ALBERT

Description: Presents the table of contents for this issue of the publication.

Description: This paper proposes an effective method of improving ship detection performance of a compact high-frequency (HF) radar system which has been primarily optimized for observing surface radial current velocities and bearings. Previously developed ship detection systems have been vulnerable to error sources such as environmental noise and clutter when they are applied in a compact HF radar optimized for observing surface current. In particular, the influences of error are reduced by applying a principle component analysis of the generated range-Doppler maps. A compact radar signal model is first developed by the data acquired from an operating compact HF radar site. The proposed method is then validated by comparing it to the conventional ship detection method in terms of detection and false alarm rates. The experimental results confirm that the proposed method shows superior performance in both simulated and practical environments.

Description: An eccentric paddle mechanism based on the epicyclic gear mechanism (ePaddle-EGM), proposed to enhance the mobility of amphibious robots for multiterrains tasks, can execute several terrestrial and aquatic gaits. In this paper, we rectify the experimental setup as well as the thrust modeling to do further research on rotational paddling gait. The effects of the rotational period and the location of paddle shaft to the thrust are analyzed. Besides the amplitude features, experimental results also illustrate the vector character of thrust generated by the rotational paddling gait.

Description: We consider random linear packet coding for fading channels with long propagation delays, such as underwater acoustic channels. We propose a scheme in which the number of coded packets to transmit is determined to achieve a prespecified outage/reliability criterion and investigate joint power and rate control with constrained resources. Using the channel state information that is obtained via feedback from the receiver, the transmitter adjusts its power and the number of coded packets so that the average energy per successfully transmitted bit of information is minimized. Two optimization constraints are imposed: 1) the transmit power should not exceed a maximum level; and 2) the number of coded packets should not exceed a maximum value dictated by the desired throughput and delay. We further extend the results to take into account the effect of inevitable channel estimation errors, and consider the case in which the transmitter has only an estimate of the channel gain. We design adaptation policies for such a case based on minimum mean square error (MMSE) channel estimation, taking into account the effect of channel estimation errors in an optimal manner to satisfy the required outage/reliability criterion. Finally, we compare the proposed technique to standard automatic repeat request (ARQ) protocols for underwater communications in terms of the throughput efficiency. Analytical results show that substantial energy savings and improvements in throughput efficiency are available from adaptive power/rate control. We also present experimental results obtained using channel gains measured during the Surface Process Acoustic Communication Experiment (SPACE-08), an at-sea underwater experiment conducted off the coast of Martha's Vineyard in fall 2008.

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

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

Description: A distributed networked underwater sensor (DNUS) system can provide ocean measurements over a wide area with a large number of sensors. This paper studies the estimation of the ocean current field observed by a DNUS system. Considering that the current field is correlated in time and space, we present a statistically-based acoustic travel time difference tomography method based on a Kalman filter (KF) to reconstruct and track the ocean current field. A spatiotemporal autoregressive (AR) model is used to describe the time evolution of the current field. In the spatiotemporal AR model, the observation region is divided into subtriangle grids. The subtriangles are partitioned into clusters and each cluster is assigned with one AR coefficient. Moreover, the AR coefficients are updated adaptively with the past estimated current velocities. The proposed method is verified with the synthetic observational data generated by a barotropic ocean model. Compared with the regular distributed processing method, the proposed ocean current field reconstruction and tracking method achieves a lower region-integrated root-mean-square error (RMSE). In addition, by making use of the spatiotemporal correlation, the proposed method is robust to the measurement link failure and burst errors in the DNUS system.

Description: Modal interferences are evident in the spectrogram of broadband signals propagated over distances and received on a single receiver. Plotted against range and frequency, the spectrogram displays striated bands of constant acoustic intensity levels as explained by the waveguide-invariant theory, the slope of which can be used to estimate the source range by the waveguide-invariant parameter “beta.” Given a vertical array of receivers, one finds, however, no frequency striation in the depth-integrated acoustic energy, as the total energy corrected for geometrical spreading remains “conserved” except for mode attenuations. When nonlinear solitary internal waves (SIWs) are present in the propagation path, one finds a different phenomenon: the depth-integrated energy will display striations with respect to time and frequency. In this case, the striation slope is related to the source-to-SIW range, and not the source–receiver range. The striation is caused predominantly by mode coupling (as opposed to mode interference), which changes the mode amplitudes as the SIWs move. Theory and simulations are developed in this paper. As an application, one can use the SIW-induced striations to monitor the positions (and wavefronts) of nonlinear internal waves on a continuous time basis. This method is evaluated using the SWARM95 experimental environment to address the real-world issues.

Description: Equation In [1], (6) of the above named work is corrected.

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

Description: This paper considers approaches to combining information from multiple arrays in matched-field processing (MFP) for underwater acoustic source localization. The standard approach is to apply conventional MFP for each array independently, and sum the resulting Bartlett ambiguity surfaces computed for each array; this approach assumes that individual arrays comprise calibrated sensors which are synchronized in time. However, if the relative calibration and/or time synchronization is known between some or all arrays, more informative multiple-array processors can be derived using maximum-likelihood methods. If the relative calibration between arrays is known, the observed variation in received signal amplitude between arrays provides additional information for matched-field localization which is absent in the standard processor. If synchronization is known between arrays, phase variations provide additional localization information. Multiple-array processors accounting for different levels of interarray information are derived and evaluated in terms of the probability of correct localization from Monte Carlo analyses for a range of signal-to-noise ratios and the number of frequencies for simulated shallow-water scenarios with multiple horizontal and/or vertical arrays. The analysis indicates that, dependent on array configurations, significant improvements in source localization performance can be achieved when including relative amplitude and/or phase information in the multiple-array processor. The improvement is reduced by environmental and array (calibration and synchronization) mismatch; however, this degradation can be partially mitigated by including additional frequencies in the processing.

Description: Studies of marine physical, chemical, and microbiological processes benefit from observing in a Lagrangian frame of reference. Some of these processes are related to specific density or temperature ranges. We have developed a method for a Tethys-class long-range autonomous underwater vehicle (LRAUV) (which has a propeller and a buoyancy engine) to track a targeted isothermal layer (within a narrow temperature range) in a stratified water column when operating in buoyancy-controlled drift mode. In this mode, the vehicle shuts off its propeller and autonomously detects the isotherm and stays with it by actively controlling the vehicle's buoyancy. The LRAUV starts on an initial descent to search for the target temperature. Once the temperature falls in the target center bracket, the vehicle records the corresponding depth and adjusts buoyancy to hold that depth. As long as the temperature stays within a tolerance range, the vehicle continues to hold that depth. If the temperature falls out of the tolerance range, the vehicle will increase or decrease buoyancy to reacquire the target temperature and track it. In a June 2015 experiment in Monterey Bay, CA, USA, an LRAUV ran the presented algorithm to successfully track a target isotherm for 13 h. Over the isotherm tracking duration, the LRAUV mostly remained in the 0.5 $^\circ $ C (peak-to-peak) tolerance range as designed, even though the water column's stratification kept changing. This work paves the way to coupling an LRAUV's complimentary modes of flight and drift—searching for an oceanographic feature in flight mode, and then switching to drift mode to track the feature in a Lagrangian frame of reference.

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

Description: A large number of transducers employed in the multibeam imaging sonar array significantly increases the hardware complexity, costs, and computational load. Although a great amount of work has been done to minimize the number of transducers in the array, one can verify that these methods do not provide optimal results when applied to the case of multiple beams. In this paper, a novel thinning method is proposed for multiple-beam conditions where the number of transducers is minimized by an iterative process. Optimal common transducers’ positions are selected by a wind-driven optimization (WDO) algorithm, and transducers’ weights are optimized by convex optimization in order to suppress the peak sidelobe level (PSLL). The performance of the proposed method is evaluated via simulations and experiments. Results show that the method is applicable to the minimization of the number of transducers under hundreds of beam conditions, and consequently reduces the complexity of the imaging sonar system with imaging performance still acceptable.

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

Description: In underwater acoustics, shallow-water environments act as modal dispersive waveguides when considering low-frequency sources, and propagation can be described by modal theory. In this context, propagated signals are composed of few modal components, each of them propagating according to its own wavenumber. Frequency-wavenumber $(f-k)$ representations are classical methods allowing modal separation. However, they require large horizontal line sensor arrays aligned with the source. In this paper, to reduce the number of sensors, a sparse model is proposed and combined with prior knowledge on the wavenumber physics. The method resorts to a state-of-the-art Bayesian algorithm exploiting a Bernoulli–Gaussian model. The latter, well suited to the sparse representations, makes possible a natural integration of prior information through a wise choice of the Bernoulli parameters. The performance of the method is quantified on simulated data and finally assessed through a successful application on real data.

Description: In this paper, a method is presented for correcting relative errors in the locations of individual sensors in a three-element bottom-moored array, using ambient signal arrivals recorded by the array. It has the benefit that knowledge of absolute source positions is not required to determine the relative shape of the array. The procedure is demonstrated using episodic ambient signal arrivals recorded by the southern array of the Wake Island hydrophone station, which is a component of the Comprehensive Nuclear-Test Ban Treaty Organization's (CTBTO) International Monitoring System (IMS). The original coordinates of sensors 2 and 3 were adjusted by 75 and 115 m, respectively, removing approximately 4 $^{\circ}$ of error in estimating the arrival angles of signals. The estimated coordinates compared favorably with the results of a position revision by the CTBTO a year after the triad installation.

Description: In this paper, the results of applying normal mode models to explain measured data collected during the 2013 Target and Reverberation Experiment (TREX13) are presented. An adiabatic, and one-way coupled, Born approximation normal mode model (Nautreverb) and an energy conserving two-way coupled mode model (Aeneid) are described. Both monostatic and bistatic reverberation time series solutions are produced with the models for a rough bottom spectrum or a realization from it. Next, monostatic reverberation time series are produced, for both the Nautreverb and Aeneid models, with the measured bottom roughness parameter values obtained from measurements made during TREX13. In addition, loss mechanisms for depth averaged transmission loss are assessed. The relative effects of bottom attenuation and loss due to scattering, for several variants of the model parameters for the TREX13 environment, are also discussed. Based on the small roughness measured at the TREX13 site, bottom attenuation appears to be the dominant loss mechanism. Adjustments of the frequency dependence of the bottom attenuation were constructed to reproduce salient features of the measured transmission loss.

Description: This paper presents an analysis of the time- and frequency-domain properties of a multiday series of background noise during the Target and REverberation eXperiment (TREX13) that took place off the coast of Panama City, FL, USA. Background noise was measured at two locations approximately 5 and 6 km south-east of the location of the R/V Sharp where monostatic reverberation was measured. The median spectral density level estimated for a frequency band centered at 2.7 kHz with a bandwidth of 1800 Hz was comparable to the median level at 3.5 kHz averaged over a bandwidth of 100 Hz; both were estimated to be 60 dB re 1 $\mu$ Pa $^2$ /Hz when averaged during the day or night. In contrast, the level at 4.5 kHz, estimated by averaging over a bandwidth of 100 Hz, increased by 14 dB at night. While the data suggest a degree of consistency in the background environment for the purposes of modeling, when assessing levels in the 80–8000 Hz frequency band, time of day was found to be influential. The median spectral levels presented may be used to assess the influence of background noise at a given time of day. Biological noise likely from fish was revealed to be a significant contributor to background noise at night, and daytime background noise levels were primarily influenced by wind.

Description: The sediment acoustic-speed measurement system is designed to measure in situ sediment sound speed and attenuation within the surficial 3 m of sediments in the frequency band of 2–8 kHz. Measurements were carried out during the Target and Reverberation EXperiment 2013 (TREX13) off Panama City, FL, USA. During TREX13, nine deployments at five selected sites were made along the 20-m isobath, termed the main reverberation track. The sediment types at the five selected sites ranged from coarse sand to a mixture of soft mud over sand, and the measured results show a spread of 80 m/s in sediment sound speed among the different types of sediments for all frequencies. Between 2–8 kHz, about 3% dispersion was observed at the sandy sites, whereas little dispersion was observed at the sites with mud. Preliminary attenuation results show 0.5–3.3 dB/m at the sandy sites, and 0.5–1.0 dB/m at the sites with mud in the same frequency band.

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

Description: In this paper, a technique is presented for frequency-smoothing the sample covariance matrix observed at the output of a set of elements or beams so as to obtain a frequency-smoothed estimate of the broadband noise covariance matrix due to underwater noise. The smoothing technique is based on a linear model that is derived through analysis of the delay-domain cross covariance of the ambient noise observed by a pair of beams or omnidirectional hydrophones. A simple and robust least squares fitting algorithm allows for extraction of a frequency-smoothed broadband noise covariance estimate even in the presence of narrowband energy. Simulated passive sonar examples demonstrate the validity and utility of the estimate. Possible applications include adaptive beamforming and measuring the vertical/horizontal directionality of diffuse broadband ambient noise sources such as wind and rain.

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

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

Description: This paper focuses on underwater acoustic (UA) communications and proposes a decentralized spectrum sharing method for noncooperative orthogonal frequency-division multiplexing systems in interference channels. The problem is formulated as a noncooperative game where the players are UA communication systems aiming at finding the power allocation on subcarriers that maximizes a utility function related to their information rate. Realistic assumptions regarding the UA context are formulated. Frequency-selective and randomly time-varying channels are considered. Each system is constrained in average power and adapts its power allocation strategy only with local knowledge of its channel statistics and noise plus interference power spectral density. This knowledge is obtained through a feedback link from the receiver. Estimation errors on the channel statistics are taken into account, thanks to a robust reformulation of the game. We show that an efficient decentralized spectrum sharing can be achieved when all players use a water-filling strategy against each other iteratively. Simulations results are obtained on synthetic but realistic channels. In configurations where the UA communication systems are in close areas, significant increases of spectral efficiencies can be expected compared to the conventional uniform power allocation. Results on channels sounded at sea support our conclusions.

Description: Recent advances in scheduling transmissions for underwater acoustic networks utilize and exploit long propagation latency of acoustic waves for achieving throughput gain. These techniques utilize the propagation delay information of the considered network geometry and schedule transmissions in time slots. Time-slotted transmissions are such that most of the interference overlaps with the transmission slots and the receiving slots are interference free. Moreover, exploiting propagation delays lead to multiple transmissions per time slot, thereby resulting in higher throughput. However, the packet duration of each transmission in the time slot is assumed to be fixed. The packet duration, however, provides a degree of freedom that, if utilized, results in strategies that can be adopted to achieve throughput closer to the established upper bound. Therefore, we consider the problem of finding unslotted transmission schedules allowing unequal packet duration. Given the propagation delay between nodes in the network and packet traffic demands, we formulate an optimization problem for minimizing the fractional idle time in a frame (or period) of the schedule as a mixed-integer linear fractional problem (MILFP). We compare our results to the recent advancements that exploit large propagation delays and result in time-slotted and unslotted schedules with fixed packet duration. We also present schedules computed for various network geometries with arbitrary packet traffic demands.

Description: Reliable SONAR-performance prediction in shallow water requires knowledge of the seabed reflectivity, or its geoacoustic properties, which is expensive and difficult to acquire in situ . This paper illustrates two sea trials conducted in different shallow water areas to investigate the feasibility of acquiring such knowledge efficiently from measurements of naturally occurring ambient noise by an array that is compact enough to be mounted on a small autonomous underwater vehicle. The system relies on a previous technique for passively estimating the bottom reflection loss from the acoustic noise field generated by wind and breaking waves at the sea surface. Results from these experiments supported by numerical modeling are presented and compared with independent measurements of the relevant seabed reflectivity properties. The results obtained from both experiments demonstrate the potential of using autonomous underwater vehicles for seabed characterization.

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

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

Description: In this work, carrier aggregation in orthogonal frequency division multiplexing (OFDM)-based underwater acoustic cellular networks is proposed, and its achievable communication rate and energy efficiency are evaluated. This technique improves the total throughput of OFDM using an expanded bandwidth, where multiple carriers are aggregated for data transmission from one transmitter. In this paper, a study is presented for a practical communication system consisting of a surface buoy station mounting single transducer and an autonomous underwater vehicle also with single transducer. The results are evaluated in a simulation environment as well as by field experiments. The simulation is first performed on a wide bandwidth up to ${\text{500}}\;{\text{kHz}}$ , where the results indicate that carrier aggregation can greatly improve the achievable underwater communication rate for distances up to ${\text{5}}\;{\text{km}}$ . It is further found by simulation that the maximum bandwidth configuration does not achieve the optimal energy efficiency; instead, there is an optimal bandwidth that can lead to such optimal energy efficiency. Further measurement results obtained in sea trials with a specifically designed sounding sequence are then presented. The achievable rate and energy efficiency are evaluated by the field measurement data. The measurement results provide insights into the performance of the system under narrow bandwidth settings.

Description: A seafloor laser scanner was deployed in the Gulf of Mexico during the 2013 Target and Reverberation Experiment (TREX13). This system collected digital elevation maps at 14 locations along the main reverberation track, and these measurements provide roughness power spectra for modeling seafloor acoustic scattering. The spectra were divided into two regimes according to the mid and high-frequency acoustic measurements made during the experiment. For the wave numbers corresponding to the midfrequency regime (2–4 kHz), the spectra could be approximated using the mean spectral exponent derived from the all of the spectra. With this spectral exponent, the best fit spectral strengths were found to be negatively correlated to the backscatter levels measured at 400 kHz using a multibeam echosounder (MBES). While the scattering mechanisms at 400 kHz are not influenced by the roughness at these low wave numbers, this correlation may be indirectly related to the bioturbation and the spatial variation of the shell content. A more pronounced correlation was found for the high wave numbers, where again a single spectral exponent could be used to a good approximation. In this case, the spectral strengths were also linearly related to the MBES backscattering level but with a positive correlation. For these wave numbers, the roughness is largely influenced by the shell content, which is also the dominant scattering mechanism at 400 kHz. The correlations between the roughness and the MBES measurements provide a means to approximate the seafloor roughness parameters in both wave number regimes throughout the experiment site. For the low wave number spectrum, an alternative approach is also proposed, which uses the spectral parameters for the mean spectrum to approximate the roughness throughout the TREX13 site.

Description: How do we self-localize large teams of underwater nodes using only noisy range measurements? How do we do it in a distributed way, and incorporating dynamics into the problem? How do we reject outliers and produce trustworthy position estimates? And what if some of the vehicles can measure angular information? The stringent acoustic communication constraints and accuracy needs of our geophysical survey application demand fast and very accurate localization methods. We address dynamic localization as a MAP estimation problem where the prior encodes kinematic information, and we apply a convex relaxation method that takes advantage of previous estimates at each measurement acquisition step. The resulting LocDyn algorithm is fast: It converges at an optimal rate for first order methods. LocDyn is distributed: There is no fusion center responsible for processing acquired data and the same simple computations are performed at each node. LocDyn is accurate: Numerical experiments attest to about 30% smaller positioning error than a comparable Kalman filter. LocDyn is robust: It rejects outlier noise, while benchmarking methods succumb in terms of positioning error.

Description: As a part of the 2013 Targets and Reverberation Experiment (TREX13), measurements of the acoustic field generated by a source used in midfrequency (1.8–3.6 kHz) reverberation experiments are studied at 5 and 6 km range. The TREX13 reverberation sources were placed off the coast of Panama City, FL, USA, in waters ∼20 m deep, and data discussed here are from a 2-h period in the late afternoon on April 28, 2013. The observed coda of the source signal is partitioned into an initial primary arrival, and a distinct second arrival delayed by roughly 2 s. Characteristics of the two arrivals are studied in terms of the effective number of modes, interference features, and the direction of acoustic intensity, which was directly measured by a vector sensor located at 5 km range. A shift in frequency within the primary arrival is observed over the 2-h measurement period. Frequency shifts are related to a change in range of dislocations, defined as points of complete destructive interference in the acoustic field, that modulate with tidal variation in the sound-speed profile. Precise frequencies are identified with the vector property called circularity, a nondimensional measure of acoustic intensity curl, that is maximal within the vortex-like intensity field within a dislocation. Using the waveguide invariant β, the frequency shift is used to estimate the tidal change in the thermocline depth. These interference features are absent in the second arrival, which is postulated to be an acoustic path horizontally refracted by the gently sloping bathymetry (∼0.4°) forming the coastal environment. A description of the refraction using modal rays is developed, and the transition of the mode from being trapped to leaky is handled as a transition to a virtual mode near the cutoff depth. Models of the primary and refracted arrivals are presented to support the conclusions.

Description: A significant driver of uncertainty in sonar performance is the variability in underwater acoustical propagation caused by environmental fluctuations and uncertainty in the position of sources, targets, and receivers. A set of echo-repeat experiments was conducted during the Target and Reverberation Experiment 2013 (TREX13), a sea trial that took place in April to May 2013 in the Gulf of Mexico near Panama City, FL, USA. The variability in measured transmission loss (TL) was characterized using two different methods: Variability with respect to a mean observed TL, and variability with respect to modeled TL. Both one-way and quasi-reciprocal two-way TL measurements at 2250 and 7500 Hz were analyzed to characterize the variability at timescales ranging from less than one second to several days, with the results indicating that the acoustic propagation fluctuates stochastically on all these time scales. The results of statistical tests suggest that the TL variability can be treated as Gaussian fluctuations about a central TL obtained from an acoustic propagation model, with standard deviations of 5 dB over timescales up to one day, or 10 dB over timescales from one to six days.

Description: In this paper, we address the problem of detecting a known signal in snapping shrimp noise. The latter dominates the ambient soundscape at medium-to-high frequencies in warm shallow waters. The noise process is impulsive, exhibits memory and is modeled effectively by stationary $\alpha$ -sub-Gaussian noise with memory order $m$ ( $\alpha$ SGN( $m$ )), which is essentially an impulsive Markov process. Robust detectors have long been used to mitigate the impact of impulsive noise on the performance of digital systems. However, conventional notions of robustness do not assume memory within the noise process. The $\alpha$ SGN( $m$ ) model offers a mathematical model to develop robust detectors that also exploit the mutual information between noise samples. Recent works in this area highlight substantial improvement in detection performance over traditional robust methods in snapping shrimp noise. However, implementing such detectors is a challenge as they are parametric and computationally taxing. To achieve more realizable detectors, we derive the locally optimal detector for $\alpha$ SGN( $m$ ). From it, we introduce the generalized memory-based sign correlator and its variants, all of which offer near-optimal performance in $\alpha$ SGN( $m$ ). The proposed detectors off- r excellent performance in snapping shrimp noise and low computational complexity. These properties make them attractive for use in underwater acoustical systems operating in snapping shrimp noise.

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

Description: Wideband and widebeam synthetic aperture sonar (SAS) can provide information on the frequency- and aspect-dependent scattering in a scene. We suggest an approach to predict the quality of the sensor data over the available frequencies and aspect angles. We relate the typical spatial domain quality metrics to their wave number domain (WD) counterpart, and use these to map the data quality in WD. Because SAS arrays often are undersampled along-track, we pay particular attention to data degradation from aliasing. We use the proposed approach to examine how three SAS image formation algorithms based on time domain backprojection (TDBP) access data of different quality from wideband SAS systems. We illustrate the results with predictions for a generic SAS design and demonstrate the findings on two experimental systems. We observe that the maximum support of high-quality data is achieved through BP onto a high-resolution grid followed by WD filtering.

Description: A method to generate vehicle maneuvering forces from a propulsor alone has been applied to a generic undersea vehicle. Open and ducted post-swirl propulsors were configured with an upstream rotor and downstream stator row. During normal operation, the downstream stator blades are all situated at the same pitch angle and generate a roll moment to counter the torque produced by the rotor. By varying the pitch angles of the stator blade about the circumference, it is possible to generate a mean stator side force that can be used to maneuver the vehicle. In addition, the side force can be increased with increasing thrust producing side forces at very low vehicle velocities enabling low-speed maneuvering capability. The viscous, 3-D Reynolds-averaged Navier–Stokes (RANS) commercial code Fluent was used to predict the vehicle and propulsor component forces as well as the velocity field. Open and ducted geometric configurations were studied and force coefficients computed and compared with currently used control surface forces. Computations predicted that the maneuvering propulsor generated side forces equivalent to those produced by conventional control surfaces with side force coefficients on the order of 0.25 for the open propulsor at the self-propulsion point. This translates to 50% larger forces than can be generated by conventional control surfaces on 21 $^{\prime \prime}$ unmanned undersea vehicles. The ducted configuration produces maximum side force coefficients on the order of 0.15, which is still sufficient for vehicle control. Both configurations produced side forces for the Bollard pull condition indicating low-speed maneuvering capability.

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

Description: Low-cost tethered buoys are important for seawater observation, coastal area monitoring, and pollution sensing. Underwater sensor networks operating at 433 MHz (ISM band) suffer high attenuation due to seawater conductivity. Significant propagation distance cannot be achieved through seawater or along the seabed. This paper reports a novel technique for communication between sensors operating in shallow water. A sensor tethered to the bottom was connected to a floating monopole antenna via an insulated wire transmission line. Experiments and calculations show that the attenuation along the transmission line was 38 dB/m. Surface propagation for buoy-to-base station was approximately 1 dB/m with a communication range of 30 m using a 10-dBm transmitter circuit with receiver sensitivity of ${-}$ 110 dBm. For buoy to buoy the surface propagation was measured as 3.5 dB/m with a communication range of 4 m. Experiments were carried out in calm water conditions. The results demonstrate that significant sensor network coverage of coastal regions is possible.

Description: An acoustic Digital MONitor (DMON) has been integrated into a Seaglider autonomous underwater vehicle to serve as a general-use tool for passive acoustic sensing of marine mammal vocalizations. The system is being developed as a complement to conventional ship-based cetacean survey methods. The acoustic system includes three omnidirectional hydrophones, one located on centerline of the aft payload hatch and one on each wingtip. An onboard real-time detector has been implemented to record an audio sample if ambient noise has risen above a user-prescribed signal-to-noise ratio (SNR) threshold level. The data size and the number of detections are available in semi-real time, and the acoustic data are retrieved upon recovery of the instrument. Because the DMON system interfaces with the Seaglider firmware, the glider pilot has the capability to modify several operational parameters governing the collection of acoustic data while the glider is deployed to tailor the data recording to the desired mission objectives. This implementation is referred to as the Seaglider Customizable Sampling Configuration (SCSC) DMON and has recorded a wide variety of cetacean vocal activity offshore the Hawaiian Islands.

Description: When an aspect-dependent target is insonified by an acoustic source, distinct features are produced in the resulting bistatic scattered field. These features change as the aspect between the source and the target is varied. This paper describes the use of these features for estimation of the target aspect angle using data collected by an autonomous underwater vehicle (AUV). An experiment was conducted in November 2014 in Massachusetts Bay to collect data using a ship-based acoustic source producing 7–9-kHz linear frequency modulation (LFM) chirps insonifying a steel pipe. The true target orientation was unknown, as the target was dropped from the ship with no rotation control. The AUV Unicorn , fitted with a 16-element nose array, was deployed in data collection behaviors around the target, and the ship was moved to create two target aspects. A support vector machine regression model was trained using simulated scattering bistatic field data. This model was then used to estimate the target aspect angle from the data collected during the experiment. The difference between the estimates was consistent with experimental observations of relative source positioning. The simulation-based model appeared successful in estimating the target aspect angle despite uncertainties in target and source location and mismatch between true environment and simulation parameters.

Description: This paper introduces a new subspace-based detection method for multichannel (high frequency and broadband) synthetic aperture sonar (SAS) imagery. An image-dependent dictionary learning method is applied to form the appropriate dictionary matrices for representing target and nontarget image snippets. The hypothesis testing is done by forming a test statistic that relies on the residual error power ratio in representing an unknown image snippet using the target and nontarget dictionary matrices. To avoid the computational bottleneck in most dictionary learning methods, a new recursive method is introduced which does not require any matrix inversion or singular value decomposition (SVD). The proposed detection method was then implemented and benchmarked against a matched subspace detection method for detecting mine-like objects. Results are then presented on two sonar imagery data sets collected in two geographically disparate locations.

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

Description: Presents the 2017 subject/author index for this publication.

Description: Ripples on the seafloor affect acoustic scattering and transmission loss, wave attenuation, and the amount of sediment transported in shallow water. Historically, seafloor roughness (a function of ripples, bedforms, sediment type, and size) is assumed to be spatially homogeneous and temporally static in hydrodynamic and acoustic models despite the often dynamic nature of the seafloor in the nearshore region. We present a spectral ripple model, Navy Seafloor Evolution Archetype (NSEA), which simulates the variations in seafloor roughness given measured or predicted wave conditions in sandy environments. NSEA simulates sand ripple formation and evolution based on bottom velocities either measured or predicted by a wave model. The time dependency is a function of equilibrium ripple geometries and the amount of sediment transport needed to reach an equilibrium state, which is dependent on the relict ripples. Spectral decay due to bioturbation is incorporated as a diffusive process. NSEA was validated with time series observations obtained in water depths of 7.5 and 20 m from April 20, 2013 to May 23, 2013 during the 2013 Target and Reverberation Experiment (TREX13) offshore of Panama City, FL, USA. The model predicted spectral ripple wavelengths that were in good agreement with observed spectral ripple wavelengths obtained using a fixed platform, high-frequency (2.25 MHz) sector scanning sonar. Likewise, the variations in the predicted normalized ripple heights and orientations were similar to the normalized spectral decay and orientations estimated from the sector scanning sonar imagery.

Description: The articles in this special section focus on research and development related to the Target and Reverberation Experiment (TREX13) conducted in the Gulf of Mexico off Panama City, FL, USA, in spring 2013. The primary goal of the experiment was to measure midfrequency (2–10 kHz), shallow-water reverberation with full companion environmental measurements such that models and data could be compared without ambiguity. While a number of other major field efforts, such as the Acoustic Reverberation Special Research Program and the Acoustic Clutter Reconnaissance Experiment, have been conducted to study low- to midfrequency reverberation, it has been difficult to measure and account for all of the relevant reverberation mechanisms in a single experiment.

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

Description: The 2013 Target and Reverberation Experiment (TREX13) sea trial was held just off the coast of Panama City, FL, USA, in May 2013. One of Defence Research and Development Canada's primary objectives was comparing the performance of continuous active sonar (CAS) and pulsed active sonar (PAS) in shallow water, where reverberation and clutter challenge sonar systems. The only option for a moving target during the trial was an echo repeater towed from a ship; however, this posed a serious challenge because before the trial there were no known echo repeater solutions that were compatible with CAS. Existing echo repeaters required short-duration waveforms as used in PAS, and even then they introduced range errors that caused a mismatch in reverberation background and made localization problematic. This paper presents three echo repeater techniques developed for TREX13 and tested on Canadian Forces Auxiliary research vessel CFAV Quest during the trial. Experimental results presented in this paper demonstrate detection of the echo repeater by a monostatic CAS system on R/V Hugh R Sharp.

Description: A growing attention has been recently devoted to the impact of the underwater noise field generated by shipping activities on the marine fauna. A key aspect for the quantification of such impact is a proper model of the source levels radiated from the vessels. At a first level of approximation, simplified formulations based on a small number of macroparameters describing the ship characteristics are needed to quantify the emission and, accordingly, assess the noise impact and evaluate strategies for its control. In fact, a few models of this kind are available in the literature, mainly based on measurements and databases developed for military purposes. Most of these models have been tuned on old ships: this poses the question whether they are still applicable to modern commercial vessels. In this work, spectra of a series of measurements of underwater noise emitted by commercial vessels measured in the framework of two EU FP7 projects (SILENV: www.silenv.eu and AQUO: www.aquo.eu) are analyzed and compared with the results obtained applying the available literature models. The analysis is carried out for noise emitted both at design and off-design conditions. In such conditions, the models are also compared with a proposed spectral parametrization. Focusing on the off-design conditions, a detailed narrow band analysis of a complete set of noise spectra is presented for a ship equipped with a controllable pitch propeller (CPP). The spectral peculiarities of such a propulsion plant, when operated at constant revolutions per minute (RPM) are highlighted. Results of the suggested parametrization applied to the same ship are presented together with the data of other CPP ships and a critical analysis is carried out discussing the limits of the existing predictive models.

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

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

Description: Presents corrections to the paper, "“A Bayesian method for localization by multistatic active sonar,” (Peters, D.J.), IEEE J. Ocean. Eng., vol. 42, no. 1, pp. 135–142, Jan. 2017, DOI: 10.1109/JOE.2015.2469935.

Description: Presents the online link whic hlists the Editorial Board for the IEEE Journal of Oceanic Engineering.

Description: Reverberation measurements obtained with towed arrays are a valuable tool to extract information about the ocean environment. By superimposing a polar plot of reverberation beam time series on bathymetry maps, bottom features (often uncharted) can be located. As part of Rapid Environmental Assessment exercises, Preston and Ellis used directional reverberation measurements to extract environmental information using model-data comparisons. This early work used range-independent (flat bottom) ray-based models for the model-data comparisons, while current work includes range-dependent models based on adiabatic normal modes. Here, we discuss a range-dependent shallow-water reverberation model using adiabatic normal modes that has been developed to handle bottom scattering and clutter echoes in a range-dependent environment. Beam time series similar to those measured on a horizontal line array can be produced. Comparisons can then directly be made with data, features identified, and estimates of the scattering obtained. Of particular interest will be data obtained on the triplet line array during the 2013 Target and Reverberation EXperiments in the Gulf of Mexico off Panama City, FL, USA, where interesting effects in sea bottom sand dunes were observed. Particular attention has been paid to calibration to get estimates of scattering strengths. In addition to the reverberation, a preliminary investigation of the target echo is presented.

Description: Awareness of underwater noise from shipping has grown significantly within the marine community over the past decade. Concerns have been raised about the levels of anthropogenic noise in the world's oceans, as well as in harbors and areas with sensitive or protected marine life. Regulatory bodies are listening, and are considering what actions are available to reduce noise. Measurements of vessel noise in various situations have been performed to better quantify the problem, and efforts have been made by some to correlate noise levels with simplified vessel parameters. Such efforts provide insights into what levels of noise are possible and how they are related to ship operation. However, to reduce underwater noise, a deeper understanding of specific vessel design and factors that lead to noise are required. While generalities of primary noise sources can be applied to different classes of vessels, design details of specific vessels must be obtained and analyzed to implement noise reductions. Armed with knowledge of vessel details, noise control strategies can be optimized using various computer-aided tools. This must be done on a case-by-case basis for different vessel designs, and is most efficient and effective for new builds. Costs for such efforts must also be considered if vessel noise is to be made inherently quieter worldwide.

Description: The contribution of shipping noise to the background noise in the seas can be estimated by shipping noise maps. Noise maps can be made by applying appropriate modeling tools for underwater sound propagation and ship source levels to information on the acoustic environment and on the ship traffic in the area of the map. The development of reliable source level models requires the availability of a variety of radiated noise data of ships. Therefore, full-scale radiated noise measurements were performed near a shipping lane in coastal waters out of the Port of Rotterdam. The results of the noise measurements are combined with information about the measured ships and their positions, derived from registered AIS data, and with propagation conditions to determine source levels. Criteria have been developed to select the data from which a reliable source level estimation can be obtained from individual ship passages at the busy shipping lanes. Source levels of different types of ships are compared.

Description: Presents the table of contents for this issue of the publication.

Description: Provides a listing of the editorial board, current staff, committee members and society officers.

Description: Reports on the paper submission process for this publication which will require an Open Researcher and Contributor ID (ORCID) for all authors who submit papers to IEEE journals. For the record, an ORCID is a persistent and unique identifier for researchers that functions similarly to an article’s Digital Object Identifier (DOI). ORCIDs enable accurate attribution and improved discoverability of an author’s published work. More information about ORCIDS can be found at http://orcid.org/content/about-orcid in the graphic on the last page of this document, or at https://vimeo.com/97150912. Authors will need a registered ORCID to submit amanuscript or review a proof in this publication.

Description: As part of the 2013 Target and REverberation eXperiment (TREX13), which took place off the coast of Panama City, FL, USA, directional wave measurements were made using two directional wave buoys separated in range by 5 km. The purpose of these measurements was to provide environmental support for the interpretation of reverberation and other active sonar experiments that were part of TREX13. During the measurement period between April22 and May 17, 2013 exclusive of a period of nondeployment May 2-6, 2013, the root-mean-square (rms) wave height H varied over the range 0.03-0.33 m, holding a median value of 0.11 m; the wind speed varied from ~1 to 10 m/s with a median value of 4.7 m/s, and the rms wave slope averaged over all directions varied from 0.01 to 0.10 with median value of 0.05. These parameters are placed in the context of midfrequency sonar propagation and reverberation prediction. One buoy operated the entire period, with the second buoy operating simultaneously over a four-day overlap period, during which there was excellent agreement between H and wave slope in two orthogonal directions, a finding relevant to describing the sea surface as spatially invariant, or homogeneous, for purposes of sonar modeling. The analysis of energy-weighted mean direction illustrates how the wave field was generally composed of a mixture of swell and wind-generated waves; in cases of purely wind-generated waves the effect of a limited fetch was also shown.

Description: Seabed reflection measurements can be used to infer highly detailed properties of marine sediments. The information content is largely contained in the interference pattern in frequency-angle arising from wave constructive and destructive interference in a plane layer. Wide-angle reflection measurements at a ridge crest and a swale site off the coast of Panama City, FL, USA, instead show interference patterns that are highly perturbed. Interface roughness was hypothesized to be the cause of the perturbations. This hypothesis is examined using numerical simulations. Measured data and simulations at the swale site show broadband peaks and troughs due to focusing/defocusing effects from boundary curvature which perturbs the interference pattern. While the hypothesis roughness is likely correct at the swale site, the roughness statistics are not known sufficiently to validate the hypothesis. At the crest site including roughness did not lead to strong similarities with the data. Interference pattern perturbations at both sites eliminated the possibility of estimating sediment parameters from inversion of broadband wide-angle data. Instead, sediment properties were estimated by inspection and forward modeling. The estimates reasonably agree with geoacoustic properties estimated from normal incidence measurements in the swale and indicate similar sound speeds and densities on two ridges ~6 km apart.

Description: In active sonar processing, the discrimination of the target versus clutter can be a significant challenge. Whereas conventional processing often uses target kinematics to limit the possible target tracks, it has recently been shown that frequency domain information (based on the waveguide invariant principle) can also be incorporated to further limit the possible target tracks in an environmentally robust fashion. This paper presents physics-based signal processing methods to extract information about the target track from striations in a target spectrogram formed from the echo spectra at each active sonar pulse repetition interval. The target tracking information is formulated as a post-track likeliness statistic that is extracted from the sonar data with image processing techniques. Results are demonstrated with shallow water sonar data collected during the 2013 Target and Reverberation Experiment (TREX13). It is expected that the physics-based signal processing algorithms discussed here will provide enhanced clutter rejection and improve tracking performance.

Description: Active sonar echo statistics (i.e., the probability density function of the match-filtered echo) are an important part of performance prediction. System design parameters such as pulse amplitude, duration and bandwidth are usually only assumed to alter the signal-to-noise-power ratio (SNR) at the detector and not the echo statistical model or its parameters. While this may be an accurate assumption for low bandwidth waveforms in a static propagation channel, it is not necessarily so when the echo is resolved or the channel is time varying. Echoes from a moored, vertical air hose were obtained during the 2013 Target and Reverberation Experiment (TREX-2013) and used to evaluate the impact of pulse duration on echo statistics. Using the scintillation index (SI) to represent the statistical character of the air-hose echo measurements, increasing pulse duration was seen to make the echoes more deterministic (i.e., to decrease SI). Assuming a time-varying Rician channel, a model was developed relating the pulse duration, channel correlation time, and ratio of coherent to incoherent power in the channel to the echo SI. Comparison with the TREX-13 air-hose measurements showed a good or excellent fit to five of the six 1-h measurement periods analyzed. Increasing pulse duration beyond the channel correlation time essentially increases the ratio of coherent to incoherent power in the echo, which improves detection performance when SNR is fixed (e.g., in a reverberation-limited scenario). If a single channel instantiation already produces a nearly deterministic echo, the performance improvement is minimal. However, when the channel is weakly Rician (e.g., a shallow-water channel at large ranges relative to water depth), the improvement can be significant.

Description: It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time–bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product–moment correlation coefficient (Pearson's $R$ ) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Description: In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo, reverberation, and echo-to-reverberation ratio an important part of sonar performance prediction. In range-dependent shallow-water environments, the echo is often calculated without considering the effect of dispersion, and reverberation predictions are often limited to Lambert's rule for seabed scattering. In this paper, analytical formulae are derived for the echo, including the effect of time dispersion, and for reverberation for non-Lambert scattering laws. More specifically, the method for calculating the short-pulse echo intensity developed by Harrison and Nielsen is extended to a range-dependent bathymetry. For reverberation, the Zhou-Harrison method is also generalized to cover an arbitrary power law for the seabed scattering coefficient, combined with a range-dependent bathymetry. These formulae are applied to range-dependent test cases from an international workshop, and compared with predictions using a normal mode sum. Neglect of time dispersion is found to result in an error of up to 16 dB. Results for a cylindrically symmetric bathymetry differ by up to 14 dB from the corresponding results with a Cartesian symmetry.

Description: Machinery vibration is a significant source of noise from marine vessels. Consequently, problematic machinery is often decoupled from the hull using resilient mounts. When radiated noise is a priority, two-stage mounting arrangements can be used which provide superior performance. This improvement, however, comes at a much higher weight and cost. A commonly overlooked issue with isolation systems is that the individual mounts suffer from internal resonances which reduce their effectiveness. This paper broadly evaluates the influence of these on the underwater noise levels from a typical ship diesel. The effect of nonrigid structural components, that is, the supported machine, the machine foundation, and the intermediate mass in two-stage mounting arrangements, is also discussed. The structureborne and airborne transmission paths are resolved individually using approximate analytical methods, and the source levels are estimated from empirical relationships for typical marine diesels. It is shown that, by mitigating the internal resonances of the mounts, a two-stage mounting arrangement may be unnecessary because of the contribution to the underwater noise by airborne transmission.

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

Description: The seismoacoustic parabolic equation is extended to handle variable topography, and a related approach is used to improve stability for problems involving sloping fluid-solid interfaces. A range-dependent medium is approximated in terms of a series of range-independent regions, and single-scattering operators are used to obtain transmitted fields across the vertical interfaces between regions. Sloping boundaries are handled using a single-scattering operator involving the normal displacement and the tangential stress. Sloping interfaces are handled using an improved single-scattering operator that provides greater stability for the fluid-solid case than the original single-scattering operator [M. D. Collins, J. Acoust. Soc. Amer., vol. 131, pp. 2638-2642, 2012]. The approach is applicable to problems in seismology involving variable topography and problems in seismoacoustics involving propagation across a coastline and ice cover with variable topography.

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

Description: In this paper, we present a method for investigating the coupling of structure-borne sound (SBS) and underwater sound (UWS) of ships based on a decomposition in optimal frequency bands. The coupling is modeled by a linear regression model for sequences of spectral power, integrated over frequency intervals and moving time windows. As part of this analysis, a novel approach to detecting frequency intervals that bear particular relevance for the relationship between SBS and UWS is introduced. Regression models are regularized by a maximum-significance constraint. Cross correlations between UWS regressands and SBS regressors seem to be promising as quantitative measures for describing the coupling between SBS and UWS time series. The method is evaluated with experimental data recorded simultaneously from fixed SBS sensors onboard the research vessel R/V PLANET and from a freely drifting buoy system during an open-sea experiment north of the Outer Hebrides, Scotland, in 2011.

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

Description: The 2013 Target and Reverberation Experiment (TREX13) sea trial was held just off the coast of Panama City, FL, USA, in May 2013. One of Defence Research and Development Canada's primary objectives was comparing the performance of continuous active sonar (CAS) and pulsed active sonar (PAS) in shallow water, where reverberation and clutter challenge sonar systems. The only option for a moving target during the trial was an echo repeater towed from a ship; however, this posed a serious challenge because before the trial there were no known echo repeater solutions that were compatible with CAS. Existing echo repeaters required short-duration waveforms as used in PAS, and even then they introduced range errors that caused a mismatch in reverberation background and made localization problematic. This paper presents three echo repeater techniques developed for TREX13 and tested on Canadian Forces Auxiliary research vessel CFAV Quest during the trial. Experimental results presented in this paper demonstrate detection of the echo repeater by a monostatic CAS system on R/V Hugh R Sharp .

Description: It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time–bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product–moment correlation coefficient (Pearson's $R$ ) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Description: Active sonar echo statistics (i.e., the probability density function of the match-filtered echo) are an important part of performance prediction. System design parameters such as pulse amplitude, duration and bandwidth are usually only assumed to alter the signal-to-noise-power ratio (SNR) at the detector and not the echo statistical model or its parameters. While this may be an accurate assumption for low bandwidth waveforms in a static propagation channel, it is not necessarily so when the echo is resolved or the channel is time varying. Echoes from a moored, vertical air hose were obtained during the 2013 Target and Reverberation Experiment (TREX-2013) and used to evaluate the impact of pulse duration on echo statistics. Using the scintillation index (SI) to represent the statistical character of the air-hose echo measurements, increasing pulse duration was seen to make the echoes more deterministic (i.e., to decrease SI). Assuming a time-varying Rician channel, a model was developed relating the pulse duration, channel correlation time, and ratio of coherent to incoherent power in the channel to the echo SI. Comparison with the TREX-13 air-hose measurements showed a good or excellent fit to five of the six 1-h measurement periods analyzed. Increasing pulse duration beyond the channel correlation time essentially increases the ratio of coherent to incoherent power in the echo, which improves detection performance when SNR is fixed (e.g., in a reverberation-limited scenario). If a single channel instantiation already produces a nearly deterministic echo, the performance improvement is minimal. However, when the channel is weakly Rician (e.g., a shallow-water channel at large ranges relative to water depth), the improvement can be significant.

Description: Seabed reflection measurements can be used to infer highly detailed properties of marine sediments. The information content is largely contained in the interference pattern in frequency-angle arising from wave constructive and destructive interference in a plane layer. Wide-angle reflection measurements at a ridge crest and a swale site off the coast of Panama City, FL, USA, instead show interference patterns that are highly perturbed. Interface roughness was hypothesized to be the cause of the perturbations. This hypothesis is examined using numerical simulations. Measured data and simulations at the swale site show broadband peaks and troughs due to focusing/defocusing effects from boundary curvature which perturbs the interference pattern. While the hypothesis roughness is likely correct at the swale site, the roughness statistics are not known sufficiently to validate the hypothesis. At the crest site including roughness did not lead to strong similarities with the data. Interference pattern perturbations at both sites eliminated the possibility of estimating sediment parameters from inversion of broadband wide-angle data. Instead, sediment properties were estimated by inspection and forward modeling. The estimates reasonably agree with geoacoustic properties estimated from normal incidence measurements in the swale and indicate similar sound speeds and densities on two ridges ∼6 km apart.

Description: In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo, reverberation, and echo-to-reverberation ratio an important part of sonar performance prediction. In range-dependent shallow-water environments, the echo is often calculated without considering the effect of dispersion, and reverberation predictions are often limited to Lambert's rule for seabed scattering. In this paper, analytical formulae are derived for the echo, including the effect of time dispersion, and for reverberation for non-Lambert scattering laws. More specifically, the method for calculating the short-pulse echo intensity developed by Harrison and Nielsen is extended to a range-dependent bathymetry. For reverberation, the Zhou–Harrison method is also generalized to cover an arbitrary power law for the seabed scattering coefficient, combined with a range-dependent bathymetry. These formulae are applied to range-dependent test cases from an international workshop, and compared with predictions using a normal mode sum. Neglect of time dispersion is found to result in an error of up to 16 dB. Results for a cylindrically symmetric bathymetry differ by up to 14 dB from the corresponding results with a Cartesian symmetry.

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

Description: Reverberation measurements obtained with towed arrays are a valuable tool to extract information about the ocean environment. By superimposing a polar plot of reverberation beam time series on bathymetry maps, bottom features (often uncharted) can be located. As part of Rapid Environmental Assessment exercises, Preston and Ellis used directional reverberation measurements to extract environmental information using model-data comparisons. This early work used range-independent (flat bottom) ray-based models for the model-data comparisons, while current work includes range-dependent models based on adiabatic normal modes. Here, we discuss a range-dependent shallow-water reverberation model using adiabatic normal modes that has been developed to handle bottom scattering and clutter echoes in a range-dependent environment. Beam time series similar to those measured on a horizontal line array can be produced. Comparisons can then directly be made with data, features identified, and estimates of the scattering obtained. Of particular interest will be data obtained on the triplet line array during the 2013 Target and Reverberation EXperiments in the Gulf of Mexico off Panama City, FL, USA, where interesting effects in sea bottom sand dunes were observed. Particular attention has been paid to calibration to get estimates of scattering strengths. In addition to the reverberation, a preliminary investigation of the target echo is presented.

Description: Ripples on the seafloor affect acoustic scattering and transmission loss, wave attenuation, and the amount of sediment transported in shallow water. Historically, seafloor roughness (a function of ripples, bedforms, sediment type, and size) is assumed to be spatially homogeneous and temporally static in hydrodynamic and acoustic models despite the often dynamic nature of the seafloor in the nearshore region. We present a spectral ripple model, Navy Seafloor Evolution Archetype (NSEA), which simulates the variations in seafloor roughness given measured or predicted wave conditions in sandy environments. NSEA simulates sand ripple formation and evolution based on bottom velocities either measured or predicted by a wave model. The time dependency is a function of equilibrium ripple geometries and the amount of sediment transport needed to reach an equilibrium state, which is dependent on the relict ripples. Spectral decay due to bioturbation is incorporated as a diffusive process. NSEA was validated with time series observations obtained in water depths of 7.5 and 20 m from April 20, 2013 to May 23, 2013 during the 2013 Target and Reverberation Experiment (TREX13) offshore of Panama City, FL, USA. The model predicted spectral ripple wavelengths that were in good agreement with observed spectral ripple wavelengths obtained using a fixed platform, high-frequency (2.25 MHz) sector scanning sonar. Likewise, the variations in the predicted normalized ripple heights and orientations were similar to the normalized spectral decay and orientations estimated from the sector scanning sonar imagery.

Description: As part of the 2013 Target and REverberation eXperiment (TREX13), which took place off the coast of Panama City, FL, USA, directional wave measurements were made using two directional wave buoys separated in range by 5 km. The purpose of these measurements was to provide environmental support for the interpretation of reverberation and other active sonar experiments that were part of TREX13. During the measurement period between April 22 and May 17, 2013 exclusive of a period of nondeployment May 2–6, 2013, the root-mean-square (rms) wave height $H$ varied over the range 0.03–0.33 m, holding a median value of 0.11 m; the wind speed varied from ${sim} $ 1 to 10 m/s with a median value of 4.7 m/s, and the rms wave slope averaged over all directions varied from 0.01 to 0.10 with median value of 0.05. These parameters are placed in the context of midfrequency sonar propagation and reverberation prediction. One buoy operated the entire period, with the second buoy operating simultaneously over a four-day overlap period, during which there was excellent agreement between $H$ and wave slope in two orthogonal directions, a finding relevant to describing the sea surface as spatially invariant, or homogeneous, for purposes of sonar modeling. The analysis of energy-weighted mean direction illustrates how the wave field was generally composed of a mixture of swell and wind-generated waves; in cases of purely wind-generated waves the effect of a limited fetch was also shown.

Description: Machinery vibration is a significant source of noise from marine vessels. Consequently, problematic machinery is often decoupled from the hull using resilient mounts. When radiated noise is a priority, two-stage mounting arrangements can be used which provide superior performance. This improvement, however, comes at a much higher weight and cost. A commonly overlooked issue with isolation systems is that the individual mounts suffer from internal resonances which reduce their effectiveness. This paper broadly evaluates the influence of these on the underwater noise levels from a typical ship diesel. The effect of nonrigid structural components, that is, the supported machine, the machine foundation, and the intermediate mass in two-stage mounting arrangements, is also discussed. The structureborne and airborne transmission paths are resolved individually using approximate analytical methods, and the source levels are estimated from empirical relationships for typical marine diesels. It is shown that, by mitigating the internal resonances of the mounts, a two-stage mounting arrangement may be unnecessary because of the contribution to the underwater noise by airborne transmission.

Description: A growing attention has been recently devoted to the impact of the underwater noise field generated by shipping activities on the marine fauna. A key aspect for the quantification of such impact is a proper model of the source levels radiated from the vessels. At a first level of approximation, simplified formulations based on a small number of macroparameters describing the ship characteristics are needed to quantify the emission and, accordingly, assess the noise impact and evaluate strategies for its control. In fact, a few models of this kind are available in the literature, mainly based on measurements and databases developed for military purposes. Most of these models have been tuned on old ships: this poses the question whether they are still applicable to modern commercial vessels. In this work, spectra of a series of measurements of underwater noise emitted by commercial vessels measured in the framework of two EU FP7 projects (SILENV: www.silenv.eu and AQUO: www.aquo.eu ) are analyzed and compared with the results obtained applying the available literature models. The analysis is carried out for noise emitted both at design and off-design conditions. In such conditions, the models are also compared with a proposed spectral parametrization. Focusing on the off-design conditions, a detailed narrow band analysis of a complete set of noise spectra is presented for a ship equipped with a controllable pitch propeller (CPP). The spectral peculiarities of such a propulsion plant, when operated at constant revolutions per minute (RPM) are highlighted. Results of the suggested parametrization applied to the same ship are presented together with the data of other CPP ships and a critical analysis is carried out discussing the limits of the existing predictive models.

Description: In active sonar processing, the discrimination of the target versus clutter can be a significant challenge. Whereas conventional processing often uses target kinematics to limit the possible target tracks, it has recently been shown that frequency domain information (based on the waveguide invariant principle) can also be incorporated to further limit the possible target tracks in an environmentally robust fashion. This paper presents physics-based signal processing methods to extract information about the target track from striations in a target spectrogram formed from the echo spectra at each active sonar pulse repetition interval. The target tracking information is formulated as a post-track likeliness statistic that is extracted from the sonar data with image processing techniques. Results are demonstrated with shallow water sonar data collected during the 2013 Target and Reverberation Experiment (TREX13). It is expected that the physics-based signal processing algorithms discussed here will provide enhanced clutter rejection and improve tracking performance.

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

Description: Awareness of underwater noise from shipping has grown significantly within the marine community over the past decade. Concerns have been raised about the levels of anthropogenic noise in the world's oceans, as well as in harbors and areas with sensitive or protected marine life. Regulatory bodies are listening, and are considering what actions are available to reduce noise. Measurements of vessel noise in various situations have been performed to better quantify the problem, and efforts have been made by some to correlate noise levels with simplified vessel parameters. Such efforts provide insights into what levels of noise are possible and how they are related to ship operation. However, to reduce underwater noise, a deeper understanding of specific vessel design and factors that lead to noise are required. While generalities of primary noise sources can be applied to different classes of vessels, design details of specific vessels must be obtained and analyzed to implement noise reductions. Armed with knowledge of vessel details, noise control strategies can be optimized using various computer-aided tools. This must be done on a case-by-case basis for different vessel designs, and is most efficient and effective for new builds. Costs for such efforts must also be considered if vessel noise is to be made inherently quieter worldwide.

Description: The contribution of shipping noise to the background noise in the seas can be estimated by shipping noise maps. Noise maps can be made by applying appropriate modeling tools for underwater sound propagation and ship source levels to information on the acoustic environment and on the ship traffic in the area of the map. The development of reliable source level models requires the availability of a variety of radiated noise data of ships. Therefore, full-scale radiated noise measurements were performed near a shipping lane in coastal waters out of the Port of Rotterdam. The results of the noise measurements are combined with information about the measured ships and their positions, derived from registered AIS data, and with propagation conditions to determine source levels. Criteria have been developed to select the data from which a reliable source level estimation can be obtained from individual ship passages at the busy shipping lanes. Source levels of different types of ships are compared.

Description: In this paper, we present a method for investigating the coupling of structure-borne sound (SBS) and underwater sound (UWS) of ships based on a decomposition in optimal frequency bands. The coupling is modeled by a linear regression model for sequences of spectral power, integrated over frequency intervals and moving time windows. As part of this analysis, a novel approach to detecting frequency intervals that bear particular relevance for the relationship between SBS and UWS is introduced. Regression models are regularized by a maximum-significance constraint. Cross correlations between UWS regressands and SBS regressors seem to be promising as quantitative measures for describing the coupling between SBS and UWS time series. The method is evaluated with experimental data recorded simultaneously from fixed SBS sensors onboard the research vessel R/V PLANET and from a freely drifting buoy system during an open-sea experiment north of the Outer Hebrides, Scotland, in 2011.

Description: Presents corrections to the paper, "“A Bayesian method for localization by multistatic active sonar,” (Peters, D.J.), IEEE J. Ocean. Eng., vol. 42, no. 1, pp. 135–142, Jan. 2017, DOI: 10.1109/JOE.2015.2469935.

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