Showing papers in "IEEE Journal of Oceanic Engineering in 1999"
TL;DR: In this article, an overview of the swimming mechanisms employed by fish is presented, with a relevant and useful introduction to the existing literature for engineers with an interest in the emerging area of aquatic biomechanisms.
Abstract: Several physico-mechanical designs evolved in fish are currently inspiring robotic devices for propulsion and maneuvering purposes in underwater vehicles. Considering the potential benefits involved, this paper presents an overview of the swimming mechanisms employed by fish. The motivation is to provide a relevant and useful introduction to the existing literature for engineers with an interest in the emerging area of aquatic biomechanisms. The fish swimming types are presented, following the well-established classification scheme and nomenclature originally proposed by Breder. Fish swim either by body and/or caudal fin (BCF) movements or using median and/or paired fin (MPF) propulsion. The latter is generally employed at slow speeds, offering greater maneuverability and better propulsive efficiency, while BCF movements can achieve greater thrust and accelerations. For both BCF and MPF locomotion, specific swimming modes are identified, based on the propulsor and the type of movements (oscillatory or undulatory) employed for thrust generation. Along with general descriptions and kinematic data, the analytical approaches developed to study each swimming mode are also introduced. Particular reference is made to lunate tail propulsion, undulating fins, and labriform (oscillatory pectoral fin) swimming mechanisms, identified as having the greatest potential for exploitation in artificial systems.
1,512 citations
TL;DR: Simulated annealing has been utilized to synthesize the positions and the weight coefficients of the elements of a linear array in order to minimize the peak of the sidelobes and to obtain a beam pattern that meets given requirements.
Abstract: In conventional beamforming systems, the use of aperiodic arrays is a powerful way to obtain high resolution employing few elements and avoiding the presence of grating lobes. The optimized design of such arrays is a required task in order to control the side-lobe level and distribution. In this paper, an optimization method aimed at designing aperiodic linear sparse arrays with great flexibility is proposed. Simulated annealing, which is a stochastic optimization methodology, has been utilized to synthesize the positions and the weight coefficients of the elements of a linear array in order to minimize the peak of the sidelobes and to obtain a beam pattern that meets given requirements. An important novelty is the fact that the latter goal can be achieved in parallel to the minimization of both the number of elements and the spatial aperture, resulting in a "global" optimization of the array characteristics. The great freedom that simulated annealing allows in defining the energy function to be minimized is the main reason for the notable versatility and the good results of the proposed method. Such results show an improvement in the array characteristics and performances over those reported in the literature.
191 citations
TL;DR: In this paper, an inversion of the ocean bottom parameters is performed with an objective function that includes the processing gain of a model-based matched filter (MBMF) receiver relative to the conventional matched filter.
Abstract: The paper discusses an inversion method that allows the rapid determination of in situ geoacoustic properties of the ocean bottom without resorting to large acoustic receiving apertures, synthetic or real. The method is based on broad-band waterborne measurements and modeling of the waveguide impulse response between a controlled source and a single hydrophone. Results from Yellow Shark '94 experiments in Mediterranean shallow waters using single elements of a vertical array are reviewed, inversion of the bottom parameters is performed with an objective function that includes the processing gain of a model-based matched filter (MBMF) receiver relative to the conventional matched filter. The MBMF reference signals incorporate waveguide Green's functions for known geometry and water column acoustic model and hypothesized bottom geoacoustic models. The experimental inversion results demonstrated that, even for complex environmental conditions, a single transmission of a broad-band (200-800 Hz) coded signal received at a single depth and a few hundred forward modeling runs were sufficient to correctly resolve the bottom features. These included the sound speed profile, attenuation, density, and thickness of the top clay sediment layer, and sound speed and attenuation of the silty clay bottom. Exhaustive parameter search proved unequivocally the low-ambiguity and high-resolution properties of the MBMF-derived objective. The single-hydrophone results compare well with those obtained under identical conditions from matched-field processing of multitone pressure fields sampled on the vertical array. Both of these results agree with expectations from geophysical ground truth. The MBMF has been applied successfully to a field of advanced drifting acoustic buoys on the Western Sicilian shelf, demonstrating the general applicability of the inversion method presented.
165 citations
TL;DR: In this article, the authors examined dynamical models for bladed-propeller-type marine thrusters, and proposed a simplified model for a bladed propeller type marine thruster.
Abstract: This paper examines dynamical models for bladed-propeller-type marine thrusters. Previously reported thruster dynamics models are reviewed, and a simplified model is proposed. Experimental testing of both the transient and steady-state performance of a marine thruster corroborates previously reported data, validates the simplified thruster model, and raises new questions. A companion paper reports preliminary experiments in the design and testing of thrust controllers which incorporate the dynamical thruster models described in the present paper.
146 citations
TL;DR: In this article, a necessary and sufficient condition for local system observability, a prerequisite to target motion analysis, is presented in this technical communication for two-dimensional manoeuvring target tracking with range-only measurements from a single observer.
Abstract: A necessary and sufficient condition for local system observability, a prerequisite to target motion analysis, is presented in this technical communication for two-dimensional manoeuvring target tracking with range-only measurements from a single observer. The approach taken in this paper utilizes the Fisher information matrix developed from the analytical treatment of system dynamics and noisy measurement equations established in a modified polar coordinate system. The analytical results of this paper are demonstrated by a series of simulation studies for applications on naval surface vehicle engagements.
137 citations
TL;DR: In this paper, a dextrous subsea robot hand incorporating force and slip contact sensing is described, where the hand uses a fluid-filled tentacle for each finger, which has inherent passive compliance, and no moving parts.
Abstract: This paper describes the mechanical design, finger modeling, and sensor signal processing for a dextrous subsea robot hand incorporating force and slip contact sensing. The hand uses a fluid-filled tentacle for each finger, which has inherent passive compliance, and no moving parts. Force sensing uses strain gauges mounted in the fingertip, potted within a silicon elastomer. Slip sensing uses a piezoelectric strip to detect vibration, embedded 1 mm below the elastomer surface. Static models of finger motion are presented and validated based on bending moments and hydraulic pressure. The design of a stochastic estimator is also described for sensor fusion of contact force magnitude and direction data, obtained using redundant strain gauges in the fingertip. Finally, linear dynamic models of the finger dynamics in contact with a rigid surface are obtained using least squares and recursive least squares parameter estimation, as a precursor to closed-loop force control during grasping.
110 citations
TL;DR: Hardware and software design and testing results of the SANS are described and it is shown that results from tilt table testing and bench testing provide an effective means for tuning filter gains.
Abstract: A Small Autonomous Underwater Vehicle Navigation System (SANS) is being developed at the Naval Postgraduate School. The SANS is an integrated Global Positioning System/Inertial Navigation System (GPS/INS) navigation system composed of low-cost and small-size components. It is designed to demonstrate the feasibility of using a low-cost strap-down inertial measurement unit (IMU) to navigate between intermittent GPS fixes. The present hardware consists of a GPS/DGPS receiver, IMU, compass, water speed sensor, water depth sensor, and a data processing computer. The software is based on a 12-state complementary filter that combines measurement data from all sensors to derive a vehicle position/orientation estimate. This paper describes hardware and software design and testing results of the SANS. It is shown that results from tilt table testing and bench testing provide an effective means for tuning filter gains. Ground vehicle testing verifies the overall functioning of the SANS and exhibits an encouraging degree of accuracy.
106 citations
TL;DR: In this article, the results of combined consideration of sound coherence and array signal processing in long-range deep-water environments are presented. But the authors focus on the horizontal and vertical array gains associated with linear and quadratic beamformers.
Abstract: This paper presents results of combined consideration of sound coherence and array signal processing in long-range deep-water environments. Theoretical evaluation of the acoustic signal mutual coherence function (MCF) of space for a given sound-speed profile and particular scattering mechanism is provided. The predictions of the MCF are employed as input data to investigate the coherence-induced effects on the horizontal and vertical array gains associated with linear and quadratic beamformers with emphasis on the optimal ones. A method of the radiation transport equation is developed to calculate the MCF of the multimode signal under the assumption that internal waves or surface wind waves are the main source of long-range acoustic fluctuations in a deep-water channel. Basic formulations of the array weight vectors and small signal deflection are then exploited to examine optimal linear and quadratic processors in comparison with plane-wave beamformers. For vertical arrays, particular attention is paid also to evaluation of the ambient modal noise factor. The numerical simulations are carried out for range-independent environments from the Northwest Pacific for a sound frequency of 250 Hz and distances up to 1000 km. It was shown distinctly that both signal coherence degradation and modal noise affect large-array gain, and these effects are substantially dependent on the processing technique used. Rough surface sound scattering was determined to cause the most significant effects.
102 citations
TL;DR: In this paper, the authors consider the role played by the sensor locations in the optimal performance of an array of acoustic vector sensors, and derive an expression for the Cramer-Rao bound on the azimuth and elevation of a single far-field source for an arbitrary acoustic vector-sensor array in a homogeneous wholespace.
Abstract: We consider the role played by the sensor locations in the optimal performance of an array of acoustic vector sensors, First we derive an expression for the Cramer-Rao bound on the azimuth and elevation of a single far-field source for an arbitrary acoustic vector-sensor array in a homogeneous wholespace and show that it has a block diagonal structure, i.e., the source location parameters are uncoupled from the signal and noise strength parameters. We then derive a set of necessary and sufficient geometrical constraints for the two direction parameters, azimuth and elevation, to be uncoupled from each other. Ensuring that these parameters are uncoupled minimizes the bound and means they are the natural or "canonical" location parameters for the model. We argue that it provides a compelling array design criterion. We also consider a bound on the mean-square angular error and its asymptotic normalization, which are useful measures in three-dimensional bearing estimation problems. We derive an expression for this bound and discuss it in terms of the sensors' locations. We then show that our previously derived geometrical conditions are also sufficient to ensure that this bound is independent of azimuth. Finally, we extend those conditions to obtain a set of geometrical constraints that ensure the optimal performance is isotropic.
99 citations
TL;DR: In this paper, the authors consider the problem of threshold signal detection when the clutter (for radar) and reverberation (for sonar) are generally non-Gaussian, and derive probability distributions (pdf's) based on counting functional techniques and decomposition principle.
Abstract: Effective signal processing in active regimes requires appropriate measurement and statistical modeling of the (usually) dominant scatter returns. Here the critical situation of threshold signal detection is considered, when the clutter (for radar) and reverberation (for sonar) are generally non-Gaussian. The latter is a common condition for operation, particularly at small grazing angles, e.g., with radar off sea and land surfaces, and for sonar in shallow water, where surface and bottom interaction are significant. Two examples of constrained optimization, namely incoherent reception employing instantaneous amplitudes and envelopes, illustrate the statistical role of the non-Gaussian returns in signal processing. The threshold detection algorithms are locally optimum Bayes and locally asymptotically normal. In particular, they are also canonical, i.e., formally independent of the particular physics involved, as are the performance measures, expressed now as probabilities of detection (P/sub D/) and false alarms (/spl alpha//sub F/). The principal emphasis here, however, is on the statistical description of the (signal-dependent) scatter process, including also accompanying ambient and system noise, and (unwanted) "large" reflectors (terrain features and wave surface structure). The derivation of probability distributions (pdf's) is based on the author's earlier counting functional techniques and Decomposition Principle (DP), which here can account for multiple scatter contributions. The resulting non-Gaussian scatter processes include the new KA (i.e., "bunched" Class A) model. This is a generalization of the earlier K-clutter models and one which permits a statistical description of many scatter scenarios not physically covered by the latter. Another significant result is the demonstration of the equivalence of the new approach to the formulations of classical scattering theory. Since, unlike the present method, the latter is not generally capable of providing analytic results for probability densities, this new approach provides a significant advance over previous methods by generating the needed physically derived pdf's for effective signal processing. This paper is in part a work in progress: it includes a discussion of method, a variety of analytical and empirical results and needed next steps, some illustrative comparisons with empirical sonar and radar data, and an Appendix on the physical justification of the gamma pdf for fluctuation scatter intensities.
94 citations
TL;DR: In this paper, a discrete-time quasi-sliding mode controller for an AUV in the presence of parameter uncertainties and a long sampling interval is presented, and experiments on depth control of the AUV are performed in a towing tank.
Abstract: This paper presents a discrete-time quasi-sliding mode controller for an autonomous underwater vehicle (AUV) in the presence of parameter uncertainties and a long sampling interval. The AUV, named VORAM, is used as a model for the verification of the proposed control algorithm. Simulations of depth control and contouring control are performed for a numerical model of the AUV with full nonlinear equations of motion to verify the effectiveness of the proposed control schemes when the vehicle has a long sampling interval. By using the discrete-time quasi-sliding mode control law, experiments on depth control of the AUV are performed in a towing tank. The controller makes the system stable in the presence of system uncertainties and even external disturbances without any observer nor any predictor producing high rate estimates of vehicle states. As the sampling interval becomes large, the effectiveness of the proposed control law is more prominent when compared with the conventional sliding mode controller.
TL;DR: In this article, comparative experiments with three different thrust control algorithms over a wide range of unsteady operating conditions suggest that model-based control algorithms offer transient thrust-control performance superior to that of their nonmodel-based counterpart.
Abstract: This paper reports comparative experiments with two novel and one conventional thrust control algorithms for the unsteady (transient) control of thrust generated by conventional bladed-propeller marine thrusters. First, comparative experiments with three different thrust control algorithms over a wide range of unsteady operating conditions suggest that model-based control algorithms offer transient thrust-control performance superior to that of their nonmodel-based counterpart. Second, hybrid simulations combining actual real-time experimental thruster responses with simulated one-dimensional real-time vehicle dynamics suggest that model-based thrust control algorithms offer vehicle position control superior to that of its nonmodel-based counterpart.
TL;DR: In this paper, the authors describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source, at which time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability.
Abstract: Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5/spl deg/C.
TL;DR: In this paper, the authors show that the propagation of 400-Hz sound through continental-shelf internal solitary wave packets is strongly influenced by coupling of normal modes, and that the directionality of energy flux between high-order acoustic modes and (less attenuated) low-order modes determines a gain factor for long-range propagation.
Abstract: Propagation of 400-Hz sound through continental-shelf internal solitary wave packets is shown by numerical simulation to be strongly influenced by coupling of normal modes. Coupling in a packet is controlled by the mode coefficients at the point where sound enters the packet, the dimensions of the waves and packet, and the ambient depth structures of temperature and salinity. In the case of a moving packet, changes of phases of the incident modes with respect to each other dominate over the other factors, altering the coupling over time and thus inducing signal fluctuations. The phasing within a moving packet varies with time scales of minutes, causing coupling and signal fluctuations with comparable time scales. The directionality of energy flux between high-order acoustic modes and (less attenuated) low-order modes determines a gain factor for long-range propagation. A significant finding is that energy flux toward low-order modes through the effect of a packet near a source favoring high-order modes will give net amplification at distant ranges. Conversely, a packet far from a source sends energy into otherwise quiet higher modes. The intermittency of the coupling and of high-mode attenuation via bottom interaction means that signal energy fluctuations and modal diversity fluctuations at a distant receiver are complementary, with energy fluctuations suggesting a source-region packet and mode fluctuations suggesting a receiver-region packet. Simulations entailing 33-km propagation are used in the analyses, imitating the SWARM experiment geometry, allowing comparison with observations.
TL;DR: The Transarctic Acoustic Propagation Experiment (TAP) as mentioned in this paper was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temperature and changes in sea ice thickness and concentration.
Abstract: In April 1994, coherent acoustic transmissions were propagated across the entire Arctic basin for the first time. This experiment, known as the Transarctic Acoustic Propagation Experiment (TAP), was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temperature and changes in sea ice thickness and concentration. CW and maximal length sequences (MLS) were transmitted from the source camp located north of the Svalbard Archipelago 1000 km to a vertical line array in the Lincoln Sea and 2600 km to a two-dimensional horizontal array and a vertical array in the Beaufort Sea. TAP demonstrated that the 19.6-Hz 195-dB (251-W) signals propagated with both sufficiently low loss and high phase stability to support the coherent pulse compression processing of the MLS and the phase detection of the CW signals. These yield time delay measurements an order of magnitude better than what is required to detect the estimated 80-ms/year changes in travel time caused by interannual and longer term changes in Arctic Ocean temperature. The TAP data provided propagation loss measurements to compare with the models to be used for correlating modal scattering losses with sea ice properties for ice monitoring. The travel times measured in TAP indicated a warming of the Atlantic layer in the Arctic of close to 0.4/spl deg/C, which has been confirmed by direct measurement from icebreakers and submarines, demonstrating the utility of acoustic thermometry in the Arctic. The unique advantages of acoustic thermometry in the Arctic and the importance of climate monitoring in the Arctic are discussed. A four-year program, Arctic Climate Observations using Underwater Sound is underway to carry out the first installations of sources and receivers in the Arctic Ocean.
TL;DR: In this article, the authors present an automatic optical station-keeping system for submersible vehicles in deep waters by exploiting the information in sea floor images. But their work is limited to the case of a three-thruster floating vehicle in a 6ft/spl times/12ft/pl times/6ft water tank.
Abstract: Maintaining a fixed position near the sea floor is a critical capability during the deployment of remotely operated or intelligent (autonomous) undersea vehicles in a variety of missions, including inspection and repair of undersea structures, data collection, and surveillance. We present an automatic optical station-keeping system for application to submersible vehicles in deep waters by exploiting the information in sea floor images. Readily measurable spatio-temporal image gradients are used to detect and compute the vehicle's translational and yaw motions using a direct motion vision technique. The vision system has been implemented on a Windows-NT Pentium platform, and the estimated positions and yaw angles are communicated via a serial link to the control system, running on a PC-386. Accurate station-keeping is demonstrated in experiments with a three-thruster floating vehicle in a 6-ft/spl times/12-ft/spl times/6-ft water tank.
TL;DR: A hybrid inversion algorithm which uses a parabolic equation propagation model and combines the downhill simplex algorithm with genetic algorithms is introduced and is demonstrated on synthetic range-dependent shallow-water data generated using the parabolic equations propagation model.
Abstract: Matched-fieId inversion (MFI) undertakes to estimate the geometric and geoacoustic parameters in an ocean acoustic scenario by matching acoustic field data recorded at hydrophone array with numerical calculations of the field. The model which provides the best fit to the data is the estimate of the actual experimental scenario. MFI provides a comparatively inexpensive method for estimating ocean bottom parameters over an extensive area. The basic components of the inversion process are a sound propagation model and matching (minimization) algorithm. Since a typical MFI problem requires a large number of computationally intensive sound propagation calculations, both of these components have to be efficient. In this study, a hybrid inversion algorithm which uses a parabolic equation propagation model and combines the downhill simplex algorithm with genetic algorithms is introduced. The algorithm is demonstrated on synthetic range-dependent shallow-water data generated using the parabolic equation propagation model. The performance for estimating the model parameters is compared for realistic signal-to-noise ratios in the synthetic data.
TL;DR: The results indicate that the approach yields performance comparable to human expert level capability, and because the described technique has the potential to rapidly process vast quantities of video data, it may prove valuable for other similar applications.
Abstract: This paper applies computer vision techniques to underwater video images of bioluminescent biota for quantifying, tracking, and identification. Active contour models are adapted for computerized image segmentation, labeling, tracking, and mapping of the bioluminescent plankton recorded by low-light-level video techniques. The system automatically identifies luminous events and extracts features such as duration, size, and coordinates of the point of impact, and uses this information to taxonomically classify the plankton species. This automatic classification can aid oceanographic researchers in characterizing the in situ spatial and temporal relationships of these organisms in their underwater environment. Experiments with real oceanographic data are reported. The results indicate that the approach yields performance comparable to human expert level capability. Furthermore, because the described technique has the potential to rapidly process vast quantities of video data, it may prove valuable for other similar applications.
TL;DR: In this article, the authors used a Monte Carlo approach to estimate the average internal wave energies as a function of depth for the SLICE89 experiment and showed that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean.
Abstract: Measurements of basin-scale acoustic transmissions made during the last four years by the Acoustic Thermometry of Ocean Climate (ATOC) program have allowed for the study of acoustic fluctuations of low-frequency pulse propagation at ranges of 1000 to 5000 km. Analysis of data from the ATOC Acoustic Engineering Test conducted in November 1994 has revealed new and unexpected results for the physics of ocean acoustic wave propagation in random media. In particular, use of traditional /spl Lambda/, /spl Phi/ methods (using the Garrett-Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observations of intensity probability density functions, intensity variance, and pulse time spread and wander suggest that the propagation is more likely near the border between the unsaturated and partially saturated regimes. Calculations of the diffraction parameter /spl Lambda/ are very sensitive to the broad-band nature of the transmitted pulse, with CW calculations differing from a simplistic broad-band calculation by 10/sup 3/. A simple model of pulse propagation using the Born approximation shows that CW and broad-band cases are sensitive to a random medium very differently and a theoretical description of broad-band effects for pulse propagation through a random media remains a fundamental unsolved problem in ocean acoustics. The observations show that, at 75-Hz center frequency, acoustic normal mode propagation is strongly nonadiabatic due to random media effects caused by internal waves. Simulations at a lower frequency of 28 Hz suggest that the first few modes might be treated adiabatically even in a random ocean. This raises the possibility of using modal techniques for ocean acoustic tomography, thereby increasing the vertical resolution of thermometry. Finally, the observation of unsaturated or partially saturated propagation for 75-Hz broad-band transmissions, like those of ATOC, suggests that ray-based tomography will be robust at basin-scales. This opens up the possibility of ray-based internal wave tomography using the observables of travel time variance, and vertical and temporal coherence. Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortment of parameters of the chi model can be formulated in terms of a mixed linear/nonlinear inverse. This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean. Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.
TL;DR: This paper presents an investigation of the robustness of an inter-frame feature measure classifier for underwater sector scan sonar image sequences, and identifies a set of features that are less susceptible to increased noise conditions and changes in the image filters.
Abstract: This paper presents an investigation of the robustness of an inter-frame feature measure classifier for underwater sector scan sonar image sequences. In the initial stages the images are of either divers or remotely operated vehicles (ROV's). The inter-frame feature measures are derived from sequences of sonar scans to characterize the behavior of the objects over time. The classifier has been shown to produce error rates of 0%-2% using real nonnoisy images. The investigation looks at the robustness of the classifier with increased noise conditions and changes in the filtering of the images. It also identifies a set of features that are less susceptible to increased noise conditions and changes in the image filters. These features are the mean variance, and the variance of the rate of change in time of the intra-frame feature measures area, perimeter, compactness, maximum dimension and the first and second invariant moments of the objects. It is shown how the performance of the classifier can be improved. Success rates of up to 100% were obtained for a classifier trained under normal noise conditions, signal-to-noise ratio (SNR) around 9.5 dB, and a noisy test sequence of SNR 7.6 dB.
TL;DR: In this paper, a model of the time/space structure of the internal wave was tuned to match that of the observations, showing that the data are consistent with a cnoidal wave hypothesis.
Abstract: Distinctive packets of periodic internal waves were observed during an experiment in the Gulf of Mexico. There was a 65-m-deep mixed layer overlying a thin strong density interface. A layer of weaker density stratification extended below the interface to the bottom, at a depth of 185 m. The waves had 2-10-m amplitudes, narrow frequency bandwidths with central frequencies of 8.5 cph, and they propagated in the upslope direction. The wave packets were observed on three consecutive days. They lasted about 3 h and were always observed at the same time of day, clearly in response to tidal forcing. A model of the time/space structure of the waves was tuned to match that of the observations, showing that the data are consistent with a cnoidal wave hypothesis. Observations of low-frequency acoustic propagation along two baselines show fluctuations that we hypothesize are due to interactions with the cnoidal waves. The fluctuations have spatial correlation scales (in the slantwise direction) on the order of 76 m. We simulate these effects using a time-step PE approach. We find that a mode-coupling resonance with the internal wave field results in elevated acoustic variability along a set of discrete spokes, emanating from the acoustic source. While acoustic variability tends to increase with range and with internal wave amplitude, tangential and radial correlation scales do not show a systematic dependence. The patterns in tangential and radial correlation scales show strong anisotropic patterns in azimuth, but little systematic trend in range.
TL;DR: In this article, the covariance matrix of sound-speed variations is determined from yo-yo CTD data collected during the SWARM 95 experiment at a fixed station, which covers approximately 2 h and were collected during a period when nonlinear solitary internal waves were absent or negligible.
Abstract: The covariance matrix of sound-speed variations is determined from yo-yo CTD data collected during the SWARM 95 experiment at a fixed station. The data covered approximately 2 h and were collected during a period when nonlinear solitary internal waves were absent or negligible. The method of empirical orthogonal functions (EOF) is applied to the sound-speed covariance matrix assuming that the internal wave modes are uncorrelated. The first five eigenvectors are found to agree well with the theoretically modeled eigenfunctions based on the measured buoyancy frequency and the internal wave eigenmode equation. The mode amplitudes for the first five modes are estimated from the corresponding eigenvalues. They agree with the Garrett-Munk model if j*=1 is used instead of j*=3. A second method is used to deduce the mode amplitudes and mode frequency spectra by projecting the sound-speed variation (as a function of time) onto the theoretical mode depth functions. The mode amplitudes estimated with this method are in agreement with the EOF results. A modified Garrett-Munk model is proposed to fit the frequency spectrum of linear internal waves in shallow water.
TL;DR: In this article, the authors used the Levitus ocean atlas to explore the effects of nonlinearities when inverting multimegameter-range acoustic data and to find accurate inversion methods.
Abstract: Ocean sound speed (a surrogate for temperature) derived from the ray travel times obtained from acoustic transmissions may be inaccurate when the reference ocean state is inadequate for linearized inversion. When the reference (e.g., the Levitus ocean atlas) is significantly different from the "true" ocean, the reference ray paths inaccurately represent the true sampling. In addition, natural oceanic variations, such as the evolution of a summer mixed layer, can significantly change the ray sampling over time. The guiding principle for inversion is that ray travel times associated with the inverse solution must match the measured travel times. A time-dependent reference ocean can reduce both the nonlinearities and the solution uncertainties since the model variances may be assumed to be less. The Levitus ocean atlas was employed to explore the effects of nonlinearities when inverting multimegameter-range acoustic data and to find accurate inversion methods. These methods were applied to acoustic data obtained in the North Pacific during the acoustic thermometry of ocean climate (ATOC) project using an acoustic source on Pioneer Seamount off the coast of California. In order to linearize the inversions, the annual cycle was removed by referencing the measured travel times to travel times computed using the Levitus monthly ocean atlas. This linearization results in a more accurate time series of range- and depth-averaged temperatures, but the solution for range- and depth-averaged temperature is only slightly different from that using a time-independent set of rays. Standard uncertainties for the 0-1000-m depth-averaged temperature are typically /spl plusmn/0.012/spl deg/C, while the annual peak to-peak temperature variation is about 0.4/spl deg/C. Because the travel time data are inherently averaging, the time series of range- and depth averaged temperature is insensitive to different assumptions made in the forward model, such as the model parameterization, variances, wavenumber spectra, and the data uncertainties.
TL;DR: In this article, an inverse of the rudder saturation (SAT) and slew rate limitation (SRL) is implemented in the controller to fully utilize the limited rudder power without introducing the controller wind-up, which may lead to undesirable long transients.
Abstract: This technical communication is concerned with the design of inland ship-steering autopilots characterized by turning rate regulation rather than heading regulation found on most ocean-going vessel autopilots. The autopilot design is based on the internal model control approach which allows clear connections between the controller structure and that of the ship model to be established. To fully utilize the limited rudder power without introducing the controller wind-up, which may lead to undesirable long transients, an inverse of the rudder saturation (SAT) and slew rate limitation (SRL) is implemented in the controller. Specifically, an inversion by feedback technique is employed by implementing the noninverted dynamics in a local feedback loop. This resolves the difficulty associated with inversion of the SAT and SRL mathematically. Consequently, high turning rate maneuver can be achieved in case of emergency encounters without causing the controller wind-up. This feature is particularly important for inland ships, since unexpected encounters with other ships are quite common and the capability of quick response is vital to ensure safe maneuvers.
TL;DR: An optical imaging system (BubbleCam) has been tested for the quantification of bubble distributions at high void fractions formed beneath breaking waves as discussed by the authors, which consists of a CCD video camera, stroboscopic light source, and optics allowing adjustable magnification, a fixed imaging volume, and the resolution of bubbles 3 pixels in radius and larger (equivalent to a minimum bubble radius of about 200 /spl mu/m).
Abstract: An optical imaging system (BubbleCam) has been tested for the quantification of bubble distributions at high void fractions formed beneath breaking waves. The instrument consists of a CCD video camera, stroboscopic light source, and optics allowing adjustable magnification, a fixed imaging volume, and the resolution of bubbles 3 pixels in radius and larger (equivalent to a minimum bubble radius of about 200 /spl mu/m in the test configuration). BubbleCam has been deployed in a shore-based configuration (data and power supplied via shore-connected cables) as well as an autonomous device in the open sea with its own power supply and data storage. The resulting images are processed using a variant of the Hough transform which allows computer-automated counting and measurement of the bubbles within the video frames. In addition, images can be qualitatively examined to provide insights into bubble plume evolution and creation mechanisms.
TL;DR: In this paper, ring-wave measurements are presented for drops of 2.2 and 2.8 mm in diameter impacting on fresh and salt water surfaces initially at rest, based on the observation of the mirror image of a sharp edge on the perturbed surface.
Abstract: Radar observations of the ocean surface can be affected by impacting raindrops. Ring-wave measurements are presented for drops of 2.2 and 2.8 mm in diameter impacting on fresh and salt water surfaces initially at rest. They are based on the observation of the mirror image of a sharp edge on the perturbed surface. The retrieved wave profiles show a rather stable characteristic wavenumber (0.2 mm/sup -1/) and very small wave amplitudes: the fraction of the incident kinetic energy converted into ring-waves is of the order of 1%.
TL;DR: The Synthetic Aperture Mapping and Imaging (SAMI) project as mentioned in this paper developed and tested at sea a wide-band synthetic aperture sonar prototype, capable of providing high-resolution seafloor images together with bathymetry maps.
Abstract: The objective of the Synthetic Aperture Mapping and Imaging (SAMI) project was to develop and to test at sea a wide-band synthetic aperture sonar prototype, capable of providing high-resolution seafloor images together with bathymetry maps. This system used the motion of a physically small array in order to synthesize a longer array, providing images with an across-track resolution independent of both range and transmit frequency. Such systems are clearly very relevant to the high-precision long-range (low-frequency) imaging of the sea bottom. The project has led to the construction of a prototype tested at sea on several well-known areas for comparison with existing images and maps. These areas included several types of sea bottom, depths, and geological structures. The results obtained in real time, on-board ship, have shown the relevance of the proposed wide-band techniques. The many profiles produced have provided high-resolution images and maps of various seafloors. Interpretation by geologists showed that the system was capable of providing the same or finer detail than a deep-sea short-range, high-frequency system and maintained a higher resolution over a wider swath. The sea data processed have shown that the system provided maps with a cubic meter voxel. The resolution cell is constant over the whole range (50 to 2500 m) thanks to the dynamic focusing of the synthetic aperture. Postprocessing of a part of the data stored during the experiments has been carried out in the laboratory. This work has shown that techniques such as autofocusing can give an increase in resolution (i.e., gain in contrast and resolution of about 3 dB). The results displayed in the paper show the relevance of the techniques developed to the provision of a complete high-performance imaging tool for the oceanographic community.
TL;DR: In this paper, it was shown that for low broad-band-signal-to-broadband-noise ratios, the estimates of power and modulation level are uncoupled from the estimation of modulation frequency and phase.
Abstract: A role of passive sonar signal processing is the detection and estimation of the parameters associated with amplitude modulated broad-band signals. An example of such signals is propeller noise. Discrete frequency lines occur at the rotational frequency of the propulsion shaft and at the blade frequency. This correspondence provides expressions for the Cramer-Rao lower bounds for the estimates of broad-band signal power, modulation level, modulation frequency, and modulation phase. It is shown that for low broad-band-signal-to-broad-band-noise ratios, the estimates of power and modulation level are uncoupled from the estimates of modulation frequency and phase.
TL;DR: In this article, the authors examined the effects of wave breaking and shadowing on the linear dispersion equation of the wave spectrum and found that shadowing significantly reduces the variance levels within the linear spectral region, while wave breaking is less directly related to changes in variance outside this region.
Abstract: Transformation of sea-surface Doppler microwave backscatter observations from the space-time domain to the wavenumber-frequency domain separates linear wave energy from nonlinear effects. Here observations and modeling are used to investigate the sources of these nonlinearities. Wave breaking and electromagnetic shadowing are examined with emphasis on their relative effects both inside and outside the region of the wavenumber-frequency spectrum associated with the linear dispersion equation. Shadowing significantly reduces the variance levels within the linear spectral region. In addition, shadowing is less directly related to changes in variance outside this region, i.e., that region associated with nonlinearity in the wave field. Wave breaking has less of an effect on the variance within the linear region than shadowing. However, the modeled wave breaking does have a greater tendency to increase variance levels at frequencies less than that of the linear wave field, for any given wavenumber. Aliasing and emphasis of crest backscatter are also explored to explain features seen in some wavenumber-frequency intensity images. Two-dimensional data allow the linear wave spectrum to be separated from nonlinear effects. This results in improved wave height spectrum estimation.
TL;DR: The Autonomous Bathymetry Survey System (AutoSurvey) as mentioned in this paper is a system that provides automation of swath sonar bathymetric surveys, which enables faster survey of an area through environmentally adaptive techniques while ensuring adequate coverage and data quality.
Abstract: This paper describes the Autonomous Bathymetry Survey System (AutoSurvey), a system that provides automation of swath sonar bathymetric surveys. This system enables faster surveying of an area through environmentally adaptive techniques while ensuring adequate coverage and data quality. AutoSurvey assesses data quality and coverage in real time and generates next-trackline waypoints based on actual system performance. The need for real-time performance assessment is discussed. A primary factor considered is the effect of the environment on swath bathymetry system performance, which is difficult to predict a priori. The system's features, design, and implementation are discussed in this paper. Simulation and sea trial results are presented, as well as an analysis of the system's ability to reduce survey time.