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Oceanic Engineering, IEEE Journal of

Issue 3 • Date Jul 1999

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Displaying Results 1 - 13 of 13
  • Stochastic optimization of linear sparse arrays

    Page(s): 291 - 299
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    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 View full abstract»

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  • Direct estimation of motion from sea floor images for automatic station-keeping of submersible platforms

    Page(s): 370 - 382
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    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×12-ft×6-ft water tank View full abstract»

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  • Internal wave spectrum in shallow water: measurement and comparison with the Garrett-Munk model

    Page(s): 333 - 345
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    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 View full abstract»

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  • Testing and evaluation of an integrated GPS/INS system for small AUV navigation

    Page(s): 396 - 404
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    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 View full abstract»

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  • Investigation of nonlinear interactions in ocean swell waves using the reduced bispectrum

    Page(s): 312 - 322
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    In this paper, a two-dimensional (2-D) reduced bicoherence function which can be calculated via the reduced bispectrum is introduced. The statistical and computational aspects of the 2-D reduced bicoherence function are compared with those of the conventional bicoherence function. It is shown that the proposed technique is less noise-sensitive than the conventional bicoherence estimate. The use of the 2-D reduced bicoherence function to investigate nonlinear interactions in ocean swell waves is also demonstrated. As the 2-D reduced bicoherence can be efficiently calculated, it is proposed to use this novel technique in oceanographic field experiments that require real-time data processing and interpretation View full abstract»

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  • A new optical instrument for the study of bubbles at high void fractions within breaking waves

    Page(s): 300 - 311
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    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 μm 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 View full abstract»

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  • Observability of target tracking with range-only measurements

    Page(s): 383 - 387
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    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 View full abstract»

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  • Observations of cnoidal internal waves and their effect on acoustic propagation in shallow water

    Page(s): 346 - 357
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    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 View full abstract»

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  • Ring-waves generated by water drops impacting on water surfaces at rest

    Page(s): 323 - 332
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    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-1) and very small wave amplitudes: the fraction of the incident kinetic energy converted into ring-waves is of the order of 1% View full abstract»

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  • A hybrid simplex genetic algorithm for estimating geoacoustic parameters using matched-field inversion

    Page(s): 358 - 369
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    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 View full abstract»

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  • New physical-statistical methods and models for clutter and reverberation: the KA-distribution and related probability structures

    Page(s): 261 - 284
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    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 (PD) and false alarms (αF). 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 View full abstract»

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  • Cramer-Rao lower bounds for sonar broad-band modulation parameters

    Page(s): 285 - 290
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    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 View full abstract»

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  • Discrete-time quasi-sliding mode control of an autonomous underwater vehicle

    Page(s): 388 - 395
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    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 View full abstract»

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Aims & Scope

The IEEE Journal of Oceanic Engineering (ISSN 0364-9059) is published quarterly by the IEEE Oceanic Engineering Society (IEEE OES). The scope of the Journal is the field of interest of the IEEE OES, which encompasses all aspects of science, engineering, and technology that address research, development, and operations pertaining to all bodies of water. This includes the creation of new capabilities and technologies from concept design through prototypes, testing, and operational systems to sense, explore, understand, develop, use, and responsibly manage natural resources.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
N. Ross Chapman
School of Earth & Ocean Sciences
University of Victoria
3800 Finnerty Road
Victoria, BC V8P 5C2 Canada
chapman@uvic.ca