ALBERT

Description: Sonar images are formed by transmitting acoustical pulses and measuring the reflected sound power from the scene surfaces. The recorded signal by a sonar device encodes information about the shape and material properties of these surfaces. In this paper, we present a detailed derivation of an image model for a new class of high-resolution lens-based 2-D forward-looking sonar systems, when the diffuse reflectance of scene/target can be characterized by the Lambertian model. A simplified single patch model is generalized to account for the finite pulse width of the transmitted beam and the simultaneous arrival of scattered signals from multiple patches at the same range in a given azimuthal direction. Validating the model using intensity measurements of isolated cylindrical targets, we then demonstrate application for multipath reflections from bottom surfaces with cylindrical and spherical targets.

Description: In India, several experimental coastal aquaculture cages have been installed on sites along the Indian coast. Many of them have been commercially successful, while some have faced technical difficulties. The construction of the Indian coastal aquaculture cage is based on ad hoc designs adopted from other countries. The performance of the coastal aquaculture cage shows that there is a need for development of a cage design with a location-specific mooring system to withstand local wave conditions for a longer period in different coastal zones of the country. This requires that designs of the cages suiting Indian conditions be made based on sound engineering principles. To design the cage, the current- and wave-induced tensions in the mooring chain and net twine and the cage motions need to be estimated. In this study, a model cage was fabricated and tested in a towing tank under different waves and towing conditions. The tension on the mooring chain was measured during model experiments along with the towing speed and wave parameter. A prototype coastal aquaculture cage with single point mooring near the coast of Visakhapatnam in South East India was identified for measurement of forces and motions. On the prototype cage, measurements of cage motions, tensions on mooring chain, and net twine and cage orientation were carried out. Characteristics of the cage response, observed in model and prototype experiments, are presented in this paper. The model cage is not a geometrically scaled down version of the prototype cage. The diameter/depth ratios of the model and the prototype are different. The mooring pattern and net twine of the model cage is the same as that of the prototype. The data presented in this paper could be used as a reference for validating the numerical model for simulating the cage forces and motions under wave and current loads.

Description: One of the challenges presented in using autonomous underwater vehicles (AUVs) for remote data collection is accurate time synchronization. In the case of bistatic acoustics, synchronization is required between AUV and source so that the time in which each ping is sent out is exactly known. There are three key obstacles to achieving this. First, the vehicle is submerged and therefore unable to access common time references directly. Second, the required accuracy in data acquisition far exceeds that possible using a computer clock to trigger data collection. Finally, to achieve accuracy in microseconds, the system must be characterized to eliminate delays introduced by filtering and analog-to-digital conversion. This paper describes the implementation and characterization of an accurate and precise timing and data acquisition system used on an AUV to collect acoustic array data. The timing was achieved using a combination of global positioning system (GPS) pulse-per-second (PPS) for synchronization on the surface and the Microsemi (Aliso Viejo, CA, USA) chip scale atomic clock (CSAC) for timing while submerged, with a PPS triggered data acquisition system. Characterization and calibration procedures were developed to ensure that the system met the experiment requirements, which included less than one percent of a wavelength error in phase, and one tenth of a meter accuracy in range. Analog and digital delays in the system were determined, and a method was demonstrated to further improve accuracy by dynamically estimating digital delays. The steps outlined in this paper for achieving precision data acquisition could be applied to many other remote systems that require similar microsecond accuracy.

Description: Underwater images suffer from blurring effects, low contrast, and grayed out colors due to the absorption and scattering effects under the water. Many image enhancement algorithms for improving the visual quality of underwater images have been developed. Unfortunately, no well-accepted objective measure exists that can evaluate the quality of underwater images similar to human perception. Predominant underwater image processing algorithms use either a subjective evaluation, which is time consuming and biased, or a generic image quality measure, which fails to consider the properties of underwater images. To address this problem, a new nonreference underwater image quality measure (UIQM) is presented in this paper. The UIQM comprises three underwater image attribute measures: the underwater image colorfulness measure (UICM), the underwater image sharpness measure (UISM), and the underwater image contrast measure (UIConM). Each attribute is selected for evaluating one aspect of the underwater image degradation, and each presented attribute measure is inspired by the properties of human visual systems (HVSs). The experimental results demonstrate that the measures effectively evaluate the underwater image quality in accordance with the human perceptions. These measures are also used on the AirAsia 8501 wreckage images to show their importance in practical applications.

Description: In this paper, an automated change detection technique is presented that compares new and historical seafloor images created with sidescan synthetic aperture sonar (SAS) for changes occurring over time. The method consists of a four-stage process: a coarse navigational alignment that relates and approximates pixel locations of reference and repeat–pass data sets; fine-scale coregistration using the scale-invariant feature transform (SIFT) algorithm to match features between overlapping data sets; local coregistration that improves phase coherence; and finally, change detection utilizing a canonical correlation analysis (CCA) algorithm to detect changes. The method was tested using data collected with a high-frequency SAS in a sandy shallow-water environment. Successful results of this multistage change detection method are presented here, and the robustness of the techniques that exploit phase and amplitude levels of the backscattered signals is discussed. It is shown that the coherent nature of the SAS data can be exploited and utilized in this environment over time scales ranging from hours through several days. Robustness of the coregistration methods and analysis of scene coherence over time is characterized by analysis of repeat pass as well as synthetically modified data sets.

Description: This paper addresses the problem of optimal acoustic sensor placement for underwater target localization in 3-D using range measurements only. By adopting an estimation theoretical framework, the optimal geometric sensor formation that will yield the best achievable performance in terms of target positioning accuracy is computed by maximizing the determinant of an appropriately defined Fisher information matrix (FIM). For mathematical tractability, it is assumed that the measurements of the ranges between the target and a set of acoustic sensors are corrupted with white Gaussian noise. For the sake of completeness, an explicit analytical result for a generic $n$ -sensor network is first obtained for the case when there is no uncertainty in the prior knowledge about the target position. The result is then extended to the practical case where the target is known to lie inside a region of uncertainty. The optimal sensor configuration thus obtained lends itself to an interesting and useful geometrical interpretation. In addition, the “spreading” of the configuration is shown to depend on the number of range measurements, target depth, and the probability distribution function that characterizes the prior knowledge about the target position. Results are also obtained for the problem of optimal sensor placement with constraints, namely, by considering that the sensors can be either located at the sea surface or distributed between the surface and the seabed. The connection between 2-D and 3-D scenarios is clarified. Simulation examples illustrate the key results derived.

Description: In this paper, experimental broadband propagation measurements in a typical sea port scenario at 5.8 GHz are presented. Measurements were performed in Cádiz Bay, Spain, by sounding the channel with a periodic pulsed signal, from which power delay profiles (PDPs) were estimated for several locations. The study focuses on propagation scenarios based on low-height antenna deployments. Experimental PDPs are found with a spiky shape that can be explained from the nature of the environment. This result is significantly different from the one expected in other regular environments (urban, indoor, etc.) where continuous shaped profiles are common. Furthermore, time-dispersive parameters, such as mean delay, delay spread, and coherence bandwidth, are extracted. The estimated values for mean delay and delay spread remain small, which indicates that the channel can be regarded as flat for typical low-bandwidth signals employed over the sea. In fact, the median value for coherence bandwidth is found to be 8 MHz, hence these channels are suitable to support relatively high data rates by using simple communication systems.

Description: In this paper, we study the problem of detection of underwater minefields amidst dense clutter as that of statistical inference under a spatial point-process model. Specifically, we model the locations (mine and clutter) as samples of a Thomas point process with parent locations representing mines and children representing clutter. Accordingly, the parents are distributed according to a homogeneous Poisson process and, given the parent locations, the children are distributed as independent Poisson processes with intensity functions that are Gaussian densities centered at the parents. This provides a likelihood function for parent locations given the observed clutter (children). Under this model, we develop a framework for penalized maximum-likelihood (ML) estimation of model parameters and parent locations. The optimization is performed using a combination of analytical and Monte Carlo methods; the Monte Carlo part relies on a birth–death–move procedure for adding/removing points in the parent set. This framework is illustrated using both simulated and real data sets, the latter obtained courtesy of Naval Surface Warfare Center Panama City Division (NSWC-PCD), Panama City, FL, USA. The results, evaluated both qualitatively and quantitatively, underscore success in estimating parent locations and other parameters, at a reasonable computation cost.

Description: The accurate modeling of underwater acoustical reflection from a wind-roughened ocean surface is a challenging problem. Some complicating factors are the presence of near-surface bubbles and the potential for shadowing of acoustical energy by parts of the surface itself. One essential factor, which is the subject of this paper, is the specular reflection of coherent plane waves at an ocean-like rough surface. We tested the accuracy of one rough surface reflection model, the small-slope approximation (SSA) approach as used by Williams (J. Acoust. Soc. Amer., vol. 116, no. 4, pp. 1975–1984, Oct. 2004), for scenarios for which scattering was entirely in the vertical plane. The SSA model was used to compute values of the coherent plane wave reflection loss per bounce for wind speeds between 5 and 12.5 m/s, frequencies between 1.5 and 9 kHz, and grazing angles between about 1 $^{circ}$ and 10 $^{circ}$ . These values were compared to those obtained from a Monte Carlo approach based on the parabolic equation (PE) method, where realistic ocean surfaces were generated based on the Pierson–Moskowitz spectrum for ocean surface heights. The SSA model compared favorably with the more rigorous PE method for most of the range of parameters considered. An approximation to the SSA model was derived for application to grazing angles less than particular values, and this approximation was shown to compare well with results from PE modeling.

Description: A general numerical model has been developed to predict the probability density function (pdf) of the magnitudes of complex pulse-compressed broadband echoes (broadband echo pdf) due to arbitrary aggregations of scatterers that are detected with a single-beam echosounder. The model is based on physics principles and rigorously accounts for the broadband frequency-dependent characteristics of the system, signal, scatterers, and the beampattern modulation effects of the sonar/radar transceiver. A key aspect to modeling the statistics of broadband echoes is accounting for the scenarios where the pulse-compressed echoes may only partially overlap. The echo statistics under those conditions will be significantly different than those associated with a narrowband system with the same center frequency but whose echoes will completely overlap given the same density of scatterers. As a result, pulse-compressed broadband echoes will generally deviate more from the Rayleigh distribution (i.e., be more “heavy-tailed”) than narrowband echoes—a feature that is critical to analyzing real-world data. As in the case of narrowband signals, the shape of the broadband echo pdf is shown to vary from strongly non-Rayleigh to Rayleigh as the number of dominant scatterers in the beam increases. The model is applied to sonar in the ocean in which the numerical density of fish is inferred using broadband echoes (30–70 kHz). The results are compared with those from conventional echo energy methods. As with narrowband systems, statistics of broadband echoes can be used to estimate the numerical density of scatterers without the need for absolute calibration of the system.

Description: All types of seismic sources have a common feature in which large energy is released into water in a short time to produce an intense acoustic pulse. For plasma sparker, the discharge time is mainly determined by the discharge circuit parameters. Providing an accurate prediction of the acoustic pulse is difficult because of the complex plasma discharge process and unclear initial bubble condition. This paper presents the first experimental study on the influence of electric pulse width on acoustic pulses, especially the expansion pulse. Three comparative experiments are conducted to investigate the effect of electric pulse and to determine the minimum acoustic pulse width produced by a plasma sparker. One of the main results is that only the amplitude of the expansion pulse can obviously be changed if the electric circuit parameters are fixed, especially the capacitance of the energy storage capacitor. Meanwhile, the smaller the energy storage capacitor is, the shorter is the electric pulse and the narrower is the expansion pulse. The maximum pressure of an expansion pulse generated by a small capacitor is larger than that generated by a large capacitor under the same energy. The expansion pulse width decreases quickly when the energy dissipation on the load speeds up, and approaches the minimum value around several microseconds when the energy dissipation rate on the load is over 2 J/ $mu$ s, and the corresponding electric pulse width is on the order of hundreds of nanoseconds.

Description: This paper considers propagation of low-frequency broadband pulses in shallow water. It focuses on across-slope propagation in wedge-shaped oceanic waveguides and on single hydrophone receiving configurations. In a low-frequency shallow-water context, propagation is dispersive and usually described by modal theory. However, in the presence of a tilted bottom, propagation is also affected by 3-D effects (horizontal refractions). The paper shows that time-frequency analysis is a suitable tool to illustrate and understand 3-D propagation. It illustrates the pertinence of the time-frequency analysis of 3-D signal by proposing an algorithm to estimate the seabed slope using a single receiver. The method is benchmarked on numerical simulations, and successfully applied on small-scale experimental data.

Description: In this paper, a new design principle is proposed for the frontend-hardware (FEHW) chain of broadband passive sonars. This principle embraces the ultimate purpose of broadband passive sonars, which is to maximize the detection of passive targets. FEHW chains were conventionally designed as noise whiteners to guarantee the data bit dynamic range in analog-to-digital converters (ADCs) rather than being optimized for target detection. The conventional design was due to the restriction of electronic hardware in the early days and is still used in modern broadband passive sonars even with the great relaxation of hardware restriction today. The proposed principle optimizes the frequency response function (FRF) of FEHW chains by means of maximizing the output signal-to-noise ratio (SNR). The optimal FRF obtained using the design principle counts for the spectral characteristics of the entire sonar system (hydrophones, FEHW chain, and beamformer) as well as those of broadband passive sources and background noise. It turns out that maximizing the SNR is equivalent to maximizing the so-called deflection ratio (DR) in broadband passive sonars, but the former is directly used as a statistical test in the sonar engineering. The relationship between the optimal FRF and the Eckart filter is also discussed, with both of them counting for spectral shapes at their input, but with the former also counting for the change of spectral shapes after its output. Design examples are included to demonstrate the merit of the optimal FRF over the noise whitener with both theoretical and simulation results.

Description: In this paper, we investigate the power amplification for underwater acoustic orthogonal frequency-division multiplexing (OFDM) systems. The maximum power delivery (MPD) and the pilot signal-to-noise ratio (PSNR) specific to the OFDM modulation are adopted as design criteria. We study the impact of key parameters associated with the pulse-width modulation (PWM), such as the modulation frequency, the number of quantization bits, and the input clipping threshold, and provide a suitable procedure to determine those parameters to increase the MPD while meeting a certain PSNR constraint. Experimental setup has been established, where the experimental results validate the key findings from the analytical and simulation results.

Description: Prospective authors are requested to submit new, unpublished manuscripts for inclusion in the upcoming event described in this call for papers.

Description: A sphere is often used as a target shape for various underwater scattering experiments. The scattering from a sphere is aspect independent (monostatically), and it can be efficiently modeled. Elastic-shelled spheres can have a complicated scattering response due to the various types of waves which can propagate within the shell. In practice, “spherical” targets are rarely perfectly spherical due to the manufacturing process. For example, the shell may be slightly thicker at the top and bottom of the sphere. In this paper, we present a straightforward numerical method to model the scattering from almost-spherical objects. It is assumed that the target is still azimuthally symmetric about the ${z}$ -axis, but the shell boundary profiles may vary as a function of polar angle. The effect that this angular dependence has on the backscattered spectra is examined for some simple examples.

Description: Broadband acoustic source transmissions recorded on Seagliders at ranges up to 700 km are used to estimate subsurface glider position. Because the sources transmitted at 9-min intervals the glider moved appreciably between source receptions. Source–glider ranges estimated from acoustic arrivals were combined using least squares analysis to estimate glider position and velocity during each reception period. The analysis was applied to 387 sets of source transmissions using three different flight models of glider subsurface motion for initial position input values. The offsets between the position estimated from the flight models and the acoustically derived position resulting from the inversions were 600–900-m root mean square (rms) depending upon the model and input parameters. The offsets were tripled if the positions from the flight models were not corrected for a dive-averaged current (DAC). Estimates of a posteriori errors ranged from 78–105-m rms and from 9.1–11.6-cm/s rms for glider position and velocity, respectively. Data residuals were on the order of 50-m rms, a dramatic reduction from 178-m rms, which was documented for the case neglecting the motion of the glider between subsequent source transmissions (Van Uffelen , J. Acoust. Soc. Amer., vol. 134, pp. 3260–3271, 2013). Overall horizontal glider speed was estimated to be approximately 21-cm/s rms.

Description: System identification of vessel steering associated with unstructured uncertainties is considered in this paper. The initial model of vessel steering is derived by a modified second-order Nomoto model (i.e., nonlinear vessel steering with stochastic state-parameter conditions). However, that model introduces various difficulties in system identification, due to the presence of a large number of states and parameters and system nonlinearities. Therefore, partial feedback linearization is proposed to simplify the proposed model, where the system-model unstructured uncertainties can also be separated. Furthermore, partial feedback linearization reduces the number of states and parameters and the system nonlinearities, given the resulting reduced-order state model. Then, the system identification approach is carried out, for both models (i.e., full state model and reduced-order state model), resorting to an extended Kalman filter (EKF). As illustrated in the results, the reduced-order model was able to successfully identify the required states and parameters when compared to the full state model in vessel steering under persistent excitation maneuvers. Therefore, the proposed approach can be used in a wide range of system identification applications.

Description: Presents the introductory editorial for this issue of the publication.

Description: This paper presents a fundamental investigation on optimizing the power takeoff (PTO) for maximizing wave energy conversion of the wave-activated bodies (WABs) wave energy converters. In this research, two relative heave motions are taken for capture power, and a linear PTO is considered in the primary analysis. For such a linear dynamic system, the frequency-domain analysis can be carried out, and an analytical formula can be derived for the optimized frequency-dependent PTO damping coefficient, which can be used to determine an optimized PTO damping for maximizing wave energy conversion in regular waves. However, when an optimized PTO is required for maximizing energy conversion from ocean waves, the PTO damping optimization may be very different and much less certain, because it may be based on one of many characteristic periods of the given sea state and the dependency may change from different sea states. For this reason, the optimized PTO damping coefficient for a given sea state must be studied carefully. Another important aspect of the research is to examine whether an optimized nonlinear PTO can take more energy out from waves than that of an optimized linear PTO. For this purpose, the maximized capture powers by the optimized linear and nonlinear PTOs are compared using the time-domain analysis. It has been shown from the examples that the maximized capture power by a nonlinear PTO system may exceed that by the linear PTO, but only marginally (less than 1%). Hence, it can be generally concluded that the maximized power using a linear PTO system can be a very good indicator for the device in extracting the maximal energy from waves regardless of the linear or nonlinear PTO in actual use. This conclusion may help simplify the analysis of the wave energy converters in terms of the energy production as well as the device optimization for improving energy conversion capacity.

Description: Time reversal (TR) exploits spatial diversity to achieve spatial and temporal focusing in complex environments. Over the last decade, the TR concept has been applied successfully to phase-coherent acoustic communications in time-varying multipath ocean environments, as an alternative to conventional adaptive multichannel equalization. Temporal focusing (pulse compression) mitigates the intersymbol interference (ISI) and subsequent single-channel equalization removes the residual ISI, thus providing nearly optimal performance in theory. The spatial focusing capability facilitates multiuser or multiple-input–multiple-output (MIMO) communications without explicit use of time, frequency, or code division multiplexing, while an adaptive TR approach can reduce further the crosstalk among users or multiple transmitters. TR communications can be extended easily to time-varying channels using a block-based approach with channel updates. This paper provides an overview of TR communications in both shallow and deep water and recent advances including bidirectional equalization, multiuser communications with mobile users, and communication with a glider serving as a mobile gateway.

Description: A fundamental successful docking operation requires the autonomous underwater vehicle (AUV) to be able to guide, navigate, and control itself into the docking station in a strategic manner and even possibly execute different maneuvers at different mission phases, depending on docking scenario, requirements, and homing sensor type. A docking station, due to environmental or mission requirements, is possibly oriented at a specific direction instead of allowing omnidirectional homing, and necessitates vehicle docking in only this direction. Depending on the operating environment, either wave or current presence or both can result in a dominating disturbance to the vehicle docking operation. In this work, an inverted ultrashort baseline (USBL) system is used as the main homing sensor to complement the existing navigation suite on the DSO-developed AUV. A docking guidance system was designed and implemented using the Sugeno fuzzy inference system (FIS). A desired heading vector field and the fuzzy rules were developed to perform the fuzzy docking maneuver. An error-state Kalman filter (KF) was designed, formulated, and implemented successfully on the AUV and has proven to perform excellent relative positioning estimation in sea trials. A software architecture was designed for the docking algorithms, and implemented onto a single board computer in the AUV. A sensor fusion approach to the software programming was adopted to ensure that navigation data from all navigation sensors are properly acquired and synchronized. A docking station was designed and eventually deployed at sea for docking trials. Successful AUV docking attempts at sea trials were demonstrated, thus showing the effectiveness of the implemented docking algorithms.

Description: How to reduce power consumption is a critical issue in underwater sensor networks (UWSNs) since sensor nodes have limited energy resource. Utilizing multiple channels and duty cycling may help conserve energy because transmission collisions and idle listening can be reduced. Bursty traffic is common in UWSN; however, we do not find satisfactory solutions that can deliver this type of traffic efficiently in a duty-cycled environment. In this paper, we propose a dynamic duty-cycled multiple-rendezvous multichannel medium access control (DMM–MAC) protocol that is suitable for transmitting bursty traffic in a duty-cycled UWSN. Built on top of multiple-rendezvous multichannel MAC (MM–MAC), a node running DMM–MAC needs only one modem. DMM–MAC can operate in a more realistic multihop environment, instead of a simplified single-hop network, without the information of relative distances or propagation delays to other nodes. Utilizing the proposed dynamic duty cycling mechanism, nodes running DMM–MAC can deliver bursty traffic efficiently. Simulation results verify that DMM–MAC conserves energy and enhances network performance when compared to MM–MAC.

Description: Experimental evaluations on autonomous navigation and collision avoidance of ship maneuvers by intelligent guidance are presented in this paper. These ship maneuvers are conducted on an experimental setup that consists of a navigation and control platform and a vessel model, in which the mathematical formulation presented is actually implemented. The mathematical formulation of the experimental setup is presented under three main sections: vessel traffic monitoring and information system, collision avoidance system, and vessel control system. The physical system of the experimental setup is presented under two main sections: vessel model and navigation and control platform. The vessel model consists of a scaled ship that has been used in this study. The navigation and control platform has been used to control the vessel model and that has been further divided under two sections: hardware structure and software architecture. Therefore, the physical system has been used to conduct ship maneuvers in autonomous navigation and collision avoidance experiments. Finally, several collision avoidance situations with two vessels are considered in this study. The vessel model is considered as the vessel (i.e., own vessel) that makes collision avoidance decisions/actions and the second vessel (i.e., target vessel) that does not take any collision avoidance actions is simulated. Finally, successful experimental results on several collision avoidance situations with two vessels are also presented in this study.

Description: The range-instantaneous Doppler (RID) algorithm has been successfully used in high-resolution inverse synthetic aperture radar (ISAR) imaging of maneuvering ship targets. Parametric technique of RID is an important method to produce the well-focused RID ISAR images by estimating the parameters of received signal modeled as multicomponent polynomial-phase signals in each range bin. However, the cross terms of signals and false maxima of time–frequency distributions cause image blurring and the loss of fine detail. In this paper, we first propose a new time–frequency transform, named multiproduct high-order matched-phase transform (MPHMT), to suppress the cross terms and amplify the auto terms of signals. Then, an adaptive Viterbi search is proposed to select the optimal model parameters by an adaptive penalization function. Finally, associating MPHMT and adaptive Viterbi search with RID, we further propose the adaptive Viterbi-based RID algorithm for ISAR ship imaging. Simulation and real-data-based results demonstrate that our proposed algorithm is valid.

Description: It is well known that the energy transport of ocean waves propagates with the group velocity and that the energy decreases exponentially with depth. Expanding this theory, we will derive expressions for the energy transport as a function of depth and the total instantaneous transport's development over time for waves in waters of finite depth. Solutions to the Laplace equation are found for plane-parallel polychromatic waves with linearized boundary conditions. A time series of wave elevation collected at Uppsala University's wave energy research test site is chosen to present the results. Solutions for waters of both infinite and arbitrary depths are presented and compared. The solutions are convolution-type integrals with the wave elevation where we have found efficient ways to calculate the kernels. The difference in group velocity between finite depth and infinite depth and its impact on the energy transport is clearly seen in the results. The use of the deep-water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. We further show that the total instantaneous energy transport can actually have a direction that is opposite to the direction of the waves as observed from a reference frame fixed to the seabed.

Description: This paper introduces a new non-Gaussian detection method for complex-valued synthetic aperture sonar (SAS) imagery. The detection method is based on a multivariate extension of the Laplace distribution derived using a scale mixture of Gaussian distributions. A goodness-of-fit test in the form of a likelihood ratio is then conducted on a sonar imagery data set consisting of high-frequency (HF) and broadband (BB) images coregistered over the same region on the seafloor showing the proposed model's applicability in sonar imagery. Detection based on testing the equality of parameters from two populations is then implemented on a database containing actual SAS images of the seafloor with synthetically generated targets inserted into the images and compared to a similar non-Gaussian technique. Detection performance in this paper is given in terms of receiver-operator characteristic (ROC) curve attributes, probability of detection, and average false alarm rate.

Description: The minimum variance distortionless response (MVDR) beamformer has recently been proposed as an attractive alternative to conventional beamformers in active sonar imaging. Unfortunately, it is very computationally complex because a spatial covariance matrix must be estimated and inverted for each image pixel. This may discourage its unnecessary use in sonar systems which are continuously being pushed to ever higher imaging ranges and resolutions. In this study, we show that for active sonar systems up to 32 channels, the computation time can be significantly reduced by performing arithmetic optimizations, and by implementing the MVDR beamformer on a graphics processing unit (GPU). We point out important hardware limitations for these devices, and assess the design in terms of how efficiently it is able to use the GPU's resources. On a quad-core Intel Xeon system with a high-end Nvidia GPU, our GPU implementation renders more than a million pixels per second (1 MP/s). Compared to our initial central processing unit (CPU) implementation, the optimizations described herein led to a speedup of more than two orders of magnitude, or an expected five to ten times improvement had the CPU received similar optimization effort. This throughput enables real-time processing of sonar data, and makes the MVDR a viable alternative to conventional methods in practical systems.

Description: Among autonomous surface vehicles, sailing robotics could be a promising technology for long-term missions and semipersistent presence in the oceans. Autonomy of such vehicles in terms of energy will be achieved by renewable solar and wind power sources. Autonomy in terms of sailing decision will be achieved by innovative perception and navigation modules. The main contribution of this paper is to propose a complete hardware and software architecture for an autonomous sailing robot. The hardware architecture includes a comprehensive set of sensors and actuators as well as a solar panel and a wind turbine. For obstacle detection, a segmentation is performed on data coming from an omnidirectional camera coupled with an inertial measurement unit and a sonar. For navigation and control of the vehicle, a potential-based reactive path-planning approach is proposed. The specific sailboat kinematic constraints are turned into virtual obstacles to compute a feasible and optimal heading in terms of cost of gybe and tack maneuver as well as safety relative to obstacle danger. Finally, field test experiments are presented to validate the various components of the system.

Description: This study proposes a state estimation and compression method for navigating multiple autonomous underwater vehicles (AUVs) toward wide area surveys near seafloors. In the proposed method, a moving AUV navigates by referencing a stationary landmark AUV on the seafloor. By alternating the landmark role, all AUVs can cover a wide area while maintaining low positioning errors. The moving AUV estimates the states (positions and headings) of both moving and landmark AUVs by a stochastic approach called a particle filter. When AUVs exchange their landmark roles, they must share their estimated states. However, state sharing is precluded by the low data rate of acoustical communications in underwater environments. To overcome the problem, this paper proposes a state compression method in which AUVs approximate their states by “particle clustering” based on a clustering method ( $k$ -means) and a model evaluation method (Akaike information criterion). The compression method enables AUVs to share their states by communicating small amounts of data. The proposed method was evaluated in simulations of two AUVs navigating over a 300 $times$ 300-m $^{2}$ seafloor area. Throughout the simulation, the proposed method maintained stable positioning and successful state sharing with small communication data size.

Description: Quantifying acoustic scattering from rough interfaces is critical for reverberation modeling, acoustic sediment characterization, and propagation modeling. In this study, a finite element (FE) scattering model is developed. The model computes the plane wave scattering strength for an ensemble of rough power-law surfaces for ocean bottoms described as fluid and elastic. The FE model is compared with two models based on approximations to the Helmholtz–Kirchhoff integral: the Kirchhoff approximation (KA) and the perturbation theory (PT). In the case of a fluid-like bottom, the KA and FE models agree except at small grazing angles. The PT and FE models deviate near specular especially at small angles. For the elastic case, the PT predicts the FE results well except at the intromission angle of the shear wave. The KA deviates for angles that are below the critical angle of the compressional wave. At the shear wave intromission angle, the FE model shows a more plausible solution likely due to multiple scattering events that are not accounted for in PT for the modeled roughness.

Description: In the development in the paper (P. J. Welton, ???Cross correlation of omnidirectional, broadband signals scattered by a random pressure-release surface,??? IEEE J. Ocean. Eng., vol. 40, no. 2, 2014, DOI: 10.1109/JOE.2013.2321511), the random incident pressure should have been represented by a Fourier???Stieltjes integral in the analysis, not a Fourier integral. This change does not affect the final results.

Description: Expressions for the cross correlation of omnidirectional, broadband continuous-wave (CW) acoustic signals scattered by a pressure-release random surface are derived for both vertically and horizontally spaced hydrophones using the Kirchhoff approximation for the boundary values and the Fresnel phase approximation. The resulting expressions for the spatial coherence are simple in form and require no approximations in the description of the random surface. As the surface becomes perfectly smooth, the expressions reduce to the image solution, thus ensuring the proper limiting behavior. Two narrowband models for vertical coherence were compared to experimental data collected during a sea test at the frequencies of 14, 16, 18, and 20 kHz with a maximum hydrophone spacing of 1.4 m. One model used the measured sound-speed profile (SSP) to estimate input parameters, and the other model assumed isospeed conditions. The agreement between the experimental data and the model based on the measured SSP is very good, and the agreement with the isospeed model is almost as good.

Description: In this study, hybrid-polarity (HP) architecture is exploited to observe sea oil slicks. HP features are interpreted in terms of sea surface scattering with or without oil slicks, under low-to-moderate wind conditions. They are shown to exhibit a different sensitivity with respect to slick-free, weak-damping slick-covered, and oil-covered sea surfaces. This sensitivity is verified against HP measurements obtained transforming actual L- and C-band quad-polarimetric synthetic aperture radar (SAR) data where both oil slicks and weak-damping look-alikes are present. Experiments demonstrate: 1) the remarkable performance of HP features to both observe oil slicks and distinguishing them from weak-damping look-alikes; 2) the marginal effect played by the sensor's noise floor on HP features performance; 3) the pronounced sensitivity of the HP features to the damping properties of the surfactants; and 4) the comparable performance that characterizes polarimetric entropy derived by HP and conventional polarimetric measurements.

Description: One of the most critical issues for ship owners, shipbuilders, and insurance companies is the operational safety. In particular, keeping damaged ships stable in waves is of great interest, because more nonconventional hull forms are being introduced for military and passenger vessels while international rules and regulations are becoming stricter. However, ship stability for damaged ships is quite different from that for intact ships as the assessment is very complicated and difficult due to the highly nonlinear behavior. Computational fluid dynamics (CFD) methods that solve the Navier–Stokes equations are acknowledged as the only viable approach to simulate and analyze these complex physical phenomena. Although there have been a number of research activities reported on damaged ship stability recently, most of them are not designed to validate CFD studies. For a data set to be valuable for CFD validation and development, model tests should eliminate unclear factors as much as possible. The main objective of this study is to establish an experimental database for CFD validation by collecting data from towing tank tests of a ship hull's six degree-of-freedom (6DOF) motion responses in regular waves for both intact and damaged conditions. A mooring system was designed to prevent drift motions of the ship model. Parametric roll was not observed when the ship was damaged, although it was observed for the intact ship in the same wave conditions. The mooring force acting on the ship model due to spring tension was also calculated.

Description: Designs for an optical sensor detector array for use in autonomous control of unmanned underwater vehicles (UUVs), or between UUVs and docking station, are studied in this paper. Here, various optical detector arrays are designed for the purpose of determining and distinguishing relative 5 degrees-of-freedom (DOF) motion between UUVs: 3-DOF translation and 2-DOF rotation (pitch and yaw). In this paper, a numerically based simulator is developed to evaluate varying detector array designs. The simulator includes a single light source as a guiding beacon for a variety of UUV motion types. The output images of the light field intersecting the detector array are calculated based on detector hardware characteristics, the optical properties of water, and expected noise sources. Using the simulator, the performance of planar and curved detector array designs (of varying size arrays) are analytically compared and evaluated. Output images are validated using empirical in situ measurements conducted in underwater facilities at the University of New Hampshire, Durham, NH, USA. Results of this study show that the optical detector array is able to distinguish relative 5-DOF motion with respect to the simulator light source. Furthermore, tests confirm that the proposed detector array design is able to distinguish positional changes of 0.2 m and rotational changes of 10 $^{circ}$ within 4–8 m range in x-axis based on given output images.

Description: The dynamic characteristic of an autonomous underwater vehicle (AUV) is affected when it is reconfigured with different payloads. It is desirable to have an updated model, such that the control and guidance law can be redesigned to obtain better performance. Hence, we develop a method to enable online identification of AUV dynamics via in-field experiments, where the AUV is commanded to execute a compact set of maneuvers under doublet excitation. The identification process has two stages. In the training stage, state variable filter and recursive least square (SVF-RLS) estimator is used to estimate the unknown parameters. In the validation stage, the prediction capability of the model is checked using a fresh data set. The parameters converged within 12 s in the experiments using five different thrusts. Validation results show that the identified models are able to explain 78% to 92% of the output variation. Next, we compare the SVF-RLS estimator with the conventional offline identification method. The comparison shows that the SVF-RLS estimator is better in terms of prediction accuracy, computational cost and training time. The usefulness of the identified models is highlighted in two applications. We use it to estimate the turning radius of the AUV at different speeds, and to design a gain-scheduled controller.

Description: This paper presents the design and performance analysis of double stator axial flux permanent magnet generators for rim-driven marine current turbines (MCT). In submarine applications, drive train reliability is a key feature for reducing maintenance requirements. Rim-driven, direct-drive, multistator generators can therefore be an interesting solution to improve reliability. In this context, the work presented here focuses on the design of a double-stator, axial flux, permanent magnet (PM) generator as a rim-driven, direct-drive, multistator generator. The models, specifications and an optimization procedure that facilitate the preliminary design of these kinds of generators for rim-driven marine turbines are presented. Validation with finite element computations and a performance analysis for a particular design of rim driven generators are conducted. The results obtained highlight some designs issues associated with PM generators for rim driven marine turbines. To assess the effectiveness of the double stator axial flux PM generator, a comparison with a surface mounted radial flux PM generator design for rim marine turbines is carried out. The comparison shows that the double stator axial flux generator has a better cooling characteristic and a reduced active parts cost and mass than the radial flux PM generator.

Description: In this paper, we investigate an approach that allows naval vessels to continuously estimate their underwater electric potential (UEP) signature strength, by converting a beforehand measured reference UEP signature to the present environmental conditions. The approach is intended to improve risk assessment during mine counter measure (MCM) operations and to be used within integrated ship signature management systems (ISSMS). It is therefore designed to depend only on data available in these scenarios and to be simple and fast, rather than just being as accurate as possible. After explaining the concept and deducing the conversion relations, the approach is tested using numerical simulations and ranged UEP data of a ship (CFAV Quest). The results show that it is well suited to convert UEP signatures between different water conductivities and sensor depths, while it is not capable of converting between different seabed conductivities. The signature conversion is best for intact hull coatings, while hull-to-anode currents lower the correlation between signature and shaft currents, and hence impair the conversion quality.

Description: Detection of mine-like objects (MLOs) in sidescan sonar imagery is a problem that affects our military in terms of safety and cost. The current process involves large amounts of time for subject matter experts to analyze sonar images searching for MLOs. The automation of the detection process has been heavily researched over the years and some of these computer vision approaches have improved dramatically, providing substantial processing speed benefits. However, the human visual system has an unmatched ability to recognize objects of interest. This paper posits a brain–computer interface (BCI) approach, that combines the complementary benefits of computer vision and human vision. The first stage of the BCI, a Haar-like feature classifier, is cascaded in to the second stage, rapid serial visual presentation (RSVP) of images chips. The RSVP paradigm maximizes throughput while allowing an electroencephalography (EEG) interest classifier to determine the human subjects' recognition of objects. In an additional proposed BCI system we add a third stage that uses a trained support vector machine (SVM) based on the Haar-like features of stage one and the EEG interest scores of stage two. We characterize and show performance improvements for subsets of these BCI systems over the computer vision and human vision capabilities alone.

Description: This paper presents the development and the experimental validation of a centralized coordination control scheme that is robust to communication constraints and individual tracking errors for a team of possibly heterogeneous marine vehicles. By assuming the existence of a lower level target tracking control layer, a centralized potential-field-based coordination scheme is proposed to drive a team of robots along a path that does not necessarily need to be defined a priori. Furthermore, the formation is allowed to hold its position (the vehicles hold their positions with regard to a static virtual leader), which is particularly appreciated in several marine applications. As it is important to guarantee stability and mission completion in adverse environments with limited communications, the centralized control scheme for coordination is constructed in a way that makes it robust to tracking errors and intermittent communication links. The study and developments presented in this paper are complemented with field experiments in which vehicles have coordinated their operation to keep in formation over a dynamic path and static points. This work considers two types of communication technologies. Firstly, standard high rate radio communications are used to drive the formation and, secondly, acoustic communications are employed to assess the performance and the robustness of the proposed approach to degraded and highly variable conditions.

Description: In the crowded underwater acoustic channels, the ability to recognize sonar transmissions based on an identifying signature is quite valuable. In this work, we propose an algorithm to embed a watermark in the time-frequency coefficients of a sonar waveform and produce a watermarked sonar. The watermark, which is a digital signature, is used to authenticate the sonar by verifying the source that is broadcasting the waveform. The embedding rule is modeled after spread spectrum modulation and driven by two secure keys. The first key consists of the spreading code that is unique to each source. The second key is an embedding mask used to select and additively modify the selected time-frequency cells of the sonar. The mask is chosen based on the frequency response of the acoustic channel (primarily, its time and frequency coherence) and the signal energy distribution in the time-frequency plane. The detector is modeled after a matched filter receiver performing a replica correlation with the spread watermark passed through the channel model. A successful detection needs access to both the spreading code and the embedding mask. Watermark detection performance is extensively evaluated based on range; watermark energy; watermark placement; ambient noise; number of multipaths; and other ocean parameters, such as depth, surface, and bottom models.

Description: A robust data assimilation scheme is presented for a wave model to predict evolving nonlinear ocean waves. The Fourier coefficients of the surface elevation and the free surface velocity potential are chosen for state variables and are propagated in time by solving numerically a set of nonlinear evolution equations using a pseudo-spectral method. The numerical solutions are then updated with noise corrupted measurements of the surface elevation with the aid of an explicit Kalman filter for which the time evolution of the error covariance matrix is found explicitly by solving analytically the linearized wave prediction model. After presenting an error analysis for this explicit data assimilation scheme, numerical simulations of the integrated nonlinear wave prediction model for long-crested waves of varying wave steepness are performed by using synthetic data with different noise characteristics. It is shown that the estimated surface wave fields agree well with the true states, and the present data assimilation scheme based on the explicit Kalman filter improves considerably the computational efficiency and stability, in comparison with a standard Kalman filter for which the error covariance matrix is found numerically.

Description: Understanding wave breaking phenomena is a challenging problem that still lacks satisfactory understanding. The present paper is concerned with the development of novel instrumentation adapted to obtain Lagrangian field measurements of essential variables that are intimately related to the physics of wave breaking. These miniature (6.4 cm) Lagrangian drifters are equipped with micro-electromechanical systems (MEMS) inertial measurement units (IMUs) that record acceleration, angular rate and magnetic field data at rapid sampling rates (100 Hz). Using a quaternion-based sensor fusion algorithm, multiple sensor data is filtered in postprocessing procedures to obtain the best possible estimates of particle orientation and acceleration. We show that the developed instrumentation is able to collect meaningful data of inertial particle dynamics in a controlled laboratory flow, where the instrumented particle trajectories are benchmarked against an analytical representation of the flow. Results indicate that dead reckoning errors grow quadratically in time and that for a typical wave overturning event trajectory estimates are expected to remain within a radius $r〈$ 0.7 m with a 95% confidence level. Also, special attention is given to gyroscope reliability during the actual wave breaking process through an empirical evaluation of gyroscope performance in plunging waves. Angular rate measurements from two separate gyroscopes mounted inside the same drifter show high agreement leading to the conclusion that gyroscope errors arising from wave induced vibrations are negligible. Finally, we report on the first ever Lagrangian field observations of inertial particle dynamics in plunging waves with heights on the order of 2 m. We expect that further development of the technology and analysis tools presented herein could revolutionize our understanding of the hydrodynamics of breaking waves.

Description: The working principle and the capabilities of a new platform called the River Drifter are here presented. This technology has applications in the study of the hydrodynamics of coastal areas, rivers, and lakes. The River Drifter was designed for shallow water applications (1 m and deeper) to collect concurrent measurements of surface currents, three-dimensional velocity profiles underneath the device, water depth, and salinity. Here, we discuss how water level displacements can be inferred and used to measure the swell characteristics and to also correct the measured velocity. We also show how the local vorticity field can be computed. As an example application, we describe a study whose goal was to investigate the fate of a polluted river plume and how two River Drifters initially following the same path are characterized by very different final trajectories. The different behaviors of the two drifters are explained in terms of the local flow dynamics, which are strongly influenced by the seabed morphology, forcing the River Drifters to move in different directions.

Description: The drive towards sustainable energy has seen rapid development of marine renewable energy devices (MREDs). The NERC/Defra collaboration FLOw, Water column and Benthic ECology 4-D (FLOWBEC-4D) is investigating the environmental and ecological effects of installing and operating wave and tidal energy devices. The FLOWBEC sonar platform combines several instruments to record information at a range of physical and multitrophic levels for durations of two weeks to capture an entire spring-neap tidal cycle. An upward-facing multifrequency Simrad EK60 echosounder is synchronized with an upward-facing Imagenex Delta T multibeam sonar. An acoustic Doppler velocimeter (ADV) provides local current measurements and a fluorometer measures chlorophyll (as a proxy for phytoplankton) and turbidity. The platform is self-contained, facilitating rapid deployment and recovery in high-energy sites and flexibility in gathering baseline data. Five 2-week deployments were completed in 2012 and 2013 at wave and tidal energy sites, both in the presence and absence of renewable energy structures at the European Marine Energy Centre (EMEC), Orkney, U.K. Algorithms for target tracking have been designed and compared with concurrent, shore-based seabird observations used to ground truth the acoustic data. The depth preference and interactions of birds, fish schools and marine mammals with MREDs can be tracked to assess whether individual animals face collision risks with tidal stream turbines, and how animals generally interact with MREDs. These results can be used to guide marine spatial planning, device design, licensing and operation, as different device types are tested, as individual devices are scaled up to arrays, and as new sites are considered.

Description: A sidescan sonar's beam function (BF) (the sonar's transmit-receive response versus inclination angle in the sonar's frame of reference) can be extracted from an ensemble of contiguous sonar traces affected by sonar vehicle roll for a uniform seabed. The rotation of the sonar's frame of reference relative to the seabed's frame of reference is employed to extract a number of discrete overlapping beam subfunctions each with angular extent corresponding to the extent of vehicle roll. These are then reconciled to form a composite sonar BF. A seabed backscatter function (SF) (the backscatter response of the seabed versus inclination angle in the seabed's frame of reference) may subsequently be inferred with respect to the BF from trace data, whether affected by roll or not. Independent sonar BF and seabed SFs allow the effect of sonar vehicle roll to be corrected when BF compensation is applied to images, and the effect of seabed slope to be corrected when SF compensation is applied. In an alternate approach to extracting a sonar BF, a seabed SF may be extracted first from the sonar traces affected by vehicle roll, by extracting a number of overlapping backscatter subfunctions, and reconciling these into a composite SF. The sonar BF may then be inferred from the trace data with respect to the SF. Seabed SFs inferred from trace data with respect to the BF may be used as a basis for seabed characterization.

Description: In December 2013, an experiment using an autonomous underwater vehicle (AUV) as a moving source for water column sound speed profile (SSP) inversion was conducted in Mogan Lake, China. With in-situ conductivity, temperature, and depth (CTD) measurements, the empirical orthogonal functions were first constructed to represent the SSP. Radiated noises of the AUV were received on a vertical line array and processed via acoustic field matching to obtain estimates of the range-independent SSP, along with the position and velocity of the AUV and the water column depth. The frequency of the radiation noise of the AUV was approximately 14 kHz. At such high frequency, even though the source motion was slow, a significant Doppler shift/broadening was observed. To incorporate the Doppler effects to better match the measured data, a forward acoustic model is derived based on the waveguide Doppler and normal mode theory. An analytical solution of the forward model is obtained for arbitrary signal integration intervals with a monochromatic source, moving radially relative to the receiver. Through simulations and experimental data processing, the feasibility of using an AUV source for water column SSP inversion has been demonstrated; it is also shown that the waveguide Doppler model is more effective compared with the model that does not consider the waveguide Doppler effect.

Description: This paper investigates a multiple-input multiple-output (MIMO) detector for underwater acoustic (UWA) communications, that uses the improved proportionate normalized least mean squares (IPNLMS) algorithm for iterative channel estimation in turbo equalization. By exploiting the sparse nature of UWA channels, the iterative IPNLMS channel estimator achieves fast convergence and fast tracking by data reuse of training sequence and decision-directed channel reestimation over each turbo iterations. The proposed channel estimation scheme is combined with the low-complexity minimum mean-square-error (LC-MMSE) equalization to detect single carrier MIMO signals in overlapped subblocks without guard intervals. The proposed MIMO detection scheme achieves high performance, high transmission efficiency, and low computational complexity, especially for large MIMO and high-constellation modulation schemes. The experimental results of the undersea 2008 Surface Processes and Acoustic Communications Experiment (SPACE08) demonstrate an order of magnitude improvement of bit error rate (BER) performance over those using block-wise MMSE channel estimation. We have also verified that the channel-estimation based turbo equalizer (CE-TEQ) outperforms the direct-adaptation based turbo equalizer (DA-TEQ) in terms of BER performance and robustness against error propagation.

Description: Acoustic remote sensing techniques are an attractive means for obtaining information on the composition of marine sediments since they have high coverage capabilities and thus allow for efficient surveying. Operating at a wide range of specific frequencies, the acoustic sensors provide insight into the sediment body at different depths. This article suggests an efficient manner to combine high- and low-frequency acoustics for obtaining a comprehensive description of fine-grained sediments in a shallow water environment, as was aimed at by the Maritime Rapid Environmental Assessment/Blue Planet trial (MREA/BP'07). This trial was carried out in the Mediterranean Sea in 2007, employing a variety of acoustic sensors, including echosounders, seismic systems, and dedicated array configurations. In a previous paper (Siemes , IEEE J. Ocean. Eng. vol. 35, no. 4, pp. 766–778), we established a three-dimensional picture of the sediment distribution in the MREA/BP'07 area by associating high-frequency echosounder data with low-frequency seismics. A classification based solely on these hydrographic measurements, however, could not provide the physical properties of the near-surface sediments. In the current article we complement this environmental picture with results from a geoacoustic inversion effort, which do provide information on the actual physical properties, such as sound speed, density, attenuation, and layer thickness. In contrast to carrying out the inversion over the complete area, only a limited number of locations was selected for inversion, to limit the computational efforts. This selection was based on the hydrographic environmental picture obtained in (Siemes , IEEE J. Ocean. Eng. vol. 35, no. 4, pp. 766–778). Inverted sediment properties obtained within similar hydrographic regions confirm the similarity in sediment type among these regions, whereas differences in sediment properties between different hydrographic regions are con- irmed as well. Variations in the inversion results within an area with a single sediment type could be attributed to the presence of gas. These results show the suitability of the proposed approach, where backscatter and seismic data discern areas that a priori would differ in their near-surface sediment properties and where geoacoustic inversion assigns actual sediment parameters to these different areas.

Description: A nominally circular 2-D broadband acoustic array of 1.3-m diameter, comprising 508 sensors and associated electronics, was designed, built, and tested for ambient noise imaging (ANI) potential in Singapore waters. The system, named Remotely Operated Mobile Ambient Noise Imaging System (ROMANIS), operates over 25–85 kHz, streaming real-time data at 1.6 Gb/s over a fiber optic link. By using sensors that are much larger than half-wavelength at the highest frequency of interest, so with some directionality, good beamforming performance is obtained with a small number of sensors compared to a conventional half-wavelength-spaced array. A data acquisition system consisting of eight single-board computers enables synchronous data collection from all 508 sensors. A dry-coupled neoprene cover is used to encapsulate the ceramic elements as an alternative to potting or oil filling, for easier maintenance. Beamforming is performed in real-time using parallel computing on a graphics processing unit (GPU). Experiments conducted in Singapore waters yielded images of underwater objects at much larger ranges and with better resolution than any previous ANI system. Although ROMANIS was designed for ANI, the array may be valuable in many other applications requiring a broadband underwater acoustic receiving array.

Description: For the imaging of multiple-receiver synthetic aperture sonar (SAS) under the nonstop-hop-stop case, an exact analytical solution for the two-dimensional (2-D) frequency spectrum is usually difficult to obtain because of the existence of the double-square-root (DSR) term in the range history equation. Several approximate solutions for the 2-D spectrum have been derived to focus the multiple-receiver SAS data. In this paper, the range history geometry for multiple-receiver SAS is newly constructed to meet the demand of the derivation of an analytical 2-D spectrum. According to the geometry-based bistatic formula (GBF) method, a quasi-analytical 2-D spectrum with an unknown variable named half quasi-bistatic angle (HQBA) is reviewed. Based on the method of series reversion (MSR), the fourth order equation with respect to the HQBA is solved and an analytical HQBA is obtained. After substituting the analytical HQBA into the quasi-analytical 2-D spectrum, a completely analytical 2-D spectrum is obtained and the problem of the 2-D spectrum derivation caused by the DSR term is solved successfully. In this paper, an extended range Doppler (RD) algorithm based on the derived 2-D spectrum is proposed for focusing the multiple-receiver SAS data under the non stop-hop-stop case. The results of simulation and real experiment data imaging have validated the effectiveness and accuracy of the proposed algorithm.

Description: Direct-sequence (DS) code-division-multiple-access (CDMA) signaling allows multiple users to communicate simultaneously over the same bandwidth at the expense of a reduced rate. Due to severe cochannel interference (CoI) in multipath rich underwater channels, no more than 4 users have been demonstrated. Multiple-input-multiple-output (MIMO) communication also allows multiple users. Using parallel/sequential interference cancellation schemes, experimental results show similar limitations on the number of users. To extend the number of users beyond four (as required for certain applications), it is proposed to combine CDMA with MIMO, utilizing the spatial-temporal processing gain of passive-phase conjugation, and the approximate-orthogonality of the spreading codes to remove the (increased) CoI. The receiver algorithm uses simple matched filters without explicit channel-equalization and CoI cancellation. The algorithm performance is studied as a function of input-SNR, using simulated data based on experimentally measured channel impulse responses. M-sequences with 127, 255 and 511 chips are used. One finds that 255/511 chips are generally required to support 8 users, and 127/255 chips are needed to support 6 users for the channel studied. Multiuser communications are demonstrated with at-sea data for 8 users, with spreading codes of 511 chips, yielding zero bit errors. The data rate is ${sim} 8$ bits/s/user, given 4 kHz bandwidth.

Description: In this paper, we consider a class of single-input–multiple-output (SIMO) underwater acoustic communication channels, where each propagation path can be characterized by a complex-valued Gaussian block-fading model. The capacity of such channels is computed and analyzed using three power allocation strategies: waterfilling, uniform, and on–off uniform power allocation across the signal bandwidth. Our analysis considers the effects of imperfect channel estimation, delayed feedback, and pilot overhead, which are found to contribute to about 1 (b/s)/Hz loss from 4 (b/s)/Hz at 20-dB signal-to-noise ratio (SNR) for the experimental channel. We find that given the long feedback delays associated with acoustic channels, all-on uniform power allocation, which does not require feedback and is simple to implement, emerges as a justified practical solution that outperforms the other strategies. Furthermore, when considering acoustic-specific propagation effects, such as frequency-dependent attenuation and colored noise, considerable gain can be achieved by selecting the frequency band according to the attenuation pattern and the available transmit power, e.g., at least 6-dB gain for a 10-km link when compared to transmission over a preselected frequency band of 10–15 kHz.

Description: Adaptive equalization is a widely used method of mitigating the effects of multipath propagation and Doppler spreading in underwater acoustic communication channels. While the structure of a multichannel equalizer and least-squares-based adaptation algorithm are extensively used in practice, little is known in how to choose the number of sensors, separation between them, and lengths of the constituent filters such that the equalization performance is optimized. This paper studies the problem of optimal multichannel equalizer design in the context of time-varying underwater acoustic communication channels. In the first part, the paper presents a theoretical characterization of the equalization performance when the number of symbols that can be received in the time period over which the channel can be considered time invariant is limited. This result is then used to develop an understanding that the optimal number of equalizer coefficients is a tradeoff between the minimum mean squared error (MMSE) requirement for longer constituent filters and the insight that the limit on the number of stationary observations also limits the number of filter coefficients that can be effectively adapted. In the second part, the paper develops a theoretical model for wideband arrivals impinging upon an array of sensors of the multichannel equalizer. This model is used to develop an understanding that the optimal sensor separation is a tradeoff between the requirement for long aperture which improves resolution, and the fact that the grating lobes, caused by spatial undersampling, limit the equalizer’s ability to estimate the transmitted signal from the received signal.

Description: Proliferation of deployed sea-going autonomous platforms, such as autonomous underwater vehicles (AUVs), unmanned surface vehicles (USV), and sensor nodes anchored to the seabed, make the deployment of true underwater acoustic networks more and more feasible. An important feature of any network is the ability to synchronize the clocks of the participants, for the purpose of, e.g., time-slotted media access control (MAC) and navigation. Terrestrial clock synchronization protocols, such as the well-established network time protocol (NTP), are not readily applicable to underwater acoustic networks, because of long propagation times, low packet delivery success rates, communication ranges that vary over time in an unpredictable manner, and, in the presence of mobile nodes, the ad hoc nature of the composition of the network. This paper proposes a continuous estimation of internode clock offset and drift, based on the continuous exchange of modem packets, possibly containing transmission and reception timestamps. The proposed solution takes explicitly into account the limitations of the acoustic communication channel and network node mobility. This robust, opportunistic clock synchronization (ROCS) is robust against modem reset, and will work even if packet delivery success rates are not optimal or if no communication is possible for extended periods of time. Experimental results are given from the COLLaborative Asw Behaviours–Next Generation Autonomous Systems (COLLAB-NGAS14) campaign, held October 19–31, 2014, off the west coast of Italy. During the sea trial, the proposed clock synchronization algorithm was deployed and successfully tested within an underwater acoustic network composed of mobile and fixed nodes.

Description: Several underwater acoustic channels exhibit impulsive ambient noise. As a consequence, communication receivers implemented on the basis of the Gaussian noise assumption may yield poor performance even at moderate signal-to-noise ratios (SNRs). This paper presents a new channel-estimate-based decision feedback equalizer (CEB–DFE) that deals with high platform mobility, exploits any sparse multipath structure, and maintains robustness under impulsive noise. The key component of this DFE is a linear-complexity sparse channel estimator, which has the ability to detect and reject impulses based on two noise models: contaminated Gaussian and symmetric alpha stable $({rm S}alpha{rm S})$ . By processing phase-shift keying (PSK) signals from three mobile shallow-water acoustic links, the gain of the proposed receiver over existing equalizers is demonstrated.

Description: Mixed integer nonlinear programming (MINLP) techniques are increasingly used to address challenging problems in robotics, especially multivehicle motion planning (MVMP). The main contribution of this paper is a discrete time, distributed receding horizon mixed integer nonlinear programming (RH–MINLP) formulation of the underwater multivehicle path coordination problem with constraints on vehicle kinematics, dynamics, collision avoidance, and acoustic communication connectivity, and the application of state-of-the-art MINLP solution techniques. Each vehicle robot starts from a fixed start point and moves toward a goal point along a fixed path, so as to avoid collisions and remain in communication connectivity with other robots. Acoustic communication connectivity constraints account for the attenuation due to signal propagation and delays arising from multipath propagation in noisy communication environments, and specify intervehicle connectivity in terms of a signal-to-noise ratio (SNR) threshold. Scenarios including up to four robots are simulated to demonstrate: 1) the effect of communication connectivity requirements on robot velocity profiles; and 2) the dependence of the solution computation time on the communication connectivity requirement. Typically the optimization improved connectivity at no appreciable cost in journey time (as measured by the arrival time of the last arriving robot). Results also demonstrate the responsive nature of robot trajectories to safety requirements with collision avoidance being achieved at all times despite overlapping and intersecting paths.

Description: In this paper, artificial flocs, similar to the ones that are found in a recently deposited cohesive seabed, were prepared using Ca-montmorillonite and kaolinite (two of the most abundant clay minerals), guar gum (a polysaccharide closely resembling natural marine organic matter), and artificial sea water, in order to investigate the effect of the composition on their rheological response. Shear rate effects on viscosity of these suspensions were also systematically addressed. These effects typically have a strong influence on the overall rheological response of materials incorporating any polymeric substance. It was found that the addition of guar gum, even as little as 5% relative to the clay content, increases the viscosity of the floc suspensions significantly. The higher the guar-gum and clay content, the greater is the viscosity of the floc suspensions, within the guar-gum loading range investigated in this study. The floc suspensions with higher solid content were found to exhibit a slight shear thinning behavior, apparently due to the breakdown of the floc structure under high shear stress. This behavior is more pronounced in montmorillonite floc suspensions than in the kaolinite suspensions due to the differences in the clay mineral surface charge properties and surface-to-mass ratios. We explore different rheological models in their applicability to describing the observed viscous response and conclude that a cross-type formulation may be most appropriate.

Description: The imaging of underwater acoustic sources using passive 3-D sonar systems has many potential applications. However, to achieve both wide bandwidth and low cost, an array that is superdirective at low frequencies and aperiodic, to avoid aliasing at high frequencies, is required. To design a sparse array layout and the related filter-and-sum beamformer, a recently proposed method for airborne acoustic cameras is applied. First, the method is generalized to include position errors. Next, the method is used to demonstrate that 49 low-cost, poorly matched hydrophones are sufficient to create a square array for an underwater acoustic camera working from 500 Hz to 8.75 kHz with a side length of 1 m. Finally, the images of a simulated acoustic scenario obtained using different passive systems are compared, which reveals the advantages of the proposed design strategy.

Description: An underwater acoustic network simulation methodology is presented that is based on lookup tables (LUTs) with physical-layer error ratios. These LUTs are prepared with a validated replay channel simulator, which is here driven by channel measurements from the Kauai Acomms MURI 2011 (KAM11) experiment. Three physical-layer candidates are considered: a coherent single-carrier scheme, a coherent multicarrier scheme, and an incoherent scheme. The three modulation schemes are operated at a fixed message size and at four different data rates. Low rates are more robust to noise, interference, and channel dispersion, but also more prone to collisions in a network with busy traffic, because the packets are longer. Error statistics for colliding packets have been measured and are incorporated in the network simulator as collision LUTs. Example simulations are presented for a reduced flooding protocol with or without retransmissions. The results demonstrate how retransmissions pose a tradeoff between performance at high and low traffic load. The best network performance is obtained if the highest data rate is selected that yields a reasonably well-connected network. Collision avoidance is more important than the extended connectivity offered by low-rate signaling. On the other hand, at a given bit rate, a physical-layer scheme with extended connectivity but relatively weak links is shown to outperform a scheme with stronger links over fewer connections.

Description: This paper presents an underwater acoustic channel simulation methodology that combines parametric modeling with stochastic replay of at-sea measured channel impulse responses. The motivation behind this approach is to extend the scope of use of replay-based methods by allowing some parameterization of the channel properties while complying with some level of realism. Such an approach is beneficial for extensive testing of communication links. The key idea is to deliberately distort the statistics of the experimental channel in order to meet some user-specified constraints. Our approach is based on a relative entropy minimization between the original time-varying channel impulse response and the simulated one. A particular attention is given to constraints on the channel Doppler spread and on the level of covariance between channel taps. The testing capabilities provided by parametric replay-based simulations are illustrated with real data collected in the bay of Brest, France.

Description: Many marine scientific, industrial, and military applications may require the deployment of underwater acoustic sensor networks for sensing and monitoring. A grid topology with multihop relaying is useful for wide area coverage as well as long distance data transmission. We investigate network architectures where data originate at one end of the grid, and are forwarded along multiple lines. We are particularly interested in transmission schedules that maximize network throughput by exploiting propagation delay to allow multiple simultaneous transmissions. We show that an optimal schedule is necessarily per-node fair, and derive the upper bound on throughput. Furthermore, we present a low-complexity algorithm to find schedules achieving the upper bound, regardless of the size of the network.

Description: In this paper, we provide a complete geometrical treatment of classical rays emanating from an underwater point source and propagating in an unbounded medium where the speed of sound has a hyperbolic cosine dependence on the depth coordinate $(z)$ . General results are derived exclusively from Snell's law and are not limited to the case in which the ray emitting source is located at a point on the minimum propagation speed plane. Explicit relations are provided for the following: 1) the ray depth coordinate $(z)$ expressed as a function of the ray horizontal range $(rho)$ and the ray source angle $(theta_0)$ ; 2) all the relations among the ray source angle $(theta_0)$ , the ray receiver angle $(theta)$ , and the travel time $(tau)$ to reach an arbitrary position of the receiver from an arbitrary position of the source; and 3) the classical wavefront coordinates $(rho,z)$ along a ray expressed as a function of the ray source angle $(theta_0)$ and travel time $(tau)$ . From the wavefront coordinates $(rho,z)$ , we construct and display ray/wavefront diagrams for a varying source depth $(z_0)$ relative to the minimum propagation speed plane. We also derive the time-averaged acoustic- energy flux carried along classical ray tubes.

Description: Long reverberation tails are often observed in shallow-water acoustic channel impulse responses (CIRs). Examples from three significantly different environments along the Norwegian coast are presented. It is shown that 2-D propagation modeling fails to reproduce the long tails. Nevertheless, inclusion of the impulse response tails is necessary in connection with model-based simulators of communication performance, to avoid too optimistic estimates of bit error rates, etc. This is demonstrated by simulation examples for one of the environments, comparing results for truncated and complete impulse responses. High wind speeds and Doppler spread signals indicate that surface reverberation must be taken into account for modeling the tail energy. In bistatic scenarios such as the ones considered here, backscattering computations in a single vertical 2-D plane are not sufficient. A 3-D ray-based model, Rev3D, is utilized to include effects of out-of-plane scattering from the sea bottom as well as the sea surface. The scattering-strength functions include azimuthal variation, with significantly enhanced scattering close to the forward direction. Rev3D modeling supports the hypothesis that out-of-plane scattering and reverberation are main mechanisms behind the observed continuous, exponentially decaying impulse responses. Modeling incorporates available environmental information concerning sound-speed profiles, bottom topography and type, surface wave spectra, etc. Good agreement of measured and modeled time series is obtained for two of the locations. For the remaining location, modeling correctly recovers a significantly lower decay rate of the reverberation tail in comparison to the other locations, but the modeled decay rate is somewhat too large.

Description: Sidescan sonars are used to provide a high-resolution 2-D image of the seafloor, but when used in shallow water, these side-looking systems are vulnerable to multipath interference. In some cases, this interference affects image interpretation and downstream processing such as target recognition or bottom classification. However, it is possible to suppress multipath interference by using a small array featuring a vertical stack of receivers. Multipath signals that arrive from the direction of the surface are easily suppressed using across-track receive beamforming, however multipath signals that arrive from the seafloor are not so easily removed. This paper investigates the use of time-varying across-track receive beamforming as a method to suppress these bottom-bounce signals. Two sidescan images are presented that illustrate the impact that bottom-bounce multipath can have on sidescan sonar images. A theoretical model is presented that gives the relative intensity of the received signals and illustrates how their intensities are changed by altering the receive beampattern. In the first example, a bottom–surface–bottom signal arriving from nadir is suppressed by simply reducing the extent of the main lobe before the signal is received. In the second example, two multipath signals arriving near broadside are suppressed by introducing a null into the main lobe. It is concluded that an array employing the proposed beam processing is capable of rejecting bottom-bounce multipath, assuming that the angle and time of arrival of the interference and bottom signal are known.

Description: The plasma sparker has been applied in oceanic high-resolution seismic exploration for decades. Normally it is towed on the water surface. This is suitable for shallow water, but if the water depth is great, the resolution will decrease dramatically, especially in the horizontal direction. This paper proposes the concept of a deep-towed plasma sparker and presents an experimental study of plasma sparker performance in terms of electric parameters, bubble behavior, and acoustic characteristics. The results show that hydrostatic pressure at a source depth ranging from 1 to 2000 m has a negligible influence on the electric parameters but a strong influence on bubble behavior, wherein both the maximum bubble radius and oscillation period are decreased. The collapse pulse vanishes when the source depth reaches 1000 m or deeper, and no bubble oscillation can be distinguished. The source level (evaluated by the expansion pulse) is also decreased as the source depth increases; moreover, the greater the discharge energy, the smaller the source level loss. The discharge energy per electrode should be greater than 20 J for the deep-towed plasma sparker, which can make the source level loss induced by hydrostatic pressure smaller than the transmission loss. The fast Fourier transform (FFT) results show that the dominant energy is around 20 kHz, which is mainly induced by the expansion pulse and its oscillation. According to the simulation results, the fundamental frequency of the acoustic waveform increases with source depth in accord with a log linear trend, and also reaches tens of kilohertz in deep water. So, before the development of deep-towed plasma sparker, a new technical solution will need to be developed to solve this problem.

Description: Ocean exploration is typically interpreted with reference to the location of a data collecting node, e.g., when reporting an event occurrence, or the location of an object itself. Underwater tracking (UT) is somewhat different from tracking using RADAR or radio-frequency signals, due to irregularities of the ocean current and depth-varying sound speed in water. Sound-speed uncertainty also makes it challenging to incorporate Doppler shift measurements into UT. In this paper, we present a new UT scheme which considers the above challenges and utilizes spatial correlation of ocean current to estimate the drift velocity of the tracked node (TN) as a combination of the drift velocities of anchor nodes. We also offer two types of unbiased confidence indexes aimed to control the use of drift velocity estimation. To evaluate the performance of our UT scheme, we employ a hybrid simulator that combines numerical models for the ocean current and the signal-power attenuation in the ocean. We also report results from two sea trials conducted in the Mediterranean Sea and in the Indian Ocean. By tracking the sound speed, and utilizing Doppler shift measurements and drift velocity information of anchor nodes, accuracy is significantly improved.

Description: Passive localization of weak sources in the presence of strong interferences is generally a difficult task. In this paper, an eigenanalysis-based adaptive interference suppression (EAAIS) method is proposed for estimating the bearing and subsequently the weak source range. Assuming the target of interest (TOI) is in a known bearing sector, first we define a contribution ratio for each eigenvector of the cross-spectral density matrix (CSDM). The eigenvectors not dominated by the TOI are adaptively identified and removed for interference suppression. The remaining eigenvectors are then used to reconstruct the CSDM for TOI bearing estimation. Finally, the acoustic source range estimation is achieved with the reconstructed CSDM, by matching the beam intensity striation pattern, where it is not required to estimate the waveguide-invariant parameter. The effectiveness of the proposed TOI localization is demonstrated using several experimental data sets.

Description: The temperature dependence of the parameters within an effective density fluid model that incorporates the effects of granularity is used to examine sound speed and attenuation of sand within the temperature range of 1 $^{circ}$ C–30 $^{circ}$ C. Analysis of the behavior of attenuation as a function of frequency and permeability over the above temperature range give some new insights. In particular, the attenuation coefficient can either increase or decrease with temperature depending on the physical parameters and the frequency range being examined. A relaxation frequency is defined that separates frequency ranges where attenuation temperature dependence changes. Below the relaxation frequency, attenuation increases with temperature, and above this frequency, attenuation decreases with temperature. The analysis of the effect of sand sediment physical parameters on acoustic properties versus temperature also reveals the unique role of permeability. An “attenuation transition permeability” is also defined. As temperature increases, attenuation increases if the static permeability is less than the attenuation transition permeability and decreases if the static permeability is greater than the attenuation transition permeability. The comparison of experimental data sets with calculations using the temperature-dependent effective density fluid model indicates the dominant role of pore water both on the temperature-dependent sound speed and attenuation of sand sediment and gives support to use of an effective-density-based model.

Description: Sperm whales (Physeter macrocephalus) generate a series of broadband impulsive echolocation signals called clicks when diving in search of prey. These acoustic signals can be divided into usual clicks and creak clicks. They have a frequency spectrum that lies primarily between 2 and 18 kHz. In this paper, an adaptive energy-based click detector called the adaptive Teager energy operator (ATEO) is developed. A windowing technique is used to account for the time-varying characteristics of these signals, and the window length is adapted by estimating the interclick interval (ICI). The usual clicks and creak clicks are classified via ICI differences. The proposed method is compared with several well-known techniques using real data provided by the Atlantic Undersea Test and Evaluation Center (AUTEC). Performance results are presented which show that the proposed technique is a robust and efficient detector that can improve the click detection true positive rate (TPR) as well as reduce the false positive rate (FPR).

Description: Cabled ocean observatory systems that provide abundant power and broad bandwidth communication enabling undersea science have been evolving during the last decade. To establish such permanent infrastructure in the ocean, the technology of cable network switching and fault isolation with very high reliability is essential. In this paper, we review existing switching methods as applied to a constant voltage tree topology network. We propose an actively controllable method that can configure each branch of the network only by changing the feeding current; the current level implicitly conveys the switching information. A laboratory prototype demonstrated the features of backbone switching with zero current and low voltage (less than 20 V), and active controllability of the switch.

Description: The accuracy of any sonar performance prediction depends on the accuracy of its environmental and system-related inputs [e.g., transmission loss (TL), ambient noise $({L}_{N})$ , and target source level $({L}_{S})$ , among others]. However, particularly with the environment, perfect temporal and spatial knowledge of the input is simply unavailable, and as a result, performance prediction is often accomplished using generic input parameters. While this method is often adequate, the uncertainty in the inputs and the effect of this uncertainty on the resulting performance prediction typically remain uncharacterized. A method of accounting for this uncertainty and quantifying the predictive probability of detection (PPD) using probability density functions (pdfs) of TL ( ${f}_{c}=$ 900 Hz, $Delta{f}=$ 200 Hz), ${L}_{N}$ , and ${L}_{S}$ is applied to data collected at two sites in the southern East China Sea northeast of Taiwan during the 2008–2009 Quantifying, Predicting and Exploiting Uncertainty (QPE) Experiment. The first (site A) is located in a relatively flat-bottomed shallow-water (100–110 m) environment 37 km north of the continental shelfbreak, and the second (site B) is located on the 130-m isobath, closer to the continental shelfbreak near the westernmost branch of North Mien Hua Canyon. Uncertainty in measured TL and ${L}_{N}$ is quantified, and curves of PPD versus range are presented for both sites. At the time scale of an individual 8-h test event, statistically significant difference- in TL were observed over time at both sites. However, longer term averages of several test events spanning up to two years showed little to no difference between the two sites. The greatest source of uncertainty in sonar performance prediction at both sites was found to be that of the ambient noise.

Description: We have installed the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) that includes 20 observatories at Kumanonada (called DONET1) for the purpose of monitoring the seismogenic zone around the Nankai Trough. During the construction of DONET1, a remotely operated vehicle (ROV) was successfully used to lay the optical-fiber submarine cables that connect the observatories with the science nodes; however, the cable-laying operation was onerous because it takes approximately 10 h and is manually performed by ROV operators. For this reason, the cable-laying operation is usually not performed on successive days, which makes it difficult to schedule the construction work. In this paper, we present an automated cable-laying system that can automatically pay out the optical-fiber submarine cable on the seafloor at a rate that keeps pace with the ground speed and also adjusts the cable tautness. The components and control strategies are first described, and the results of some experiments and the first mission are then presented. The experiments and the first mission demonstrate that the automated cable-laying system can reduce the physical and mental burdens on the ROV operator and speed up the installation of DONET2 in 31 observatories. DONET2 is planned to be deployed off the Kii Peninsula (on the western side of DONET1).

Description: This index covers all technical items - papers, correspondence, reviews, etc. - that appeared in this periodical during the year, and items from previous years that were commented upon or corrected in this year. Departments and other items may also be covered if they have been judged to have archival value. The Author Index contains the primary entry for each item, listed under the first author's name. The primary entry includes the co-authors' names, the title of the paper or other item, and its location, specified by the publication abbreviation, year, month, and inclusive pagination. The Subject Index contains entries describing the item under all appropriate subject headings, plus the first author's name, the publication abbreviation, month, and year, and inclusive pages. Note that the item title is found only under the primary entry in the Author Index.

Description: The Editor-in-Chief reports a list of articles (including all types of contributions published in the JOURNAL) that have been published on IEEE Xplore as Early Access papers in the last three months. To remind you, Early Access enables publication of papers that are accepted in the review process when final approval is given by the Editor-in-Chief. Thus, the paper is available in an electronic archive in advance of its appearance in a regular issue of the JOURNAL. Unfortunately, the delay can be several months, so Early Access provides a distinct benefit for authors and readers alike. However, there are still some problems that offset the general benefit, notably there is not widespread dissemination of the publication event on the archive. We are continuing to work with IEEE to improve the system and reduce the delays. The list of Early Access papers published during the time since the January issue is given.

Description: Provides a listing of the editors, board members, and current staff for this issue of the publication.

Description: In this paper, we propose a class of methods for compensating for the Doppler distortions of the underwater acoustic channel for differentially coherent detection of orthogonal frequency-division multiplexing (OFDM) signals. These methods are based on multiple fast Fourier transform (FFT) demodulation, and are implemented as partial (P), shaped (S), fractional (F), and Taylor (T) series expansion FFT demodulation. They replace the conventional FFT demodulation with a few FFTs and a combiner. The input to each FFT is a specific transformation of the input signal, and the combiner performs weighted summation of the FFT outputs. The four methods differ in the choice of the pre-FFT transformation (P, S, F, T), while the rest of the receiver remains identical across these methods. We design an adaptive algorithm of stochastic gradient type to learn the combiner weights for differentially coherent detection. The algorithm is cast into the multichannel framework to take advantage of spatial diversity. The receiver is also equipped with an improved synchronization technique for estimating the dominant Doppler shift and resampling the signal before demodulation. An additional technique of carrier sliding is introduced to aid in the post-FFT combining process when residual Doppler shift is nonnegligible. Synthetic data, as well as experimental data from a recent mobile acoustic communication experiment (few kilometers in shallow water, 10.5–15.5-kHz band) are used to demonstrate the performance of the proposed methods, showing significant improvement over conventional detection techniques with or without intercarrier interference equalization (5–7 dB on average over multiple hours), as well as improved bandwidth efficiency [ability to support up to 2048 quadrature phase-shift keying (QPSK) modulated carriers].

Description: In a wireless network, where propagation delay is high and communications are sporadic, some kind of reservation protocol is generally used. Reservation access protocols were proposed earlier in earth stations-to-satellite communication with known propagation delay. However, optimality of the number of access slots with respect to the system performance parameters, such as system utilization, blocking probability, and delay, were not thoroughly studied. Besides, the effect of propagation delay uncertainty, which predominantly happens in underwater communications, are yet to be addressed. In this paper, we first analyze the system performance in many-to-one multiaccess data transfer scenario in underwater wireless ad hoc sensor networks with a fixed number of access slots and with the assumption of perfect propagation delay information. We propose two system state aware dynamic approaches to suitably adjust the number of access slots, and investigate the optimum slotting strategy to maximize the system utilization. Next, by accounting the propagation delay uncertainty, we relook into the optimality criteria on the number of access slots, where we apply a modified receiver-synchronized slotted Aloha principle to maximize the access performance. Via mathematical analysis, supported by discrete event simulations, we show that the system utilization and blocking probability performances with our proposed dynamic reservation protocols are consistently better compared to the competitive reservation protocols with fixed as well as variable access slots. Further, we conduct NS3 simulations to study the protocol performances under more realistic channel and traffic conditions, which also demonstrate that the proposed optimized dynamic slotting offers a much better system utilization performance compared to a similar underwater reservation multiaccess protocol.

Description: Underwater acoustic channels are typically characterized by long propagation delay, time-varying multipath, as well as very limited bandwidth, thus making it very challenging to develop efficient underwater medium access control (MAC) protocols. In this paper, we propose a new MAC protocol for underwater acoustic networks (UANs), called the hybrid collision-free medium access (HCFMA) scheme. The operation of the HCFMA scheme is based on the combination of the multichannel technique, the channel reservation mechanism, and the juggling-like stop-and-wait (JSW) transmission scheme. With the multichannel technique, the exchange of control signaling and data transmission can be separated, consequently, increasing the throughput of the network. The channel reservation mechanism which is carried out by the temporarily chosen control nodes guarantees no collision in data transmission. This is different from those whose channel reservations rely on a dedicated central controller. The JSW scheme provides an error-control mechanism during data transmission. Together, these approaches greatly improve the performance of UAN in throughput and delay. This paper describes the operation of the HCFMA scheme and analyzes its performance. We further conduct simulation to validate our analytical results. Our results show encouraging throughput and delay performance compared with other popular solutions, especially under heavy load conditions.

Description: In this paper, Alamouti space–frequency block coding, applied over the carriers of an orthogonal frequency-division multiplexing (OFDM) system, is considered for obtaining transmit diversity in an underwater acoustic channel. This technique relies on the assumptions that there is sufficient spatial diversity between the channels of the two transmitters, and that each channel changes slowly over the carriers, thus satisfying the basic Alamouti coherence requirement and allowing simple data detection. We propose an adaptive channel estimation method based on Doppler prediction and time smoothing, whose decision-directed operation allows for reduction in the pilot overhead. System performance is demonstrated using real data transmitted in the 10–15-kHz acoustic band from a vehicle moving at 0.5–2 m/s and received over a shallow-water channel, using quadrature phase-shift keying (QPSK) and a varying number of carriers ranging from 64 to 1024. Results demonstrate an average mean squared error gain of about 2 dB as compared to the single-transmitter case and an order of magnitude decrease in the bit error rate when the number of carriers is chosen optimally.

Description: The main objective of this paper is detection, recognition, and abundance estimation of objects representing the Prorocentrum minimum (Pavillard) Schiller (P. minimum) species in phytoplankton images. The species is known to cause harmful blooms in many estuarine and coastal environments. The proposed technique for solving the task exploits images of two types, namely, obtained using light and fluorescence microscopy. Various image preprocessing techniques are applied to extract a variety of features characterizing P. minimum cells and cell contours. Relevant feature subsets are then selected and used in support vector machine (SVM) as well as random forest (RF) classifiers to distinguish between P. minimum cells and other objects. To improve the cell abundance estimation accuracy, classification results are corrected based on probabilities of interclass misclassification. The developed algorithms were tested using 158 phytoplankton images. There were 920 P. minimum cells in the images in total. The algorithms detected 98.1% of P. minimum cells present in the images and correctly classified 98.09% of all detected objects. The classification accuracy of detected P. minimum cells was equal to 98.9%, yielding a 97.0% overall recognition rate of P. minimum cells. The feature set used in this work has shown considerable tolerance to out-of-focus distortions. Tests of the system by phytoplankton experts in the cell abundance estimation task of P. minimum species have shown that its performance is comparable or even better than performance of phytoplankton experts exhibited in manual counting of artificial microparticles, similar to P. minimum cells. The automated system detected and correctly recognized 308 (91.1%) of 338 P. minimum cells found by experts in 65 phytoplankton images taken from new phytoplankton samples and erroneously assigned to the P. minimum class 3% of other objects. Note that, due to large variations of texture and size of P. minimum cells as well as- background, the task performed by the system was more complex than that performed by the experts when counting artificial microparticles similar to P. minimum cells.

Description: Underwater acoustic channels are fast varying spatially and temporally according to environmental conditions. Adaptive modulation and coding (AMC) is appealing for underwater acoustic communications to improve the system efficiency by matching transmission parameters to channel variations. In this paper, we construct an AMC system with a finite number of transmission modes in the context of underwater orthogonal frequency-division multiplexing (OFDM). We propose the effective signal-to-noise ratio (SNR) computed after channel estimation and channel decoding as a new performance metric for mode switching, which is shown to predict the system performance more consistently than the input SNR and the pilot SNR. Real-time AMC tests have been conducted in a recent sea experiment to maximize the transmission rate with a given transmission power.

Description: Underwater vehicles that travel inside a bubble or supercavity offer possibilities for high-speed and energy-efficient transportation of cargo and personnel. Validation and testing of mathematical models and control systems for these vehicles is a challenge due to the cost and complexity of experimental facilities and testing procedures. A cost-efficient and low-complexity approach to the experimental validation of mathematical models and control systems for a supercavitating test vehicle is presented in this paper. The proposed method enables the testing of control algorithms subject to steady and unsteady flows in a high-speed water tunnel. The method combines a real-time simulation of the vehicle dynamics, force measurements from an experimental scale vehicle, and flight control computer to reproduce the vehicle motion subject to realistic flow conditions and hardware constraints as actuator saturation and time delay. The model of the vehicle dynamics, used for the validation infrastructure and control design, is derived using experimental data. A controller designed to track pitch angle reference commands was tested on the experimental platform. The test cases validated the operation of the vehicle and controller subject to steady and unsteady flows.

Description: The goal of this study is to verify the applicability of fuel cells for powering surface vessels by developing a small catamaran-type unmanned surface vehicle (USV) powered by a hydrogen fuel cell. This study encompasses the entire process of designing a fuel-cell-powered USV, as well as sea trial tests with a 1.487-m full-scale prototype. To improve the performance and system efficiency of the USV, a simulation-based hull form design and integrated control systems were implemented during the design process. First, the optimum hull form of the catamaran, with the optimum separation, was determined based on hydrodynamic optimization using parametric models to achieve good fuel economy. Second, an automatic control system integrated with the global positioning and inertial navigation systems was implemented in the designed catamaran so that it had the ability to perform waypoint, departure, station keeping, and turning circle maneuvers. Next, the gain parameters of the fuzzy proportional–integral–derivative (PID) controller were adjusted to assure reliable tracking control. Automatically controlled trial tests were then conducted to validate the performance of the autonomous surface vehicle with respect to the fuel cell power requirements and its dynamic motion. Based on the results of the trial tests and the transient response of the fuel cell, a hybrid power control algorithm for the fuel cell and the batteries was implemented for more efficient power utilization. This catamaran-type unmanned surface vehicle powered with a fuel cell can be utilized for ocean environment monitoring and multifunctional missions.

Description: This paper addresses the problem of array shape estimation for passive towed sonar systems during platform maneuvers. Directional noise fields due to distant shipping lanes can be exploited as sources of opportunity for online array shape calibration. In this paper, a nonparametric noise field model is used to form field directionality maps for time-varying array shapes to exploit point and spatially spread sources. This formulation requires neither the number nor location of sources in the field to be known or estimated. Using acoustic data, a maximum-likelihood array shape estimate is derived where the shape is modeled as a polynomial in heading. Additionally, a method for fusing the shape estimate with heading sensor data is introduced. Heading sensors may permanently fail or suffer from high levels of noise during turns; thus acoustic data can be used to compensate for malfunctioning heading sensors during turns. The combined estimate is filtered using a dynamical model that is valid for sharp turns and accounts for motion of the array perpendicular to tow heading. Multisource simulations are used to demonstrate the performance of the acoustic-based estimate and robustness of the combined estimate.

Description: Provides a listing of the editors, board members, and current staff for this issue of the publication.

Description: This work is motivated by the needs in the doubly dispersive underwater acoustic (UWA) communication channels. We propose and develop the use of filterbank multicarrier (FBMC) technique for communications in the channels of interest. A novel cost function for optimization of the filterbank prototype filter, to achieve a robust performance in doubly dispersive channels, is proposed. A design algorithm that optimizes the proposed cost function is then developed. The developed FBMC technique is compared with the celebrated orthogonal frequency-division multiplexing (OFDM). Our study shows that in doubly dispersive channels, in terms of signal-to-interference-plus-noise ratio (SINR), FBMC outperforms OFDM by several decibels. The theoretical results are validated and confirmed to be applicable to UWA communications, by examining real data from an at-sea experiment (ACOMM10), off the coast of New Jersey in July 2010.

Description: Provides a listing of current committee members and society officers.