Showing papers in "IEEE Journal of Oceanic Engineering in 2017"

ESO-Based Line-of-Sight Guidance Law for Path Following of Underactuated Marine Surface Vehicles With Exact Sideslip Compensation

Abstract: This paper presents a novel extended state observer (ESO)-based line-of-sight guidance law for path following of underactuated marine surface vehicles in the presence of time-varying sideslip angle. A reduced-order ESO is employed to identify the vehicle sideslip angle caused by constant ocean disturbances when following a curved path or time-varying ocean disturbances. This guarantees that the sideslip angle can be timely and exactly estimated, and thus contributes to following the desired path with higher accuracy regardless of external disturbances induced by wind, waves, and ocean currents. The input-to-state stability of the closed-loop system is established via cascade theory. It is proven that the transient learning process can be shorten by increasing the bandwidth of the ESO. Both simulation and experimental results are provided to validate the effectiveness of the method.

The Watermark Benchmark for Underwater Acoustic Modulation Schemes

Abstract: Watermark is a freely available benchmark for physical-layer schemes for underwater acoustic communications. It allows researchers to test and compare algorithms for the physical layer under realistic and reproducible conditions. The benchmark is a shell around the validated channel simulator Mime, which is driven by at-sea measurements of the time-varying impulse response. The first release of Watermark is issued with a library of channels measured in Norway (two sites), France, and Hawaii, offering three frequency bands (4–8, 10–18, and 32.5–37.5 kHz), single-hydrophone and array receivers, and play times varying from 33 s to 33 min.

Software-Defined Underwater Acoustic Modems: Historical Review and the NILUS Approach

Abstract: Flexible/adaptive acoustic modems that are reprogrammable/reconfigurable at all layers of the communication stack, either by a user or by means of autonomous decisions, are considered as an important enabler for interoperability and cognitive networking in the underwater domain. In this paper, we review the existing literature on software-defined acoustic modems (SDAMs) for underwater communications and networking, considering past and ongoing academic efforts, as well as industrial developments and European collaborations centered on software-defined modem structures and functionalities. We then zoom in on relevant R&D efforts currently taking place in a defense cooperation between The Netherlands and Norway, targeting the design of a software-defined modem for the NILUS MK 2 sensor node. This modem is built using general-purpose (GP) computing architectures running open-source operating system and tools, thereby making a further step toward software-defined open-architecture underwater acoustic modems. The first field tests of the NILUS MK 2 node in multihop underwater acoustic networks are presented, showing satisfactory performance in shallow and open waters.

Control of an Unmanned Surface Vehicle With Uncertain Displacement and Drag

Abstract: Experimental testing of an unmanned surface vehicle (USV) has been performed to evaluate the performance of two low-level controllers when displacement and drag properties are time varying and uncertain. The USV is a 4.3-m-long, 150-kg wave adaptive modular vessel (WAM-V) with an inflatable twin-hull configuration and waterjet propulsion. Open-loop maneuvering tests were conducted to characterize the dynamics of the vehicle. The hydrodynamic coefficients of the vehicle were determined through system identification of the maneuvering data and were used for simulations during control system development. The resulting controllers were experimentally field tested on-water. Variable mass and drag tests show that the vehicle is best controlled by a model reference adaptive backstepping speed and heading controller. The backstepping controller developed by Liao et al. (2010) is modified to account for an overprediction of necessary control action and motor saturation. It is shown that when an adaptive algorithm is implemented for the surge control subsystem of the modified backstepping controller, the effects of variable mass and drag are mitigated.

Manipulation and Transportation With Cooperative Underwater Vehicle Manipulator Systems

Abstract: Autonomous underwater manipulation has been a topic of interest since the early 1990s. In the past few years, several milestone projects such as SAUVIM and TRIDENT have demonstrated autonomy capabilities for a single underwater vehicle manipulator system (UVMS) in performing simple manipulation tasks, e.g., the recovery of an object from the seafloor. The Italian funded MARIS project aims to extend some of these results to multiple UVMSs performing a cooperative transportation task of a long object such as a pipe. This paper presents the results achieved in developing a unifying architecture for the control of both individually and cooperatively operating UVMSs which explicitly makes use of a limited amount of information exchange between the agents, which is needed due to the severe bandwidth limitations of the underwater acoustic communications. A complete execution of the reference transportation mission is presented to support the proposed distributed algorithm. Furthermore, hydrodynamic simulations of the cooperative transportation phase are presented and an analysis of the achievable performances as different communication schemes are employed is given.

A USV-Based Automated Launch and Recovery System for AUVs

Abstract: In this paper, a concept design for automated launch and recovery (L&R) of a small autonomous underwater vehicle (AUV) from an unmanned surface vehicle (USV) is described and its feasibility is assessed through modeling and simulation. The design is being implemented on a selected catamaran USV for deployment of a REMUS 100 AUV. The concept for launch involves lowering the AUV into the water from the center of the USV while the latter is in motion. Recovery and retrieval involves aligning the two vehicles together through acoustic positioning, lowering of a thin line with an outrigger-type depressor wing from a winch on the USV, latching of the line by the AUV using a custom pincer-type onboard mechanism, and subsequent reeling of the line for vehicle retrieval. The USV being considered can operate effectively in low to moderate sea states. The length of the recovery line can be adjusted to facilitate AUV recovery from deeper locations beneath the USV in higher sea state. The L&R process is modeled in OrcaFlex to assess its feasibility and to conduct a parametric study in support of developing optimal operations of the launch and recovery system (LARS). The resulting system is being implemented on a USV.

Robust Adaptive Control for Dynamic Positioning of Ships

Abstract: In this paper, a robust adaptive control scheme with the global asymptotic stability with respect to positioning errors is proposed for dynamic positioning (DP) of ships in the presence of time-varying unknown bounded environmental disturbances. The unknown environmental disturbances are expressed as the outputs of a linear exosystem with unknown parameters and all eigenvalues of system matrix lying on the imaginary axis. On the basis of this exosystem, the disturbances are further represented as the outputs of a linear model of canonical form with unknown disturbances being inputs by a multivariate linear regression model whose regressor is the state vector of the linear model and whose regression parameters depend on unknown parameters of the linear exosystem. This representation allows us to construct an observer to estimate the unavailable state vector (regressor) in the linear model and hence convert the disturbance compensation control for the DP of ships to an adaptive control problem. Then, a robust adaptive control law for the DP of ships is designed incorporating the constructed observer and the projection algorithm into the vectorial backstepping method. The global asymptotic stability with respect to positioning errors of the DP closed-loop control system is proved applying Lyapunov stability theory and Barbalat's lemma. Finally, simulation results on a supply ship Northern Clipper in two different disturbance cases and simulation comparisons with an existing DP adaptive robust control scheme demonstrate more effectiveness and less conservativeness of our proposed control scheme.

Task Space Control of an Autonomous Underwater Vehicle Manipulator System by Robust Single-Input Fuzzy Logic Control Scheme

Abstract: In this paper, a robust single-input fuzzy logic control Robust Single Input Fuzzy Logic Controller (RSIFLC) scheme is proposed and applied for task-space trajectory control of an autonomous underwater vehicle manipulator system (AUVMS) employed for underwater manipulation tasks. The effectiveness of the proposed control scheme is numerically demonstrated on a planar underwater vehicle manipulator system [consisting of an underwater vehicle and a two link rotary (2R) serial planar manipulator]. The actuator and sensor dynamics of the system are also incorporated in the dynamical model of an AUVMS. The proposed control law consists of a feedforward term to exaggerate the control activity with immoderation from the known desired acceleration vector and an estimated perturbed term to compensate for the unknown effects namely external disturbances and unmodeled dynamics as a first part and a single-input fuzzy logic control as a feedback portion to enhance the overall closed-loop stability of the system as a second part. The primary objective of the proposed control scheme is to track the given end-effector task space trajectory despite of external disturbances, system uncertainties, and internal noises associated with the AUVMS. To show the efficacy of the proposed control scheme, comparison is made with conventional fuzzy logic control (CFLC), sliding mode control (SMC), and proportional–integral–derivative (PID) controllers. Simulation results confirmed that with the proposed control scheme, the AUVMS can successfully track the given desired spatial trajectory and gives better and robust control performance.

Satellite-Based Offshore Wind Resource Assessment in the Mediterranean Sea

Abstract: Identification of prominent sea areas for the efficient exploitation of offshore wind energy potential requires primarily the assessment and modeling of several aspects of the long-term wind climate. In this work, the offshore wind speed and wind direction climate of the Mediterranean Sea is analytically described, the corresponding offshore wind energy potential is estimated on an annual and seasonal basis, and candidate areas for potential offshore wind farm development are identified. The analysis is based on ocean surface wind fields obtained from the Blended Sea Winds product, provided by the U.S. National Oceanic and Atmospheric Administration (NOAA), from 1995 to 2014. The satellite data are evaluated with reference to buoy wind measurements in the Spanish and Greek Seas. Wind data analysis reveals areas in the western and eastern Mediterranean Sea with high mean annual wind speed combined with rather low temporal variability. The obtained results suggest that offshore wind power potential in the Mediterranean Sea is fairly exploitable at specific suitable locations, such as the Gulf of Lions (with mean annual wind power density up to $\sim $ 1600 W/m ${}^{2}$ ) and the Aegean Sea (with mean annual wind power density up to $\sim $ 1150 W/m ${}^{2}$ ), that are certainly worth further in-depth assessment for exploiting offshore wind energy. Finally, based on the available offshore wind resource potential and the water depth suitability, three specific sites (in the Gulf of Valencia and the Adriatic and Ionian Seas) are selected and the average wind power output for a specific wind turbine type is estimated.

Throughput-Efficient Super-TDMA MAC Transmission Schedules in Ad Hoc Linear Underwater Acoustic Networks

Abstract: Underwater acoustic (UWA) sensor network deployments may be used in many applications for environmental, scientific, military, and commercial purposes. Several topologies are in use, but the most widely used topology is linear. Indeed, placing nodes on a single line offers more opportunities in terms of large coverage and high-rate services. The UWA channel is a shared medium. Thus, a medium access control (MAC) protocol is necessary, primarily to regulate and coordinate nodes’ access. MAC protocol design should take into consideration large propagation delays to favor better network throughput. Performance of most developed protocols in linear topologies does not exceed 1 in terms of normalized network throughput, or equivalently, channel utilization. We explore transmission schedules in three important contexts. 1) Single collision domain with unicast traffic. In an $N$ -node network, we develop transmission schedules achieving a normalized network throughput of $2 - (2/N)$ . This is the best that can be done in such a context, as demonstrated using a general greedy approach combined with an exhaustive search for small-size networks. 2) Single collision domain with broadcast traffic. We propose a periodic per-node fair schedule with the shortest period. Achievable throughput in such conditions is close to $N/2$ . Likewise, we prove that the throughput is upper bounded by $N - 1$ under the per-node fairness constraint. 3) Partially overlapping collision domains with unicast traffic. We consider a simple illustration of such a configuration. The proposed transmission schedule depicts a scenario where messages originate at one end of the network, and are sequentially relayed node by node (i.e., hop by hop) in the direction of the final destination located at the other end of the network. Furthermore, for all three discussed contexts, we build up computationally efficient algorithms that generate transmission schedules regardless of network size. We explore the idea of exploiting nonzero propagation delays for linear topologies to improve network throughput. In recent UWA sensor networks, the linear topology is a fundamental component that may be used to build more complex network configurations. This study would then serve as a base for future research into this area.

Toward the Development of Secure Underwater Acoustic Networks

Abstract: Underwater acoustic networks (UANs) have been recognized as an enabling technology for various applications in the maritime domain. The wireless nature of the acoustic medium makes UANs vulnerable to various malicious attacks, yet, limited consideration has been given to security challenges. In this paper, we outline a hybrid architecture that incorporates aspects of physical layer security, software defined networking, node cooperation, cross-layering, context-awareness, and cognition. The proposed architecture envisions strategies at the node as well as at the network level that adapt to environmental changes, the status of the network and the possible array of attacks. Several examples of attacks and countermeasures are discussed while deployment and functionality issues of the proposed architecture are taken into consideration. This work is not intended to represent a whatsoever proven solution but mainly to suggest future research directions to the scientific community working in the area of UANs.

Multisensor Acoustic Tracking of Fish and Seabird Behavior Around Tidal Turbine Structures in Scotland

Abstract: Despite rapid development of marine renewable energy, relatively little is known of the immediate and future impacts on the surrounding ecosystems. Quantifying the behavior and distribution of animals around marine renewable energy devices is crucial for understanding, predicting, and potentially mitigating any threats posed by these installations. The Flow and Benthic Ecology 4D (FLOWBEC) autonomous seabed platform integrated an Imagenex multibeam echosounder and a Simrad EK60 multifrequency echosounder to monitor marine life in a 120 $^{\circ}$ sector over ranges up to 50 m, seven to eight times per second. Established target detection algorithms fail within MRE sites, due to high levels of backscatter generated by the turbulent physical dynamics, limiting and biasing analysis to only periods of low current speed. This study presents novel algorithms to extract diving seabirds, fish, and fish schools from the intense backscatter caused by turbulent dynamics in flows of 4 m s $^{{-1}}$ . Filtering, detection, and tracking using a modified nearest neighbor algorithm provide robust tracking of animal behavior using the multibeam echosounder. Independent multifrequency target detection is demonstrated using the EK60 with optimally calculated thresholds, scale-sensitive filters, morphological exclusion, and frequency-response characteristics. This provides sensitive and reliable detection throughout the entire water column and at all flow speeds. Dive profiles, depth preferences, predator–prey interactions, and fish schooling behavior can be analyzed, in conjunction with the hydrodynamic impacts of marine renewable energy devices. Coregistration of targets between the acoustic instruments increases the information available, providing quantitative measures including frequency response from the EK60, and target morphology and behavioral interactions from the multibeam echosounder. The analyses draw on deployments at a tidal energy site in Scotland to compare the presence and absence of renewable energy structures across a range of physical and trophic levels over complete spring-neap tidal cycles. These results can be used to inform how animals forage in these sites and whether individuals face collision risks. This quantitative information can de-risk the licensing process and, with a greater mechanistic understanding at demonstration scales, its predictive power could reduce the monitoring required at future arrays.

Superposition Coding for Downlink Underwater Acoustic OFDM

Abstract: Superposition coding (SC) is a nonorthogonal scheme for downlink communications, in which all users are allowed to use the full degrees of freedom of the channel and successive interference cancellation (SIC) is adopted for user decoding. Combining SC with orthogonal frequency-division multiplexing (OFDM) modulation, the optimal resource allocation depends on the perfect channel state information (CSI) of all users at the transmitter, which is hard to obtain for underwater acoustic (UWA) channels. In this paper, we propose a practical OFDM-modulated SC scheme for downlink UWA communications, where the transmitter splits the power between two users based on statistical CSI. The expressions to characterize the boundary of the ergodic rate region achievable by the proposed scheme over long codewords are presented first, followed by the analysis of outage probability when coding is applied within one OFDM block. Then we examine the performance of SC in an OFDM-modulated system with practical coding and modulation pairs. Simulation results show that the OFDM-modulated SC scheme outperforms the orthogonal frequency-division multiple access (OFDMA) in performance of both block error rate (BLER) and spectral efficiencies under different data rate pairs. Recorded data from both medium-range and short-range sea tests verify that channel statistics are stable over a long period of time and can be used to assist resource allocation for the proposed scheme. Compared with OFDMA, considerable increase of spectral efficiencies has been found for SC based on experimental data when both users with disparate channels have nonzero data rates.

Three-Dimensional Target Reconstruction From Multiple 2-D Forward-Scan Sonar Views by Space Carving

Abstract: Building 3-D object models from 2-D images is a key capability for target classification and identification, reacquisition, and environmental mapping, among many applications in underwater with poor visibility. We present a novel approach in utilizing multiple 2-D forward-look sonar images from known sonar poses to localize an acoustically opaque target and reconstruct its 3-D shape. Based on projections onto various images, the 3-D space not occupied by an imaged target within the sonar field of view is sequentially carved out, leaving the remaining volume as the estimate of the 3-D object region. The estimation generally improves with information from new distinct views, and moreover with images acquired through sonar roll motions, rather than circumnavigating the target. Computer simulations allow assessing the convergence properties and performance of the approach for convex and concave polygons. Additionally, results from experiments with real images of amorphous coral rocks and a miniature wood table demonstrate performance in the 3-D modeling of small objects with varying reflectance properties.

Predictive Evaluation of Ship Collision Risk Using the Concept of Probability Flow

Abstract: This paper presents a semianalytical approach for evaluating the collision risk between two moving surface ships. The concept of probability flow is introduced to develop an analytically sound problem formulation, which allows for an accurate estimation of collision probability considering time-varying ship trajectory uncertainties. For efficient computation, the flow of collision probability is separated into diffusion and drift components. These two probability components are combined to obtain the instantaneous collision probability, and this instantaneous probability is integrated in time to quantify the expected risk of collision. To demonstrate the feasibility of the proposed approach, traffic simulations are performed for several representative maritime traffic scenarios and the obtained simulation results are discussed.

A Pressure Sensory System Inspired by the Fish Lateral Line: Hydrodynamic Force Estimation and Wall Detection

Abstract: Studies have shown that many behavioral decisions of fish are facilitated by the lateral line system which provides hydrodynamic information about the surrounding fluid. Inspired by the functionality of the system, a distributed pressure sensory system is developed. The system is intended for use on autonomous underwater vehicles to aid station keeping and accurate maneuvering by allowing the vehicle to react to the changes in the fluid environment before they result in body perturbation. The system can also be used for mission level decision making such as obstacle detection. This paper presents a prototype sensory system using differential, as opposed to absolute or gauge, pressure sensors as the sensing elements, which allows for higher measurement precision. Experimental tests are designed to characterize the system's ability to estimate the hydrodynamic force and to detect the presence of a wall. The hydrodynamic force estimated by the system is validated with an independent force measuring apparatus. The impending wall could also be detected by analyzing the pressure distribution obtained from the sensory system. Particularly, it is found that the wall distance and angle may be inferred by comparing the amplitude and phase of the Fourier components in the pressure distribution against those without the obstacle.

JANUS-Based Services for Operationally Relevant Underwater Applications

Abstract: This paper presents the design, test, and experimentation at sea of four JANUS-based services for operationally relevant underwater applications: 1) first contact and language switching; 2) transmission of automatic identification system data to submerged assets; 3) transmission of meteorological and oceanographic data to underwater vessels; and 4) support in distressed submarine operations. On March 24, 2017, JANUS was promulgated as a NATO standard (STANAG 4748) [1] , marking the first time that a digital acoustic communications protocol is adopted at international level. JANUS is an open, simple, and robust modulation and coding scheme developed by the NATO STO Centre for Maritime Research and Experimentation, in collaboration with academia and industry. The implementation of the services presented in this paper is based on a standardized protocol and offers the potential to widely increase the safety and efficiency in maritime operations. The objective of this paper is to demonstrate that JANUS can be used in support of maritime operations, potentially increasing their efficiency. Such demonstration is achieved through experimentation at sea of four operationally relevant JANUS-based services. The four JANUS-based services discussed were validated during two different sea trials: REP15-Atlantic and REP16-Atlantic. During those trials, various heterogeneous configurations were considered, including the use of a state-of-the-art diesel-electric submarine, guaranteeing maximum relevance for validation and evaluation of the designed solutions. The collected results demonstrate that JANUS is a viable solution for operationally relevant underwater applications, thus validating the objective of this paper. Additionally, encouraging feedback has been provided by the operational community participating to the trials. The capabilities demonstrated served as an initial proof of concept and will certainly lead to newer requirements and eventually even more functionalities.

Bounds for Low Probability of Detection for Underwater Acoustic Communication

Abstract: Most military underwater activities require stealth operations, and hydroacoustic transmissions might temper the mission. For this reason, military underwater acoustic transmission aims for low probability of detection (LPD). LPD communication systems are characterized by low transmission power and reception at low signal-to-noise power ratio (SNPR). As a side effect, LPD decreases acoustic noise pollution. In this paper, we study the LPD capability of underwater acoustic communication (UWAC) systems. We consider an interceptor with target false alarm and detection probabilities and a legitimate receiver with required detection and packet error probabilities. For the interceptor, we focus on an energy detector and calculate the minimal SNPR which satisfies its required performance. For the legitimate communication link, we consider spread-spectrum signaling with several modulation techniques, and calculate the minimal required SNPR to allow reliable detection and decoding of a data packet. Based on these minimal SNPR terms and using bounds on power attenuation in the channel, we quantify the LPD capability of the system and analyze the effect of channel and communication parameters. We also present results from a sea trial conducted in the Saanich Inlet off the coast of Vancouver Island, where we conducted LPD experiments for different transmission powers and carrier frequencies. We provide bounds for the minimal bandwidth and maximal transmission rate for LPD communication. Our results also show that LPD UWAC performs better in shallow and warm water, and for transmission at low carrier frequencies.

Synthetic Aperture Sonar Track Registration Using SIFT Image Correspondences

Abstract: Repeat–pass synthetic aperture sonar (SAS) imaging for change detection and interferometry relies on accurate image registration. SAS image registration is usually performed using a slow correlation-based method, whereas feature-based registration is becoming more prevalent for synthetic aperture radar due to its relative computational efficiency. In this paper we propose a sonar track registration method, from which the estimated track parameters can be used to compute an image registration. The proposed method uses the scale-invariant feature transform (SIFT), a popular algorithm for feature detection and matching, and is demonstrated on a simulated repeat–pass pair of SAS images. A model of the ideal sonar track geometry is presented. A least squares method is formulated for estimating the track registration from a set of noisy feature correspondences, where random sample consensus (RANSAC) is first used to reject outliers. The resulting track parameter estimate corresponds to an image registration aligned to within 0.03 pixels over the scene, which is within the 0.1-pixel resolution recommended for interferometric processing. The ground truth for the simulated ideal data is precisely known, which allowed for the subpixel localization accuracy of SIFT correspondences to be objectively evaluated. The interaction between the RANSAC error threshold and the misregistration error was also evaluated and found to be chaotic. Overall, our results indicate the potential for features to be used for faster coarse registration to reduce the computation time of a more accurate subsequent correlation-based registration.

Mitigation of Underwater Radiated Noise Related to Shipping and Its Impact on Marine Life: A Practical Approach Developed in the Scope of AQUO Project

Abstract: There is increasing concern within the scientific community about the underwater noise due to anthropogenic activity and its impact on marine life, with negative consequences on biodiversity and sea resources. In that context, the European Marine Strategy Framework Directive stated in 2008 that the anthropogenic noise due to shipping was to be mitigated. To address this issue, the European Union (EU) project “Achieve QUieter Oceans by shipping noise footprint reduction” (AQUO) ( www.aquo.eu ) started in October 2012 with a duration of four years. The project brought together experts from shipbuilding, underwater acoustics, and bioacoustics, with a multidisciplinary approach. In this paper, after giving a brief overview of the project structure, the methodology proposed by the AQUO project to set guidelines for controlling the underwater noise from commercial shipping is presented in more detail. Such a methodology is aimed at identifying the most promising strategies for the mitigation of the impact of shipping noise on marine fauna. Different technical as well as operational solutions are evaluated by taking into account the impact on marine life, the feasibility in terms of ship design, and the cost effectiveness, also considering fuel efficiency. While technical solutions are usually more effective at the design stage both in terms of costs and performance, operational solutions can potentially be adopted without any modification to the existing fleet. Furthermore, operational prescriptions can be set by national/local authorities who cannot directly intervene on ship configurations. The different solutions have been evaluated by means of numerical modeling carried out by using a Noise Footprint Assessment Model derived from the Quonops tool.

An Autonomous Water Monitoring and Sampling System for Small-Sized ASVs

Abstract: In recent years, sensorized autonomous vehicles (either AUVs or ASVs) have been increasingly used for in situ water measurements. However, collection of water samplings at depth by small-sized ASVs and their subsequent physical/chemical analysis onboard remains difficult due to size and weight constraints. This paper addresses this issue and describes the design and testing of an autonomous water monitoring and sampling system intended for operations onboard small-sized man-portable ASVs. The system is designed to collect water samples up to 50 m in depth and to measure physical water parameters along the water column. The system is composed of a probe lowered by a winch measuring physical water parameters and able to collect water samples at different selectable depths. Once the probe is returned onboard, a water distribution system transfers the collected water samples to sensors lodged in the ASV for the monitoring of chemical parameters, or into containers to transport the samples to laboratories on the mainland. The system combines small dimensions, self-cleaning capabilities, low weight, and limited power consumption allowing it to be easily installable and used on an autonomous small-sized ASVs. The sampling system was installed and tested in a robotic small-sized catamaran belonging to the HydroNet ASV class.

Design and Deployment of a Wireless Sensor Network for the Mar Menor Coastal Observation System

Abstract: The Coastal Ocean Observation System of Murcia Region (OOCMUR) was established in 2008 as a major scientific and technological infrastructure in Spain with the main objective of studying the impact of global climate change in the Mediterranean. The coastal lagoon of Mar Menor in southeast Spain was chosen as the first region to be monitored because it is one of the most hypersaline coastal lagoons in the Mediterranean, with a limited exchange of water with the open sea, and it is the largest in Europe. Wireless sensor networks (WSNs) offer an efficient and innovative solution for oceanographic monitoring, allowing a higher density sensor deployment, at a lower cost. This paper presents the design of an ad hoc WSN system and a control software for Mar Menor monitoring using a buoy structure with sensors, energy harvesting, and communications platform. The study focuses on the oceanographic interest of the selected marine area, details of network deployment, the custom-designed sensor nodes, and the results of system operation.

Automatic Detection of Marine Gas Seeps Using an Interferometric Sidescan Sonar

Abstract: There is a significant need for reliable, cost-effective, and preferably automatic methods for detecting and monitoring marine gas seeps. Seeps at the seafloor may originate from natural sources including sediments releasing biogenic methane and volcanoes releasing CO2, or from man-made constructions such as pipelines or well heads, and potentially also from subseafloor CO2 storage sites. Improved seep detection makes it possible to estimate the amount of greenhouse gases entering the oceans, and to promptly detect and address potential leaks to reduce environmental and economical consequences. Sonar is an excellent tool for seep detection due to the strong acoustic backscatter properties of gas-filled bubbles in water. Existing methods for acoustic seep detection include multibeam and sidescan surveying, as well as active and passive sensors mounted on a stationary platform. In this work, we develop a new method for automatic seep detection using an interferometric sidescan sonar. We apply signal processing techniques combined with knowledge about acoustical and spatial properties of seeps for improved detectability. The proposed method fills an important gap in existing technology—the ability to automatically detect a seep during a single pass with an autonomous underwater vehicle (AUV) equipped with an interferometric sidescan sonar. Results from simulations as well as field data from two leaking abandoned wells in the North Sea indicate that small seeps are consistently detected on a sandy seafloor even when the observation time is limited (a single pass with the AUV). We explore the detection capability for different seafloor types ranging from silt to gravel.

Frequency-Domain Turbo Equalization With Iterative Channel Estimation for MIMO Underwater Acoustic Communications

Abstract: This paper proposes a new iterative receiver for single-carrier multiple-input–multiple-output (SC-MIMO) underwater acoustic (UWA) communications, which utilizes frequency-domain turbo equalization (FDTE) and iterative channel estimation. Soft-decision symbols are not only fed back to the equalizer to cancel the intersymbol interference (ISI) and cochannel interference (CCI), but also used as training signals in the channel estimator to update the estimated channel state information (CSI) after each turbo iteration. This iterative channel estimation scheme helps to combat the problem commonly suffered by block-processing receivers in fast time-varying channels. Compared with time-domain turbo equalization, FDTE achieves comparable performance with significantly reduced computational complexity. Using soft-decision symbols to reestimate the time-varying channels, iterative channel estimation further improves the accuracy of the estimated CSI. The proposed iterative receiver has been verified through undersea experimental data collected in the Surface Processes and Acoustic Communications Experiment 2008 (SPACE08).

Coherent Multipath Direction-of-Arrival Resolution Using Compressed Sensing

Abstract: For a sound field observed on a sensor array, performance of conventional high-resolution adaptive beamformers is affected dramatically in the presence of coherent multipath signals, but the directions-of-arrival (DOAs) and power levels of these arrivals can be resolved with compressed sensing (CS). When the number of multipath signals is sufficiently small, a CS approach can be used by formulating the problem as a sparse signal recovery problem. CS overcomes the difficulty of resolving coherent arrivals at an array by directly processing the sensor outputs without first estimating a sensor covariance matrix. CS is compared to the adaptive minimum-variance-distortionless-response (MVDR) spatial processor with spatial smoothing. Though spatial smoothing produces improved results by preprocessing the sensor array covariance matrix to decorrelate the coherent multipath components, it reduces the effective aperture of the array and hence reduces the resolution. An empirical study with a uniform linear array (ULA) demonstrates that CS outperforms MVDR beamformer with spatial smoothing in terms of spatial resolution and bias and variance of DOA and power estimates. Analysis of the shallow-water HF97 ocean acoustic experimental data shows that CS is able to recover the DOAs and power levels of the multipath signals with superior resolution compared to MVDR with spatial smoothing.

A Bayesian Method for Localization by Multistatic Active Sonar

Abstract: The question of localizing a target with multistatic active sonar is reexamined from the perspective of finding a peak in a probability distribution function. The probability distribution function is constructed using straightforward Bayesian principles. Both a position estimate and a covariance matrix can be found, provided that an implementation of a numerical algorithm for finding a local maximum is available. The localization method developed herein can account for transmitter and receiver location errors, sound-speed errors, time errors, and bearing errors. A Monte Carlo test is conducted to compare the accuracy of the proposed method to that of a more conventional method used as a baseline. In each iteration, a transmitter, several receivers, and a target are positioned randomly within a square region, and the target is localized by both methods. The proposed method is generally more accurate than the baseline method, within the range of parameters considered here. The degree of improvement over the baseline is greater with a larger region area, with a larger bearing measurement error, and with a smaller time-of-arrival measurement error, and slightly greater with a larger number of receivers.

Kernel-Function-Based Models for Acoustic Localization of Underwater Vehicles

Abstract: This paper proposes a novel design for the localization system of autonomous underwater vehicles (AUVs) using acoustic signals. The solution presented exploits models based on kernel functions with two main purposes: 1) to reject outliers; and 2) to correct or improve accuracy of measurements. The localization system discussed is based on well-established techniques such as support vector data description (SVDD) and autoassociative kernel regression (AAKR) derived from machine learning theory that utilizes heuristic models for classification and regression tasks, respectively. By coupling the algorithm to the navigation system, we seek to reduce the sensitivity of the localization scheme to the reflected acoustic waves or fluctuations of underwater channel properties without modifying the solution used for data fusion or overloading the algorithm embedded in the vehicle. Data collected in the field with a light underwater vehicle (LAUV) were used to demonstrate the advantages of the proposed approach.

Spatially Distributed MIMO Sonar Systems: Principles and Capabilities

Abstract: Multiple-input–multiple-output (MIMO) sonar systems offer new perspectives for target detection and area surveillance. This paper introduces a unified formulation for sonar MIMO systems and focuses on the target detection and recognition capability of these systems. The multiplication of the number of transmitters and receivers not only provides a greater variety in terms of target view angles but also provides meaningful statistics on the target itself. Assuming that views are independent and the MIMO system is large enough, we demonstrate that target recognition is possible with only one MIMO snapshot. By studying the detection performance of MIMO sonars we also demonstrate that such systems solve the speckle noise and decorrelate individual scatterers inside one cell resolution, leading to super-resolution imaging. We also show that, if carefully designed, MIMO systems can surpass the resolution of a synthetic aperture sonar (SAS) system using the same bandwidth. All the discussed properties are derived from the independent view assumption. Fulfilling this assumption drives the design and efficiency of such systems.

Modeling of Generic Offshore Vessel in Crane Operations With Focus on Strong Rigid Body Connections

Abstract: Both marine surface vehicles and underwater vehicles are often equipped with cranes, robotic manipulators, or similar equipment. Much attention is given to modeling of both the dynamics of marine vehicles and the dynamics of manipulators, cranes, and other equipment. However, less attention is given to the interconnected behavior of the vehicle and equipment, even though such equipment may have a considerable impact on the vehicle dynamic behavior, and therefore risk, or conversely, the vehicle may have a considerable impact on the equipment dynamic behavior. With main focus on ships equipped with cranes, this paper presents a framework for modeling the interconnected dynamics of rigid body systems, based on Lagrangian dynamics. The resulting equations of motion are implemented as a bond graph template to which any subsystem of interest, such as actuators, hydrodynamics, and controllers, may be interfaced. An example on how this framework can be used to develop a high-fidelity simulator of an offshore installation vessel with a heavy duty crane is presented. This work represents the first bond graph implementation of crane and vessel dynamics where the interconnections are modeled according to true physical rigid body principles without nonphysical limitations such as diagonal mass-inertia matrix.

Surface Current Characteristics in the Taiwan Strait Observed by High-Frequency Radars

Abstract: In this paper, we focus on the characteristics of the surface currents in the Taiwan Strait observed by two high-frequency surface wave radars (HFSWRs). The current velocities correlate well with the in situ buoy records, which show the reliability of the radar data. Tidal constituents are then calculated from the 80-day-long sea current data. The current pattern in this area consists of a dominant semidiurnal tidal component and a mixed semidiurnal one. The influence of the shallow-water constituents is significant, and the bathymetry also produces an obvious effect on the spatial distribution characteristics of the tidal currents. The directions of the major axes of the ${M}_{2}$ tidal current ellipses coincide well with that of the strait axis. Besides, the east–west components of the residual current velocity and the wind speed are found to be highly correlated with a correlation coefficient up to 0.808, while the north–south components are nearly uncorrelated with a correlation coefficient down to 0.238. This study shows the great capability of the HFSWR in oceanographic researches and applications, especially for the tidal analysis and a further study of the wind–current relation.