Showing papers in "IEEE Journal of Oceanic Engineering in 2005"

Recovery of underwater visibility and structure by polarization analysis

Abstract: Underwater imaging is important for scientific research and technology as well as for popular activities, yet it is plagued by poor visibility conditions. In this paper, we present a computer vision approach that removes degradation effects in underwater vision. We analyze the physical effects of visibility degradation. It is shown that the main degradation effects can be associated with partial polarization of light. Then, an algorithm is presented, which inverts the image formation process for recovering good visibility in images of scenes. The algorithm is based on a couple of images taken through a polarizer at different orientations. As a by-product, a distance map of the scene is also derived. In addition, this paper analyzes the noise sensitivity of the recovery. We successfully demonstrated our approach in experiments conducted in the sea. Great improvements of scene contrast and color correction were obtained, nearly doubling the underwater visibility range.

Trends in biorobotic autonomous undersea vehicles

Abstract: The emergence of biorobotic autonomous undersea vehicle (AUV) as a focus for discipline-integrated research in the context of underwater propulsion and maneuvering is considered within the confines of the Biorobotics Program in the Office of Naval Research. The significant advances in three disciplines, namely the biology-inspired high-lift unsteady hydrodynamics, artificial muscle technology and neuroscience-based control, are discussed in an effort to integrate them into viable products. The understanding of the mechanisms of delayed stall, molecular design of artificial muscles and the neural approaches to the actuation of control surfaces is reviewed in the context of devices based on the pectoral fins of fish, while remaining focused on their integrated implementation in biorobotic AUVs. A mechanistic understanding of the balance between cruising and maneuvering in swimming animals and undersea vehicles is given. All aquatic platforms, in both nature and engineering, except during short duration burst speeds that are observed in a few species, appear to lie within the condition where their natural period of oscillation equals the time taken by them to travel the distance of their own lengths. Progress in the development of small underwater experimental biorobotic vehicles is considered where the three aforementioned disciplines are integrated into one novel maneuvering device or propulsor. The potential in maneuvering and silencing is discussed.

Chemical plume tracing via an autonomous underwater vehicle

Abstract: Olfactory-based mechanisms have been hypothesized for biological behaviors including foraging, mate-seeking, homing, and host-seeking. Autonomous underwater vehicles (AUVs) capable of such chemical plume tracing feats would have applicability in searching for environmentally interesting phenomena, unexploded ordinance, undersea wreckage, and sources of hazardous chemicals or pollutants. This article presents an approach and experimental results using a REMUS AUV to find a chemical plume, trace the chemical plume to its source, and maneuver to reliably declare the source location. The experimental results are performed using a plume of Rhodamine dye developed in a turbulent, near-shore, oceanic fluid flow.

Correlation-based decision-feedback equalizer for underwater acoustic communications

Abstract: The purpose of this paper is to develop a decision-feedback equalizer (DFE) using a fixed set of parameters applicable to most shallow oceans with minimal user supervision (i.e., a turn key system). This work is motivated by the superior performance [bit error rate (BER)] of the multichannel DFE compared with other methods, such as passive-phase conjugation (PPC), at the same time noting its sensitivity to different acoustic environments. The approach is to couple PPC, utilizing its adaptability to different environments, with a single-channel DFE. This coupling forms an optimal processor for acoustic communications in theory, but it has never been implemented in practice. By coupling with DFE, the method achieves the same spatial diversity as conventional multichannel DFE, without requiring a large number of receivers as does PPC. The correlation-based DFE in terms of the autocorrelation functions of the channel impulse responses summed over the receiver channels (the Q function) is derived. This paper shows in terms of waveguide physics, further supported by real data, the many desirable features of the Q function that suggest, given adequate sampling of the water column, a general applicability of the correlation-based equalizer to different environments, irrespective of the sound speed profiles, bottom properties, and source-receiver ranges/depths. This property can be expected to hold approximately for a small number of receivers with spatial diversity. This paper demonstrates the robustness of the new equalizer with moving source data despite the range change (which modifies the impulse response) and symbol phase change due to time-varying Doppler

Underwater acoustic communications using time reversal

Abstract: This paper contains theoretical and experimental results on the application of the time-reversal process to acoustic communications in order to improve data telemetry in the ocean. A coherent underwater acoustic communication system must deal with the inter-symbol interference caused by the time-varying, dispersive, shallow-water ocean environment. An approach is demonstrated that takes advantage of the focal properties of time reversal. The spatial and temporal compression available at the time-reversal focus mitigates channel fading, reduces the dispersion caused by the channel, and increases the signal strength. Thus, a time-reversal communication system does not require spatial diversity at the receiver, i.e., an array of receiving sensors, but takes advantage of spatial diversity at the transmitter. The time-reversal communications system concept is demonstrated using experimental data collected in shallow water. Data telemetry bit rates of 500 bps (BPSK) and 1000 bps (QPSK) with bit error rates of 0 out of 4976 bits and 254 out of 9953 bits, respectively, were obtained when transmitting to a receiver at a distance of 10 km, with a carrier frequency of 3500 Hz, and a 500 Hz bandwidth. In a shallow-water upslope region, bit error rates of 15 out of 4976 bits and 14 out of 4976 bits were achieved over the same distance. In neither case was complex processing at the receiver used (i.e., channel equalization, error correction coding). Time-reversal transmissions are intercompared with single source and broadside transmissions and shown to have superior results in both range independent and dependent bathymetries. The time-reversal performance appears limited by self-generated inter-symbol interference. In addition, an initial look at the application of a single channel adaptive channel equalizer to received time-reversal communication sequences is presented. The same properties that are beneficial to a single channel receiver are also beneficial to adaptive channel equalization. A single channel RLS DFE equalizer is cascaded with the received time-reversal sequences and shown to further reduce scatter in the I/Q plane. The bit error rate decreased in all but one of the cases

Ship sources of ambient noise

Abstract: The rapid increase in world shipping results in an increase in low-frequency ambient noise at an average rate of about 1/2 dB per year. During the past 10 years there has been a virtual revolution in the sizes and speeds of merchant ships, resulting in significant increases in the noise radiated by the average ship. This trend is continuing. In this paper, the trends in world merchant shipping will be presented, including important changes in propulsion plants as well as in numbers and sizes of ships. The need for radiated noise measurements of these new ship types will be stressed. Ambient noise is also dependent on the geographical distribution of shipping. The LRAPP-sponsored program to establish standard shipping distributions for the Northern Hemisphere will be discussed, and the reliability of current information will be assessed.

Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Abstract: Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of /spl plusmn/30cm (2-/spl sigma/) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of /spl plusmn/8 cm (2-/spl sigma/) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to /spl plusmn/10 cm (2-/spl sigma/) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to /spl plusmn/1 m (2-/spl sigma/). Simulations show that /spl plusmn/30 cm (2-/spl sigma/) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.

Experimental investigation of forces and geometry of a net cage in uniform flow

Abstract: A scale model of a flexible circular net with different weights attached to the bottom was tested in a flume tank. Global forces and net deformation were measured for different steady current velocities. Three different sizes of bottom weights were used in the tests. The results from these tests are presented and discussed with the emphasis on the dependency between the forces and the geometry. Comparison is also made to empirical based formulas for calculation of drag and lift forces on net structures. Findings show that i) the forces on, and deformation of a flexible net structure are mutually highly dependent on each other; ii) estimates of global forces on a flexible net structure calculated using simple drag formulas derived from stiff net panel experiments give large errors when compared to experimental measurements; iii) numerical models taking into account the dependency between force and deformation should be used to obtain accurate estimates of forces on flexible net structures; and iv) the forces on a flexible net structure are dependent on Reynolds number, and their dependency are similar to that of a regular cylinder.

Performance of an AUV navigation system at Arctic latitudes

Abstract: In October 2001, the Monterey Bay Aquarium Research Institute (MBARI) operated an autonomous underwater vehicle (AUV) in the Arctic at latitudes exceeding 80/spl deg/. The navigation instruments consisted of a ring-laser gyro inertial navigation system (INS) coupled with a DVL and GPS, a separate fiber-optic-based gyro-compass, and a traditional flux-gate AHRS system. The instruments were tested on deck, in open water, and under ice. This paper describes the performance of these instruments at high latitudes.

Localization of autonomous underwater vehicles by floating acoustic buoys: a set-membership approach

Abstract: This paper addresses localization of autonomous underwater vehicles (AUVs) from acoustic time-of-flight measurements received by a field of surface floating buoys. It is assumed that measurements are corrupted by unknown-but-bounded errors, with known bounds. The localization problem is tackled in a set-membership framework and an algorithm is presented, which produces as output the set of admissible AUV positions in a three-dimensional (3-D) space. The algorithm is tailored for a shallow water situation (water depth less than 500 m), and accounts for realistic variations of the sound speed profile in sea water. The approach is validated by simulations in which uncertainty models have been obtained from field data at sea. Localization performance of the algorithm are shown comparable with those previously reported in the literature by other approaches who assume knowledge of the statistics of measurement uncertainties. Moreover, guaranteed uncertainty regions associated to nominal position estimates are provided. The proposed algorithms can be used as a viable alternative to more traditional approaches in realistic at-sea conditions.

Hydrodynamic implications of large offshore mussel farms

Abstract: Around 3000 Ha of New Zealand's coastal waters are presently allocated to mussel farming of the Greenshell mussel Perna canaliculus mainly in small inshore farms of 30 000 to 80 000 m/sup 2/ each. The farms are constructed from blocks of parallel moored long-lines, with loops of spat-impregnated line hanging vertically in the upper 10-15 m of the water column (provided sufficient water depth). When fully laden, the mussel loops can present a rough diameter of 0.20 m or more to the flow. Industry expansion is likely to be in the form of larger offshore farms. Some of the larger proposed farms will have more than 1000 km of mussel line. This presents a potentially significant drag to currents and waves. In this paper, observations are used from one of the first large offshore farms constructed in New Zealand, in conjunction with scaling estimates of energy loss, to investigate the effect of the farm on waves, currents, and stratification. The farm, consisting of 230 long-lines arranged into 20 blocks, measured 650 m /spl times/ 2450 m. The hanging mussel loops at this site reached an average depth of 8 m, and water depth at midtide varied across the site from 10-12 m. Transects were taken through the farm using both conductivity, temperature, depth (CTD), and Acoustic Doppler Profiler (ADP) instruments. A second ADP was moored inside the farm providing velocity time series. The average current was reduced within the farm by 36%-63%. The moored ADP detected an undercurrent beneath the farm with velocities nearly twice that within the farm. Sharp changes in stratification coincide with the start of fully stocked mussel long-lines. Furthermore, other than at the very upstream end of the farm, stratification appears to have prevented significant vertical mixing between the fluid within the farm and that flowing under the farm. Wave energy was low during the 7-d deployment, with significant wave heights peaking at 0.25 m. The attenuation of transitional waves through the farm is examined by comparing simultaneous pressure sensor measurements from offshore and inshore of the farm. The observations show that wave energy attenuation was frequency dependent. Observed wave energy attenuation across the 650 m wide farm was approximately 5%, 10%, and 17% at wave frequencies of 0.1, 0.2, and 0.25 Hz, respectively.

Spatial modulation experiments in the underwater acoustic channel

Abstract: A modulation technique for increasing the reliable data rate achievable by an underwater acoustic communication system is presented and demonstrated. The technique, termed spatial modulation, seeks to control the spatial distribution of signal energy such that the single physical ocean channel supports multiple parallel communication channels. Given a signal energy constraint, a communication architecture with access to parallel channels will have increased capacity and reliability as compared to one with access to a single channel. Results from two experiments demonstrate higher obtainable data rates and power throughput for a system employing spatial modulation than for one that does not. The demonstrated benefits were characterized by an equivalent SNR gain of over 5 dB in the first experiment. In the second experiment, using two element source and receiver arrays with apertures of 0.9 m, a coherently modulated signal was shown to offer nearly 50% greater capacity by using spatial modulation than by using temporal modulation alone.

Using ocean ambient noise for array self-localization and self-synchronization

Abstract: Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.

Measurement of ocean surface winds using synthetic aperture radars

Abstract: A methodology for retrieving high-resolution ocean surface wind fields from satellite-borne synthetic aperture radar (SAR) data is introduced and validated. The algorithms developed are suited for ocean SAR data, which were acquired at the C band of either vertical (VV) or horizontal (HH) polarization in transmission and reception. Wind directions are extracted from wind-induced streaks that are visible in SAR images of the ocean at horizontal scales greater than 200 m. These wind streaks are very well aligned with the mean surface wind direction. To extract the orientation of these streaks, two algorithms are introduced, which are applied either in the spatial or spectral domain. Ocean surface wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated SAR images, together with the local SAR-retrieved wind direction. Therefore, several C-band models (CMOD IFR2, CMOD4, and CMODS) are available, which were developed for VV polarization, and have to be extended for HH polarization. To compare the different algorithms and C-band models as well as demonstrate their applicability, SAR-retrieved wind fields are compared to numerical-model results considering advanced SAR (ASAR) data from Environmental Satellite (ENVISAT), a European satellite.

Extraction of coastal ocean wave fields from SAR images

Abstract: Wave spectra derived from synthetic aperture radar (SAR) images acquired by ENVISATs are compared to in situ measurements by seven sensors, deployed in a field experiment carried out on the French coast of La Manche (English Channel). The wave spectra inversion scheme is adapted for shallow water from the European Space Agency (ESA)'s operational processing techniques used for level 2 ocean wave products. Under the low to moderate wind speed observed conditions, overall good agreement is found between in situ and SAR observations over a wide range of wave heights and directions, including waves propagating in the radar azimuth direction and incidence angles different from the standard imagette products.

Operational oil-slick characterization by SAR imagery and synergistic data

Abstract: A methodology is proposed for the semiautomatic detection, characterization, and classification of slicks detected in C-band Synthetic Aperture Radar (SAR). For the first detection step, automatic algorithms were tested on Environmental Research Satellite (ERS) and Environmental Satellite (EnviSat) images acquired during the Prestige tanker accident. These tests reveal that simple filter or segmentation methods efficiently detect slicks with high contrasts and simple shapes, while a new and more complex multiscale method is able to detect a wider range of slicks. The characteristics of automatically detected slicks are then combined with meteooceanic data in order to eliminate slicks related to wind anomalies and current fronts. The data suggest that slicks in cold upwelling waters are natural, and confirm that slicks are heavy oils when high sea states are present. This detection-classification methodology is validated with aircraft slick-tracking maps. In most cases, joint SAR and environmental data are sufficient to classify the slicks.

Directional wave information from the SeaSonde

Abstract: This paper describes methods used for the derivation of wave information from SeaSonde data, and gives examples of their application to measured data. The SeaSonde is a compact high-frequency (HF) radar system operated from the coast or offshore platform to produce current velocity maps and local estimates of the directional wave spectrum. Two methods are described to obtain wave information from the second-order radar spectrum: integral inversion and fitting with a model of the ocean wave spectrum. We describe results from both standard- and long-range systems and include comparisons with simultaneous measurements from an S4 current meter. Due to general properties of the radar spectrum common to all HF radar systems, existing interpretation methods fail when the waveheight exceeds a limiting value defined by the radar frequency. As a result, standard- and long-range SeaSondes provide wave information for different wave height conditions because of their differing radar frequencies. Standard-range SeaSondes are useful for low and moderate waveheights, whereas long-range systems with lower transmit frequencies provide information when the waves are high. We propose a low-cost low-power system, to be used exclusively for local wave measurements, which would be capable of switching transmit frequency when the waveheight exceeds the critical limit, thereby allowing observation of waves throughout the waveheight range.

Active learning for detection of mine-like objects in side-scan sonar imagery

Abstract: A data-adaptive algorithm is presented for the selection of the basis functions and training data used in classifier design with application to sensing mine-like targets with a side-scan sonar. Automatic detection of mine-like targets using side-scan sonar imagery is complicated by the variability of the target, clutter, and background signatures. Specifically, the strong dependence of the data on environmental conditions vitiates the assumption that one may perform a priori algorithm training using separate side-scan sonar data collected previously. In this paper, a novel active-learning algorithm is developed based on kernel classifiers with the goal of enhancing detection/classification of mines without requiring an a priori training set. It is assumed that divers and/or unmanned underwater vehicles (UUVs) may be used to determine the binary labels (target/clutter) of a small number of signatures from a given side-scan collection. These sets of signatures and associated labels are then used to train a kernel-based algorithm with which the remaining side-scan signatures are classified. Information-theoretic concepts are used to adaptively construct the form of the kernel classifier and to determine which signatures and associated labels would be most informative in the context of algorithm training. Using measured side-looking sonar data, the authors demonstrate that the number of signatures for which labels are required (via diver/UUV) is often small relative to the total number of potential targets in a given image. This procedure designs the detection/classification algorithm on the observed data itself without requiring a priori training data and also allows adaptation as environmental conditions change.

A behavior-based scheme using reinforcement learning for autonomous underwater vehicles

Abstract: This paper presents a hybrid behavior-based scheme using reinforcement learning for high-level control of autonomous underwater vehicles (AUVs). Two main features of the presented approach are hybrid behavior coordination and semi on-line neural-Q/spl I.bar/learning (SONQL). Hybrid behavior coordination takes advantages of robustness and modularity in the competitive approach as well as efficient trajectories in the cooperative approach. SONQL, a new continuous approach of the Q/spl I.bar/learning algorithm with a multilayer neural network is used to learn behavior state/action mapping online. Experimental results show the feasibility of the presented approach for AUVs.

Numerical modeling of nonlinear elastic components of mooring systems

Abstract: This paper deals with the finite-element analysis of compliant structures containing nonlinear elastic components and subjected to loading by waves and currents. An efficient procedure to model such components is developed and implemented in a computer program. It can be applied to analyze nonlinear elastic ropes, rubber tethers, risers, and hoses. The elastic modulus is assumed to depend on the amount of strain in a component. The Newton-Raphson iteration scheme is modified to account for the change of elastic properties on each step of time integration procedure. The approach is applied to develop feed buoy mooring systems for the University of New Hampshire Open Ocean Aquaculture site. The complex nonlinear and viscoelastic load-elongation behavior of suitable high stretch mooring elements for this application is modeled to obtain realistic numerical predictions at sea conditions. Various designs and environmental loading scenarios are considered. Numerical simulations provide predictions of the overall dynamics of the system and maximum values of tensions in critical components.

Wind- and wave-field measurements using marine X-band radar-image sequences

Abstract: This paper describes two algorithms for the retrieval of high-resolution wind and wave fields from radar-image sequences acquired by a marine X-band radar. The wind-field retrieval algorithm consists of two parts. In the first part, wind directions are extracted from wind-induced streaks, which are approximately in line with the mean surface wind direction. The methodology is based on the retrieval of local gradients from the mean radar backscatter image and assumes the surface wind direction to be oriented normal to the local gradient. In the second part, wind speeds are derived from the mean radar cross section. Therefore, the dependence of the radar backscatter on the wind vector and imaging geometry has to be determined. Such a relationship is developed by using neural networks (NNs). For the verification of the algorithm, wind directions and speeds from nearly 3300 radar-image sequences are compared to in situ data from a colocated wind sensor. The wave retrieval algorithm is based on a methodology that, for the first time, enables the inversion of marine radar-image sequences to an elevation-map time series of the ocean surface without prior calibration of the acquisition system, and therefore, independent of external sensors. The retrieved ocean-surface elevation maps are validated by comparison of the resulting radar-derived significant wave heights, with the significant wave heights acquired from three colocated in situ sensors. It is shown that the accuracy of the radar-retrieved significant wave height is consistent with the accuracy of the in situ sensors.

Simulation of wave-energy converter with octagonal linear generator

Abstract: To extract electrical energy from sea waves in a commercially and technologically acceptable manner, a number of issues have to be solved. Electricity generation by means of direct conversion of the oscillating gravitational potential energy of a floating buoy can be anticipated, provided a proper design of a generator could be made. This paper deals with the simulation of a novel design for a linear generator aimed for the extraction of energy from ocean waves. The ocean waves are modeled by 4-m-height sinusoidal waves with a characteristic period of 7 s. A wide range of the geometrical sizes, permanent magnets, stator winding, and spring forces acting on the buoy are possible. This paper presents simulations of octagonal three-phase linear generators in the 100-kW power range. The beneficial effects of a stator of octagonal shape are briefly investigated, but not studied in depth. The main emphases in the present study have been to decrease power fluctuations and suppress voltage harmonics. In conventional rotating machines, well-known measures are to use a fractional number of slots per pole and phase, and an additional method is to make the pole edges smoother. These methods are here simulated for the first time on a linear machine aimed for ocean wave-energy conversion and a substantial reduction in power fluctuations and voltage harmonics are predicted.

A case study of a modified gravity type cage and mooring system using numerical and physical models

Abstract: A modified gravity-type cage, developed by SADCO Shelf Ltd., was examined using numerical and physical models to determine if the cage and mooring system is suitable for an exposed site south of the Isles of Shoals, NH. The 3000-m/sup 3/ SADCO Shelf Submersible Fish Cage has angled stays between the upper framework and the ballasted bottom rim (in addition to net) to resist the horizontal shear deformation. The mooring system consists of three legs-each made up of a taut vertical chain and an angled rope, both leading to deadweight anchors. Normalized response amplitudes (response amplitude operators) were found for motion response in heave, surge and pitch, and load response in the anchor and bridle lines, in regular (single frequency) waves. In addition, a stochastic approach was taken to determine the motion and load transfer functions in random waves using a spectrum representative of seas at the selected site. In general, the system motion had a highly damped response, with no resonant peaks within the wave excitation range of 0.05 to 0.45 Hz. The anchor line force response was at all frequencies below 5 kN per meter of wave amplitude. The physical model tests showed consistently more conservative (larger) results compared to those for the numerical model.

Adaptive port-starboard beamforming of triplet sonar arrays

Abstract: For a low-frequency active sonar (LFAS) with a triplet receiver array, it is not clear in advance which signal processing techniques optimize its performance. Here, several advanced beamformers are analyzed theoretically, and the results are compared to experimental data obtained in sea trials. Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides the ability to solve the notorious port-starboard (PS) ambiguity problem of ordinary single-array receivers. More importantly, the PS rejection can be so strong that it allows to unmask targets in the presence of strong coastal reverberation or traffic noise. The theoretical and experimental performance of triplet array beamformers is determined in terms of two performance indicators: array gain and PS rejection. Results are obtained under several typical acoustic environments: sea noise, flow noise, coastal reverberation, and mixtures of these. A new algorithm for (beam space) adaptive triplet beamforming is implemented and tuned. Its results are compared to those of other triplet beamforming techniques (optimum and cardioid beamforming). These beamformers optimize for only one performance indicator, whereas in theory, the adaptive beamformer gives the best overall performance (in any given environment). The different beamformers are applied to data obtained with an LFAS at sea. Analysis shows that adaptive triplet beamforming outperforms conventional beamforming algorithms. Adaptive triplet beamforming provides strong PS rejection, allowing the unmasking of targets in the presence of strong directional reverberation (e.g., from a coast) and at the same time provides positive array gain in most environments.

A physical approach for the observation of oil spills in SAR images

Abstract: In this paper, a physical approach to support oil spills observation over synthetic aperture radar (SAR) images is presented. Electromagnetic model is based on an enhanced damping model that takes into account oil viscoelastic properties and wind speed. As a matter of fact, a multisensor approach is considered and a constant false alarm rate (CFAR) filter is used to minimize speckle effect. A set of experiments is presented and discussed. They show that oil spill processing is effective over single-look SAR images using mean input data.

Current measurements by airborne along-track InSAR: measuring technique and experimental results

Abstract: Since the first demonstration of high-resolution mapping of surface currents by airborne along-track interferometric synthetic aperture radar (along-track InSAR) in the late 1980s, theoretical models of the along-track InSAR imaging mechanism and recommendations for ideal instrument parameters, measuring strategies, and data processing and interpretation techniques have been discussed in a number of publications. However, due to the experimental nature of existing instruments and algorithms and a very limited reference database from actual experiments, potential users have not recognized the along-track InSAR as a readily available tool for current measurements until now. In order to promote the use of InSAR and to validate and demonstrate current measurements on the basis of instrument parameters and data processing techniques proposed earlier, the authors have carried out experiments with an airborne X-band along-track InSAR over spatially varying current fields at two test sites in the German Bight of the North Sea. In this paper, an overview of the experimental scenarios and the acquired data is given, and the newly implemented algorithms for the retrieval of two-dimensional (2-D) surface current fields from the InSAR raw data are described. Using acoustic Doppler current profiler (ADCP) data and predictions of a numerical circulation model as reference, a root mean square (rms) error of spatial variations in the InSAR-derived current fields on the order of 0.1 m/s at an effective resolution of about 100 m is obtained, which is consistent with theoretical expectations. Furthermore, it is shown that the proposed iterative correction scheme for nonlinearities of the InSAR imaging mechanism on the basis of numerical simulations works well and leads to a significant improvement. Altogether, it is concluded that the proposed technique for current measurements by along-track InSAR is efficient, robust, and sufficiently mature for applications that require high-resolution snapshots of surface current fields within areas of some square kilometers, such as the monitoring of bathymetric changes in coastal waters.

Classification of transient sonar sounds using perceptually motivated features

Abstract: This paper describes a novel framework for classifying underwater transient signals recorded by passive sonar. The proposed approach involves two key ideas. Firstly, a feature-selection algorithm is used to identify those acoustic features that optimally model each class of transient sound. Secondly, features that are perceptually motivated are proposed, i.e., they encode information that human listeners are likely to use in transient classification tasks. Three perceptual features are proposed, which encode timbre, the physical material of the sound source, and the temporal context (pattern) in which the transient occurred. The authors show how these features, which are computed over different temporal windows, can be combined to make classification decisions. The performance of the proposed classifier is evaluated on a corpus of transient signals extracted from passive sonar recordings. Specifically, the performance of the perceptual features is compared with spectral features and with those that encode statistics of time, frequency, and power. The present results show that the perceptual features provide valuable cues to the class of a transient. However, the best performing classifier was obtained by selecting a subset of perceptual, spectral, and statistical features in a class-dependent manner.

Moored fish cage dynamics in waves and currents

Abstract: Recent work in the area of open ocean aquaculture system dynamics has focused separately upon either the response of fish cages in waves or the steady drag response due to ocean currents. In reality, however, forcing on these open ocean structures is a nonlinear, multidirectional combination of both wave and current profiles. At the University of New Hampshire-operated Open Ocean Aquaculture site, data were collected from a wave measurement buoy and a downward-looking Acoustic Doppler Current Profiler to characterize the surface elevation and water velocity profiles during an extreme northeast storm event. In addition to waves and currents, fish cage motion response in heave, surge, and pitch was inferred from accelerometer measurements during the same storm. The environmental data sets obtained during the peak of the storm were processed, analyzed, and used as input to a dynamic finite-element model. Simulations were performed using three load case scenarios: 1) in both waves and currents; 2) in waves only; and 3) in currents only. Model motion response results in both the time and frequency domain were compared with data obtained in situ . In addition to the motion response tests, the wave and current forcing influencing the mooring line tension response was also investigated. Analysis shows that in this case, the currents do not severely influence the oscillatory motion response, but do cause the cage to tilt, layback, and sink. The wave and current interaction effect did, however, influence the anchor line loads with a portion being attributed to nonlinear effects.

SAR-Derived coastal and marine applications: from research to operational products

Abstract: The synthetic aperture radar (SAR) has now successfully demonstrated its capacity to uniquely provide valuable high-resolution information for coastal applications (oil-spill monitoring, ship detection, shallow-water bathymetry mapping, sea-ice monitoring, high-resolution wind fields, coastal wave fields). However, it appears that the operational use of SAR-derived products still remains limited, particularly in Europe. Although costs and sampling rate are often invoked to explain this limitation, it also appears that the SAR-instrument capabilities are generally poorly known within the operational community. Consequently, no real initiative currently involves the sustainable use of SAR in the main European projects for operational oceanography nor meteorology. Conversely, other countries such as Norway and United States are now moving into the use of SAR on an operational basis for coastal ocean monitoring. Significant efforts are being led by these countries to develop and harmonize such a monitoring system and extend the number of locations. In order to promote the potential of SAR to routinely scrutinize our coastal environment, the objective of this paper is to provide an overview of current SAR-related issues, including a brief technical system description (coverage, revisit time, etc.) and qualitative and quantitative descriptions of operational marine products. The prospects for achieving true operational usage and improving these products will then be considered in terms of requirements (satellite receiving station, revisit time, low data costs).

Analysis of swath bathymetry sonar accuracy

Abstract: The practical limitations of many bottom mapping sonars lie in their ability to accurately estimate the angle of arrival. This paper addresses the accuracy of angle estimation when employed to determine the location of an extended target such as the bottom. A Gaussian model is assumed for the bottom backscatter and the corresponding Cramer-Rao lower bound for the variance of the angle estimate is determined for multi-element linear arrays. The paper focuses on determining the performance of high-resolution swath bathymetry sonars and, therefore, concentrates on the ability to determine bottom location with short pulses. Two error mechanisms, footprint shift and uncorrelated noise, are identified as important contributors to measurement errors. The two-element interferometric sonar configuration is investigated in detail. It is shown through the use of probability distributions, the Cramer-Rao bound, and simulation that it is difficult to get a good estimate of performance through simulation alone. Performance enhancement through pre-estimation and post-estimation averaging of multiple snapshots and changes in performance with pulse length and pulse rise time are also considered. Bottom estimation performance employing multi-element arrays is compared and contrasted with that of the two-element interferometric array. It is determined that there is little benefit associated with the multi-element array in terms of angle estimation performance alone. However, when other considerations such as angle ambiguities, multiple angles of arrival, and physical shortcomings associated with practical arrays are taken into account, the multi-element array is favored.