well as several existing MAC protocols. We demonstrate that BiC-MAC can significantly increase channel utilization and offer performance gains in terms of both throughput and delay, while achieving a stable saturation throughput. Our study highlights the value of adopting bidirectional, concurrent transmission in underwater networks.
60 citations
TL;DR: In this article, a nonlinear dynamic model for the motion of a rigid vehicle in a dense fluid flow that comprises a steady, nonuniform component and an unsteady, uniform component is presented.
Abstract: In this paper, we present a nonlinear dynamic model for the motion of a rigid vehicle in a dense fluid flow that comprises a steady, nonuniform component and an unsteady, uniform component. In developing the basic equations, the nonuniform flow is assumed to be inviscid, but containing initial vorticity; further rotational flow effects may then be incorporated by modifying the angular rate used in the viscous force and moment model. The equations capture important flow-related forces and moments that are absent in simpler models. The dynamic equations are presented in terms of both the vehicle's inertial motion and its flow-relative motion. Model predictions are compared with exact analytical solutions for simple flows. Applications of the motion model include controller and observer design, stability analysis, and simulation of nonlinear vehicle dynamics in nonuniform flows. As illustrations, we use the model to analyze the motion of a cylinder in a plane laminar jet, a spherical Lagrangian drifter, and a slender underwater vehicle. For this last example, we compare predictions of the given model with those of simpler models and we demonstrate its use for flow gradient estimation. The results are applicable to not only underwater vehicles, but also to air vehicles of low relative density such as airships and ultralights.
TL;DR: In this paper, measurements and analysis of acoustic propagation effects demonstrate the shortcomings of narrowband channel models and demonstrate that correlative channel sounders preserve wideband properties, which renders them suitable for UWB channel simulation based on the principle of direct replay.
Abstract: Traditional channel models for communications research are designed for narrowband systems. Underwater acoustic communication systems use a bandwidth that is not small compared with the center frequency of the signal and qualify as ultrawideband (UWB) in a relative sense. In this paper, measurements and analysis of acoustic propagation effects demonstrate the shortcomings of narrowband channel models. These effects are frequency-dependent fluctuation rates and frequency-dependent attenuation, where the frequency dependence of the attenuation differs between paths. This frequency selectivity of the medium violates the assumption of uncorrelated taps and requires a UWB channel model. It is also shown that correlative channel sounders preserve wideband properties, which renders them suitable for UWB channel simulation based on the principle of direct replay.
TL;DR: In this paper, the authors developed effective joint policies involving routing and node replacement decisions to minimize the replacement costs per unit time, and compared the node replacement policies with the routing and routing policies.
Abstract: Ocean-bottom seismic systems are emerging as superior information-acquisition methods in seismic monitoring of petroleum reservoirs beneath ocean beds. These systems use a large network of sensor nodes that are laid on the ocean floor to collectively gather and transmit seismic information. In particular, underwater wireless sensor networks are gaining prominence in continuous seismic monitoring of undersea oilfields. They are autonomous and use wireless acoustic transmission for transferring data. However, the deployment period of such networks extends well beyond the battery lifetimes of the nodes. Hence, to ensure continuous monitoring from all node locations, it is required to replace the energy-depleted nodes on the ocean floor. Replacing these nodes at remote undersea locations is very expensive, and hence, the total node replacement cost in large seismic node networks is extremely high. In this paper, we develop effective joint policies involving routing and node replacement decisions to minimize the replacement costs per unit time. Our routing approach is simple and suitable for this application, and it strives for energy efficiency at all times. We propose a few node-replacement policies and compare their performances when they are combined with our proposed routings.
TL;DR: The first Chinese coastal acoustic tomography (CAT) experiment for mapping the tidal currents in Zhitouyang Bay near Zhoushan Island was successfully performed with seven acoustic stations from July 12 to 13, 2009 as discussed by the authors.
Abstract: The first Chinese coastal acoustic tomography (CAT) experiment for mapping the tidal currents in Zhitouyang Bay near Zhoushan Island was successfully performed with seven acoustic stations from July 12 to 13, 2009. Using CAT, the horizontal distributions of the tidal currents in the tomography domain were calculated by the inverse analysis, in which the travel time differences for sound traveling reciprocally between the station pairs are used as data. The specified tidal current structures, such as the strong east-west oscillation of the tidal current, the branched current, and the tidal vortices, were reconstructed as snapshots at the successive tidal phases. The relative vorticity calculated from the inverted current fields served to specify the current structures, such as tidal vortices. The inversion-estimated uncertainty of (0.02-0.08) m s-1 narrowed the root-mean-square difference (RMSD) of (0.00-0.11) m s-1 between the 3-min interval original data and the hourly mean data for all the sampled data, which may be regarded as a measure of error. Throughout the tidal phases, the divergence from the inverted current showed a positive (negative) distribution in the shallow (deep) region as an overall view. However, the divergence for the entire tomography domain was nearly equal to zero, corresponding to no net transport. This result implies that the observational errors are quite small for the present experiment. This experiment reaffirms that coastal acoustic tomography is an accurate and efficient observational method for continuously mapping tidal current structures in coastal regions that are characterized by heavy shipping traffic and active fishing.
TL;DR: Good agreement is found between bit error rates and packet error rates of in situ transmissions and simulated transmissions and the implication is that this type of channel simulator can be employed to test new modulation schemes in a realistic fashion without going to sea, except for the initial data collection.
Abstract: This paper discusses validation methods for underwater acoustic communication channel simulators, and validates direct and stochastic replay of underwater acoustic communication channels as implemented in a channel simulator called Mime. Direct replay filters an input signal directly with a measured time-varying impulse response, whereas stochastic replay filters an input signal with a synthetic impulse response consistent with the scattering function of the measured channel. The validation uses data from two sea experiments and a diverse selection of communication schemes. Good agreement is found between bit error rates and packet error rates of in situ transmissions and simulated transmissions. Long-term error statistics of in situ signaling are also reproduced in simulation when a single channel measurement is used to configure the simulator. In all except one comparison, the packet error rate in simulation is within 20% of the packet error rate measured on location. The implication is that this type of channel simulator can be employed to test new modulation schemes in a realistic fashion without going to sea, except for the initial data collection.
TL;DR: In this paper, the authors proposed a novel navigation method for underwater vehicles based on a single seafloor station (SS), with which the vehicles can estimate their positions and orientations with respect to the SS without the need of expensive inertial navigation system or time-consuming calibration.
Abstract: In this paper, we propose a novel navigation method for underwater vehicles based on a single seafloor station (SS), with which the vehicles can estimate their positions and orientations with respect to the SS without the need of expensive inertial navigation system or time-consuming calibration. This method is suitable for near-seafloor applications requiring real-time and accurate positioning, such as seafloor imaging and sampling. The method is also suitable for autonomous underwater vehicles (AUVs) since no other external aid is necessary other than SS. The key idea is to utilize mutual acoustical measurements between the vehicle and the SS. Simply explained: 1) the vehicle starts by interrogating the SS acoustically and measures the range between the two units as well as the bearing of the SS with respect to the vehicle in the vehicle reference frame; and then, 2) the SS computes the bearing of the vehicle with respect to the SS and transmits this information back to the vehicle using a similar acoustical device. By combining this information and inputting it into a nonlinear filter structure that includes measurements of the vehicle's ground velocity and yaw angular velocity, the vehicle computes its position and heading estimates. A pair of acoustical devices named acoustical localization and communication (ALOC) devices that can communicate and calculate their relative positions have been built. Sea trials were carried out in October 2011 using the AUV Tri-Dog1 (TD) and a trial SS at Kagoshima Bay in Japan. The AUV successfully navigated around the SS based on the measurements of the ALOC device mounted on both the AUV and the SS. The performance of the method was verified through simulations based on the experimental results.
TL;DR: It is shown how a software diagnosis model can be built automatically that can be integrated with the hardware model to diagnose the complete system and empirically that on Autosub 6000 this allows us to diagnose real vehicle faults that could potentially lead to the loss of the vehicle.
Abstract: This paper reports our results in using a discrete fault diagnosis system Livingstone 2 (L2), onboard an autonomous underwater vehicle (AUV) Autosub 6000. Due to the difficulty of communicating between an AUV and its operators, AUVs can benefit particularly from increased autonomy, of which fault diagnosis is a part. However, they are also restricted in their power consumption. We show that a discrete diagnosis system can detect and identify a number of faults that would threaten the health of an AUV, while also being sufficiently lightweight computationally to be deployed onboard the vehicle. Since AUVs also often have their missions designed just before deployment in response to data from previous missions, a diagnosis system that monitors the software as well as the hardware of the system is also very useful. We show how a software diagnosis model can be built automatically that can be integrated with the hardware model to diagnose the complete system. We show empirically that on Autosub 6000 this allows us to diagnose real vehicle faults that could potentially lead to the loss of the vehicle.
TL;DR: In this paper, the adaptive beamforming on the receiver side was used to reduce multipath interference and hence improve the signal-to-noise ratio (SNR) of a small, dense receiver array, and the results showed that with a small vertical array, a narrow mainlobe width is more important than low sidelobe levels, in terms of maximizing the SNR.
Abstract: Multipath propagation degrades the performance of active, bottom-imaging sonars in shallow-water environments. One way to avoid multipath interference is to use a vertical array with a narrow enough angular response to separate the direct bottom return from the multipath. However, this requires a large array and is often infeasible for practical reasons. In this study, we focus on the use of adaptive beamforming on the receiver side to reduce multipath interference and hence improve the signal-to-noise ratio (SNR). Using a small, dense receiver array, we apply classical and adaptive beamformers to real data collected by the NATO Undersea Research Centre in a shallow-water environment. Our results show that the adaptive minimum variance distortionless response (MVDR) beamformer offers an improvement in the estimated SNR compared to a conventional beamformer in most cases. However, the MVDR beamformer is suboptimal when the receiver consists of only a few elements. We propose using the low complexity adaptive (LCA) beamformer, which is based on the same optimization criteria as the MVDR beamformer, but is robust in a coherent environment without the need for spatial smoothing. For two to 4-element receivers, we observe an improvement of about 0.5-2.5 dB in the estimated SNR when using the LCA beamformer. In cases where the model indicates that the direct bottom return and the dominating multipath arrive from nearly the same angle, little or no improvement is observed. This is typically the case for first- or second-order multipaths reflected off the seafloor toward the receiver. The results from this study also show that with a small vertical array, a narrow mainlobe width is more important than low sidelobe levels, in terms of maximizing the SNR. Consequently, an unweighted conventional beamformer performs better than a conventional beamformer with a Hanning window applied for sidelobe suppression.
TL;DR: The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network.
Abstract: Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.
TL;DR: In this paper, a multiobjective optimization algorithm is employed to search for the optimal geometries where two cost criteria are used as the objective functions to be minimized, i.e., the submerged surface area to the maximum absorbed power, and the maximum reaction force to the maximal absorbed power.
Abstract: This paper deals with the optimization of wave absorbers oscillating about a fixed submerged horizontal axis. A multiobjective optimization algorithm is employed to search for the optimal geometries where two cost criteria are used as the objective functions to be minimized. The two cost criteria are the ratios, integrated over a specified frequency range, of the submerged surface area to the maximum absorbed power, and of the maximum reaction force to the maximum absorbed power. Geometric configurations with uniform simple cross-sectional shapes, viz. line, circle, and elliptical sections, are considered. For each configuration, the body dimensions and submergence, as well as the submergence of the rotation axis, are the variables to be optimized. It is found that most of the optimal geometries have their rotation axes close to the sea bottom and their bodies close to the free surface. The optimal size of the geometries varies depending on the selected wave frequency range, but the optimal cross-sectional dimensions are generally less than one third of the water depth when optimized over a uniform distribution of wave frequencies from 0.4 to 1.3 rad/s. Among the cross sections considered, the elliptical one performs best.
TL;DR: In this paper, the development of a numeric simulation for predicting ocean current turbine performance is presented along with performance predictions. Butler et al. used an unsteady blade element momentum (BEM) rotor model to calculate the rotor forces and seven degree-of-freedom (DOF) equations of motion to calculate coupled effects between the rotor and the main body.
Abstract: The development of a numeric simulation for predicting ocean current turbine performance is presented in this paper along with performance predictions. This numeric model uses an unsteady blade element momentum (BEM) rotor model to calculate the rotor forces and seven degree-of-freedom (DOF) equations of motion to calculate the coupled effects between the rotor and the main body. For the results presented in this paper, this simulation is set to model a 20-kW experimental ocean current turbine, and performance predictions are made for environmental condition that it will likely operate when deployed in the Gulf Stream off Southeast Florida. This model predicts that this turbine will have a maximum rotor power coefficient of 0.45 and that the vertical current gradient will only minimally affect the system performance. This simulation is also used to quantify the cyclic loadings that will be induced for misalignments between the rotor axis and the incoming flow, and it predicts the system motions and the forces on the rotor when the system is operating in a wave field.
TL;DR: In this article, an intelligent underwater vehicle (AUV) simulator with path-planning capability was developed, which automatically generates continuous-curvature paths of a cubic B-spline class constrained by the minimum turning radius of the marine vehicle and waypoints.
Abstract: An intelligent autonomous underwater vehicle (AUV) simulator with path-planning capability was developed. A guidance system of the simulator automatically generates continuous-curvature paths of a cubic B-spline class constrained by the minimum turning radius of the marine vehicle and waypoints. The simulator of the guidance system includes a line-of-sight (LOS) algorithm and a horizontal proportional-derivative (PD) controller, adapting the Euler-Rodriguez quaternion method on the base of 3-D Euler-Lagrange formulation. A web-based interactive simulation system can animate the attitudes and position of the AUV in real time. A 3000-T AUV was used to test the guidance system. Comparisons of linear and cubic path-planning strategies were discussed, including a straight line and a conventional cubic spline method, three parametric methods for planning cubic B-spline paths, and an iterative method for improving and expanding the function of the path generator. Simulation results of the tracking performance tests show that the AUV can precisely approach targets and waypoints using the proposed method. The improvement in the cross-tracking error was approximately 80%, whereas reduction in traveling time was 5%.
TL;DR: Two new algorithms for modeling the propagation of a known time series in a restricted class of time-varying environments are presented, which can address steady motion of the source and/or receiver and a moving sea surface that satisfies some simple constraints.
Abstract: In recent years, there have been notable technical advances in modulation schemes for underwater acoustic communications, and inexpensive commercial modems are now readily available. This has generated a renewed interest in modeling the effects of the underwater sound channel on the transmission of a known time series. The previously developed Virtual Timeseries Experiment (VirTEX) algorithm addressed the need for such models. It utilizes a sequence of ray-tracing computations on temporal snapshots of the environment. This approach can handle practical environments with arbitrary source, receiver, or sea-surface motion. While VirTEX can model the transmission of a known time series to any desired accuracy, its utility is offset by the computational resources required. In this paper, we present two new algorithms for modeling the propagation of a known time series in a restricted class of time-varying environments. The first algorithm can address steady motion of the source and/or receiver. The second algorithm can address a moving sea surface that satisfies some simple constraints. While more restrictive and less accurate than VirTEX, these new algorithms are significantly faster and more efficient. This makes them much more attractive for applications involving the modeling of extensive “what-if” scenarios. The algorithms can be implemented in software by postprocessing of the output from popular ray-tracing computer programs.
TL;DR: In this article, the authors developed a prototype of a 3000m class underwater glider for virtual mooring, which glides back and forth between the sea surface and the seabed collecting ocean data at a specific point.
Abstract: We are now developing a prototype of a 3000-m class underwater glider for virtual mooring. The vehicle glides back and forth between the sea surface and the seabed collecting ocean data at a specific point. Hydrodynamic forces acting on the half-size model were measured to determine the optimal wing shape. Next, to obtain the dynamical-hydrodynamic coefficients, forced oscillation tests were carried out using the optimally shaped model. Finally, the motions of the glider were simulated using the hydrodynamic coefficients obtained from these model experiments. The experimental and calculated results are shown in this paper.
TL;DR: In this article, the relationship between conductivity and sediment concentration was calibrated using known masses of neutrally suspended sediment in a heavy liquid and agreed well with existing linear and power-law relationships (r2 >; 0.98).
Abstract: A new sensor for measuring sediment concentration under sheet flow conditions is presented. Electrical conductivity, measured using a four-electrode method, is used as a proxy for sediment concentration. The relationship between conductivity and sediment concentration was calibrated using known masses of neutrally suspended sediment in a heavy liquid and agrees well with existing linear and power-law relationships (r2 >; 0.98). A 29-point conductivity profile at 1-mm resolution is generated by multiplexing through a vertical array of 32 plate electrodes. Numerical simulations of the voltage field around the sensor indicate that the horizontal extent of measurement volume is 1.5 times the sensor width. The finite extent of the measurement volume leads to smoothing of the vertical concentration profile. The sensor resolves sheet flow layers with a thickness greater than 3.5 mm, and a correction formula is introduced to correct the measured sheet thickness for the smoothing effect. Initial field results in the swash zone of a natural beach quantify sheet flow processes with unprecedented detail. Short-lived sheet flow with a maximum thickness of 19 mm was observed during the uprush, and a longer duration sheet flow with a maximum thickness of 8 mm was observed during the backwash.
TL;DR: In this article, the authors investigated the application of light field rendering (LFR) to images taken from a distributed bistatic nonsynchronous LLS imager using both line-of-sight and non-line of sight imaging geometries to create multiperspective rendering of an unknown underwater scene.
Abstract: Over the last several decades, developments in underwater laser line scan (LLS) serial imaging sensors have resulted in significant improvements in turbid water imaging performance. In the last few years, there has been renewed interest in distributed, truly multistatic time-varying intensity (TVI) (i.e., multiple transmitter nonsynchronous LLS) sensor configurations. In addition to being capable of high-quality image acquisition through tens of beam attenuation lengths, while simultaneously establishing a non-line-of-sight free-space communications link, these system architectures also have the potential to provide a more synoptic image coverage of larger regions of seabed and the flexibility to simultaneously examine a target from different perspectives. A related issue worth investigation is how to utilize these capabilities to improve rendering of the underwater scenes. In this regard, light field rendering (LFR)-a type of image-based rendering (IBR) technique-offers several advantages. Compared to other IBR techniques, LFR can provide signal-to-noise ratio (SNR) improvements and the ability to image through obscuring objects in front of the target. On the other hand, multistatic nonsynchronous LLS can be readily configured to acquire image sequences needed to generate LFR. This paper investigates the application of LFR to images taken from a distributed bistatic nonsynchronous LLS imager using both line-ofsight and non-line-of-sight imaging geometries to create multiperspective rendering of an unknown underwater scene. The issues related to effectively applying this technique to underwater LLS imagery are analyzed and an image postprocessing flow to address these issues is proposed. The results from a series of experiments at the Harbor Branch Oceanographic Institute at the Florida Atlantic University (HBOI-FAU, Fort Pierce, FL, USA) optical imaging test tank demonstrated the capability of using bistatic/multistatic nonsynchronous LLS system to generated LFR and, therefore, verify the proposed image processing flow. The benefits of LFR to underwater imaging in challenging environments were further demonstrated via imaging against a variety of obstacles such as mesh screens, bubbles, and water at different turbidity. Image quality metrics based on mutual information and texture features were used in the analysis of the experimental results.
TL;DR: This paper presents two iterative receivers, termed as multiuser detection (MUD)- based receivers and single-user detection (SUD)-based receivers, for a distributed orthogonal frequency-division multiplexing (OFDM) system with multiple quasi-synchronous users.
Abstract: This paper studies a distributed orthogonal frequency-division multiplexing (OFDM) system with multiple quasi-synchronous users, where different users may transmit different numbers of parallel data streams. The distinction from most existing work is that the multipath channels for different users have significantly different Doppler scales. Such a setting with two single-transmitter users was first studied in a recent publication by Tu et al. (2010). This paper presents two iterative receivers, termed as multiuser detection (MUD)-based receivers and single-user detection (SUD)-based receivers. The MUD-based receiver adopts a frequency-domain-oversampling front end on each receive element, then performs joint channel estimation and multiuser data detection iteratively. The SUD-based receiver adopts conventional single-user processing modules, but adds a critical step of multiuser interference (MUI) cancellation, where the MUI reconstruction explicitly considers different resampling factors used by different users. Experimental data sets from the 2010 Mobile Acoustic Communications Experiment (MACE10, Martha's Vineyard, MA) and the 2008 Surface Processes and Acoustic Communications Experiment (SPACE08) are used to emulate a distributed OFDM system with different numbers of users and different numbers of data streams per user. Performance results in different settings validate the effectiveness of the proposed iterative receivers.
TL;DR: The NEutrino Mediterranean Observatory-Submarine Network 1 (NEMO-SN1) seafloor observatory is located in the central Mediterranean Sea, Western Ionian Sea, off Eastern Sicily (Southern Italy) at 2100m water depth, 25 km from the harbor of the city of Catania as discussed by the authors.
Abstract: The NEutrino Mediterranean Observatory-Submarine Network 1 (NEMO-SN1) seafloor observatory is located in the central Mediterranean Sea, Western Ionian Sea, off Eastern Sicily (Southern Italy) at 2100-m water depth, 25 km from the harbor of the city of Catania. It is a prototype of a cabled deep-sea multiparameter observatory and the first one operating with real-time data transmission in Europe since 2005. NEMO-SN1 is also the first-established node of the European Multidisciplinary Seafloor Observatory (EMSO), one of the incoming European large-scale research infrastructures included in the Roadmap of the European Strategy Forum on Research Infrastructures (ESFRI) since 2006. EMSO will specifically address long-term monitoring of environmental processes related to marine ecosystems, climate change, and geohazards. NEMO-SN1 has been deployed and developed over the last decade thanks to Italian funding and to the European Commission (EC) project European Seas Observatory NETwork-Network of Excellence (ESONET-NoE, 2007-2011) that funded the Listening to the Deep Ocean-Demonstration Mission (LIDO-DM) and a technological interoperability test (http://www.esonet-emso.org). NEMO-SN1 is performing geophysical and environmental long-term monitoring by acquiring seismological, geomagnetic, gravimetric, accelerometric, physico-oceanographic, hydroacoustic, and bioacoustic measurements. Scientific objectives include studying seismic signals, tsunami generation and warnings, its hydroacoustic precursors, and ambient noise characterization in terms of marine mammal sounds, environmental and anthropogenic sources. NEMO-SN1 is also an important test site for the construction of the Kilometre-Cube Underwater Neutrino Telescope (KM3NeT), another large-scale research infrastructure included in the ESFRI Roadmap based on a large volume neutrino telescope. The description of the observatory and its most recent implementations is presented. On June 9, 2012, NEMO-SN1 was successfully deployed and is working in real time.
TL;DR: In this paper, a model-based channel simulation is performed by applying a ray tracer, together with a toolbox for generation of rough sea-surface evolutions, which can provide realistic results for both stationary and mobile communication nodes by capturing specific features observed in experiments.
Abstract: Sea-surface scattering by wind-generated waves and bubbles is regarded to be the main nonplatform related cause of the time variability of shallow acoustic communication channels. Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modeling of these dynamic sea-surface effects. For frequencies in the range of 1-4 kHz , there is an important effect of bubbles on sea-surface reflection loss due to refraction, which can be modeled with a modified sound-speed profile (SSP) accounting for the bubble void fraction in the surface layer. The bubble cloud then acts as an acoustic lens, enhancing the rough-surface scattering by the resulting upward refraction. It is shown here that, for frequencies in the considered range of 4-8 kHz, bubble extinction, including both the effects of bubble scattering and absorption, provides a significant additional contribution to the surface loss. Model-based channel simulations are performed by applying a ray tracer, together with a toolbox for generation of rough sea-surface evolutions. This practical simulation framework is demonstrated to provide realistic results for both stationary and mobile communication nodes by capturing specific features observed in experiments, such as time variability, fading reverberation tails, and wind-speed dependence of the Doppler power spectrum.
TL;DR: In this paper, an acoustic and visual system was developed to perform high-resolution surveys of the volumetric distribution of manganese crusts from an underwater vehicle, which was used to identify areas of exposed crust using the 3D reconstructions, and subsequently determine the thickness of the crusts based on the acoustic measurements.
Abstract: This paper describes acoustic and visual instruments developed to perform high-resolution surveys of the volumetric distribution of manganese crusts from an underwater vehicle. The instruments consist of an acoustic device, developed to perform in situ measurements of manganese crust thickness at depths of up to 3000 m, and a vision-based mapping system that generates 3-D color reconstructions of the seafloor. Methods to process the information obtained by these sensors to automatically identify areas of exposed crust using the 3-D reconstructions, and subsequently determine the thickness of the crusts based on the acoustic measurements, are described. Sea trials were performed at #5 Takuyo seamount with the systems mounted onboard the remotely operated vehicle Hyper-Dolphin during the NT10-11 cruise of the R/V Natsushima. The results are that the first time in situ measurements of manganese crust thickness have been performed, and it is demonstrated that, for the types of substrate dominant in the surveyed area, continuous acoustic measurement of manganese crust thickness is possible. The work described in this paper indicates that the proposed instruments and data processing algorithms can form useful tools to enable more efficient survey of manganese crusts.