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ABSTRACT: A physics-based method for beamforming signals measured on a horizontal array is developed with an application to underwater active sonar systems. The proposed striation-based beamformer coherently combines the pressure from each element in the array at different frequencies, and these frequencies are selected based on a striation hypothesis. The linear frequency shift and corresponding phase term introduced in the array weight vector accounts for multipath-induced fading, producing beam output with increased signal gain. The method is demonstrated using data collected on an array towed in the North Atlantic. The combination of the striation-based beamformer with the waveguide invariant concept to improve tracker performance is discussed.
The Journal of the Acoustical Society of America 10/2012; 132(4):EL264-70. · 1.55 Impact Factor
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ABSTRACT: Synthetic time reversal (STR) is a technique for blind deconvolution in an unknown multipath environment that relies on generic features (rays or modes) of multipath sound propagation. This paper describes how ray-based STR signal estimates may be improved and how ray-based STR sound-channel impulse-response estimates may be exploited for approximate source localization in underwater environments. Findings are based on simulations and underwater experiments involving source-array ranges from 100 m to 1 km in 60 -m-deep water and chirp signals with a bandwidth of 1.5-4.0 kHz. Signal estimation performance is quantified by the correlation coefficient between the source-broadcast and the STR-estimated signals for a variable number N of array elements, 2 ≤ N ≤ 32, and a range of signal-to-noise ratio (SNR), -5 dB ≤ SNR ≤ 30 dB. At high SNR, STR-estimated signals are found to have cross-correlation coefficients of ∼90% with as few as four array elements, and similar performance may be achieved at a SNR of nearly 0 dB with 32 array elements. When the broadband STR-estimated impulse response is used for source localization via a simple ray-based backpropagation scheme, the results are less ambiguous than those obtained from conventional broadband matched field processing.
The Journal of the Acoustical Society of America 04/2012; 131(4):2599-610. · 1.55 Impact Factor
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ABSTRACT: The 2009 cooperative array performance experiment (CAPE'09) was designed to compare performance between vector- and pressure-sensor arrays. The experiment was a joint effort of Chinese and American investigators; both arrays were designed and assembled by the Hangzhou Applied Acoustics Research Institute (HAARI), while the source systems and signal processing/recording systems were designed and assembled by Applied Physics Laboratory, University of Washington (APL-UW). The two arrays, both approximately 7 m in length, were deployed vertically off the stern of the APL-UW's R/V Robertson in Lake Washington, Seattle. Various transmitted signals in the 1.5-4 kHz band were recorded simultaneously on the two arrays at ranges between 10 m and 4 km. The signals included repeated linear frequency-modulated chirps and communications sequences. The pressure- and vector-sensor arrays had 32 and 8 uniformly spaced elements, respectively. Because each element in the vector-sensor array recorded both pressure and the three components of particle velocity, the two arrays made the same number of measurements over a similar vertical aperture. In the present talk, the design features of the vector-sensor array are emphasized. Sample results for both arrays are presented. [Work supported by ONR.].
The Journal of the Acoustical Society of America 10/2011; 130(4):2540. · 1.55 Impact Factor
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ABSTRACT: The waveguide invariant summarizes the pattern of constructive and destructive interference between acoustic modes propagating in the ocean waveguide. For many sonar signal-processing schemes, it is essential to know the correct numerical value for the waveguide invariant. While conventional beamforming can estimate the ratio between the waveguide invariant and the range to the source, it cannot unambiguously separate the two terms. In the present work, striation-based beamforming is developed. It is shown that the striation-based beamformer can be used to produce an estimate for the waveguide invariant that is independent of the range. Simulation results are presented.
The Journal of the Acoustical Society of America 08/2011; 130(2):EL76-81. · 1.55 Impact Factor
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ABSTRACT: A demodulation algorithm for coherent underwater acoustic communications may start by applying a finite impulse response (FIR) filter matched to the response of the channel. Relevant design parameters are the length of the FIR filter and the rate at which it must be updated. The performance of the algorithm as a function of these design parameters can then be quantified in terms of the mean-squared error (MSE) in the soft demodulation output. In terms of propagation physics, the length of the FIR filter is related to the number of acoustic paths retained as usable signal, and the required update rate is related to the time variation of the channel. In the present study, scattering by the sea surface is the assumed source of time variation. A model is developed for the MSE as a function of the design parameters. The Kirchhoff approximation is used to model acoustic paths that may have undergone multiple reflections by the sea surface. Required model inputs include the grazing angle, the significant wave height, and the dominant period for the surface waves. Model predictions are compared to published experimental results. [Work supported by ONR.].
The Journal of the Acoustical Society of America 10/2010; 128(4):2432. · 1.55 Impact Factor
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ABSTRACT: The effect on the ambient noise level in shallow water of the ocean growing more acidic is modeled. Because most noise sources are near the surface, high-order acoustic modes are preferentially excited. Linear internal waves, however, can scatter the noise into the low-order, low-loss modes most affected by the changes in acidity. The model uses transport theory to couple the modes and assumes an isotropic distribution for the noise sources. For a scenario typical of the East China Sea, the noise at 3 kHz is predicted to increase by 30%, about one decibel, as the pH decreases from 8.0 to 7.4.
The Journal of the Acoustical Society of America 06/2010; 127(6):EL235-9. · 1.55 Impact Factor
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ABSTRACT: The cooperative array performance experiment (CAPEx) was performed in Lake Washington near Seattle in September 2009. Acoustic transmissions in the 1.5-4 kHz band were recorded simultaneously on two vertical arrays: one a conventional 32-element pressure-sensor array and the other an 8-element array that measured both pressure and the three orthogonal components of acoustic particle velocity at each element. The present talk is an overview of both the data collected and the hardware used during CAPEx. Data were collected on the stationary arrays for both stationary- and towed-source scenarios in water 60 m deep. The source-receiver range varied between 10 m and 4 km. The data collected at short range demonstrate the relationship between the pressure and particle velocity fields. At more distant ranges, the particle velocity data are used to estimate the bearing to the source. Experimental results are compared to predictions generated using numerical models. [Work supported by ONR.].
The Journal of the Acoustical Society of America 03/2010; 127(3):1820. · 1.55 Impact Factor
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ABSTRACT: Reconstructing the signal originally broadcast from a remote source in an unknown multipath environment is a task commonly known as blind deconvolution. At frequencies of several kilohertz and above, multipath shallow-ocean sound propagation may be adequately described by ray acoustics. This presentation describes results from the application of ray-based artificial time reversal (ATR) to underwater sound propagation measurements. The receiving array was vertical and it recorded signals with center frequencies and bandwidths of a few kHz at source-receiver ranges up to 3 km in a water depth of approximately 60 m. Ray-based ATR uses a simple-beam-former-determined ray-arrival direction to construct a frequency-dependent phase correction at the receiving array that allows the Green's function of the sound channel and the original source waveform to be separately estimated. Here, the correlation coefficient between the original signal and the ATR-reconstructed signal is presented as a function of range and signal-to-noise ratio. In addition, the effect of reducing the number elements of the receiving array and the use of a coherent combination of reconstructed results for various ray arrival directions on cross correlation coefficient will be discussed. [Work supported by ONR.].
The Journal of the Acoustical Society of America 03/2010; 127(3):1963. · 1.55 Impact Factor
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ABSTRACT: Waveguide invariant theory is applied to horizontal line array (HLA) beamformer output to localize moving broadband noise sources from measured acoustic intensity striation patterns. Acoustic signals emitted by ships of opportunity (merchant ships) were simultaneously recorded on a HLA and three hydrophones separated by 10 km during the RAGS03 (relationship between array gain and shelf-break fluid processes) experiment. Hough transforms are used to estimate both the waveguide invariant parameter "beta" and the ratio of source range at the closest point of approach to source speed from the observed striation patterns. Broadband (50-150-Hz) acoustic data-sets are used to demonstrate source localization capability as well as inversion capability of waveguide invariant parameter beta. Special attention is paid to bathymetric variability since the acoustic intensity striation patterns seem to be influenced by range-dependent bathymetry of the experimental area. The Hough transform method is also applied to the HLA beam-time record data and to the acoustic intensity data from three distant receivers to validate the estimation results from HLA beamformer output. Good agreement of the results from all three approaches suggests the feasibility of locating broadband noise sources and estimating waveguide invariant parameter beta in shallow waters.
The Journal of the Acoustical Society of America 01/2010; 127(1):73-83. · 1.55 Impact Factor
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ABSTRACT: The performance of a communications equalizer is quantified in terms of the number of acoustic paths that are treated as usable signal. The analysis uses acoustical and oceanographic data collected off the Hawaiian Island of Kauai. Communication signals were measured on an eight-element vertical array at two different ranges, 1 and 2 km, and processed using an equalizer based on passive time-reversal signal processing. By estimating the Rayleigh parameter, it is shown that all paths reflected by the sea surface at both ranges undergo incoherent scattering. It is demonstrated that some of these incoherently scattered paths are still useful for coherent communications. At range of 1 km, optimal communications performance is achieved when six acoustic paths are retained and all paths with more than one reflection off the sea surface are rejected. Consistent with a model that ignores loss from near-surface bubbles, the performance improves by approximately 1.8 dB when increasing the number of retained paths from four to six. The four-path results though are more stable and require less frequent channel estimation. At range of 2 km, ray refraction is observed and communications performance is optimal when some paths with two sea-surface reflections are retained.
The Journal of the Acoustical Society of America 11/2009; 126(5):2359-66. · 1.55 Impact Factor
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ABSTRACT: Nonlinear internal waves depress the high-gradient region of the thermocline. After the waves have passed, it may take several hours for the thermocline to rise to its prewave level. To examine consequent acoustic effects, two acoustic data sets collected 18-19 August in SW06 were analyzed with one at a fixed range of 550 m and the other along a tow track out to 8.1 km. Both data sets allow the effects of the rising thermocline on acoustic propagation to be examined over a range of temporal and spatial scales. Signals transmitted are in the midfrequency band 1.5-10.5 kHz. The gradual rising of the thermocline is shown to change acoustic intensity by 5 and 2 dB for the fixed and towed source data, respectively. Using nearby oceanographic moorings, a simple plane wave ocean model is developed to provide time-dependent environmental input to broadband acoustic modeling assuming range-independency. Modeling results are shown to produce mean transmission loss results consistent with experimental observations at both the short and the long ranges. The results suggest that the rising thermocline effect of nonlinear internal waves on mean transmission loss is observable, significant, and predictable. The result has implications on geoacoustic inversion. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2511. · 1.55 Impact Factor
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ABSTRACT: The sonar simulation toolset (SST) is a ray-based propagation model capable of generating time series realizations with stochastic spreading of acoustic sequences. The model supports varying environments and geometries including moving sources andor receivers. The simulated data were compared with experimental data collected at the Pacific Missile Range Facility, Kauai, Hawaii in 2003 (KauaiEx2003) and 2008 (KAM08). Models generated with appropriate sound speed profiles and wind speeds are shown to have similar channel impulse response functions to those observed during the experiments. The high-frequency communications sequences simulated from those models have been demodulated to demonstrate analogous results. These results suggest that underwater acoustic communications and networking can be effectively simulated with sonar simulation toolset to aid in experiment planning and data analysis. [Work supported by the Office of Naval Research.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2580. · 1.55 Impact Factor
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Daniel Rouseff
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ABSTRACT: Coherent underwater communications is hampered by the time spread caused by multi-pathing and the Doppler spread caused by the rapidly changing shallow water environment. In practice, a time-varying impulse response model will retain only a finite number of paths as useful signal and will be updated to compensate for the changing environment at a finite rate. In the present study, communications performance is examined as a function of the number of retained paths and the model-update interval. Results are quantified in terms of the mean squared error in the soft demodulation output. A physics-based performance model is developed. The model uses as input acoustic quantities like the Rayleigh parameter and environmental quantities like the significant wave height and the dominant period for surface waves. Model predictions for performance are compared to experimental results for data collected near the Hawaiian Island of Kauai. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2580. · 1.55 Impact Factor
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ABSTRACT: When mapped versus range and frequency, acoustic intensity often displays a regular pattern of striations, ribbons of high intensity. The trajectory of these striations may be described by the waveguide invariant, commonly designated as beta. While beta may be formally an invariant quantity, it is not necessarily a constant, particularly in highly variable shallow water environments. To study fluctuations in beta due to fluctuations in the environment, it is useful to generalize from an invariant scalar to the waveguide invariant distribution. Through data analysis and simulation, the waveguide invariant distribution is calculated for different locations including the New Jersey shelf off the United States and the Kamchatka shelf off Russia. The effects of isotropic and anisotropic internal waves are quantified. [Work supported by ONR and the Russian Foundation for Basic Research.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2703. · 1.55 Impact Factor
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ABSTRACT: Preliminary results are presented from an analysis of mid-frequency acoustic transmission data collected at range 550 m during the Shallow Water 2006 Experiment. The acoustic data were collected on a vertical array immediately before, during, and after the passage of a nonlinear internal wave on 18 August, 2006. Using oceanographic data collected at a nearby location, a plane-wave model for the nonlinear internal wave's position as a function of time is developed. Experimental results show a new acoustic path is generated as the internal wave passes above the acoustic source.
The Journal of the Acoustical Society of America 10/2008; 124(3):EL73-7. · 1.55 Impact Factor
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ABSTRACT: Mid-frequency (1-10 kHz) sound propagation was measured at ranges 1-9 km in shallow water in order to investigate intensity statistics. Warm water near the bottom results in a sound speed minimum. Environmental measurements include sediment sound speed and water sound speed and density from a towed conductivity-temperature-depth chain. Ambient internal waves contribute to acoustic fluctuations. A simple model involving modes with random phases predicts the mean transmission loss to within a few dB. Quantitative ray theory fails due to near axial focusing. Fluctuations of the intensity field are dominated by water column variability.
The Journal of the Acoustical Society of America 10/2008; 124(3):EL85-90. · 1.55 Impact Factor
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ABSTRACT: The scintillation index and the intensity cumulative distribution function of mid-frequency (2-10 kHz) sound propagation are presented at ranges of 1-9 km in a shallow water channel. The fluctuations are due to water column sound speed variability. It is found that intensity is only correlated over a narrow frequency band (50-200 Hz) and the bandwidth is independent of center frequency and range. Furthermore, the intensity probability distribution peaks at zero for all frequencies, and follows an exponential distribution at small values.
The Journal of the Acoustical Society of America 10/2008; 124(3):EL91-6. · 1.55 Impact Factor
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ABSTRACT: During the Shallow Water 2006 Experiment (SW06), mid-frequency acoustic transmission data were collected on a vertical array over a continuous 7-hour period at range 550 m. The relatively short range was deemed desirable for studying the effects of internal waves; individual waves in a packet of nonlinear internal waves might be isolated between the acoustic source and receiver. Of present interest are data immediately before, during and after the passage of a non-linear internal wave on 18 August 2006. Among other features, the data show a new acoustic path being generated as the internal wave passes the acoustic source. A ray-based model is developed for the observed effect that uses as input nearby oceanographic measurements. [Work supported by ONR.].
The Journal of the Acoustical Society of America 06/2008; 123(5):3587. · 1.55 Impact Factor
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ABSTRACT: As part of the ONR-sponsored SW06 experiment, mid-frequency sound propagation was measured at ranges 1-10 km in the frequency band of 2-10 kHz in August, 2006. The water depth is 80 m and the source depth is 30 m, close to the minimum of a duct with a thermocline above and a warm salty water below. The receivers are clustered into two groups, one at 25 m depth, the other at 50 m. The region has active internal wave activity during this time. Because the source is near the axis of the sound channel, it is observed that propagation is dominated by trapped modes and behaves similar to sound propagation in a deep water duct. Amplitude fluctuations and cross-frequency correlations are estimated. The scintillation index as a function of frequency and bandwidth is calculated.
The Journal of the Acoustical Society of America 06/2008; 123(5):3433. · 1.55 Impact Factor
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ABSTRACT: Active sonar systems can provide good target detection potential but are limited in shallow water environments by the high level of reverberation produced by the interaction between the acoustic signal and the ocean bottom. The nature of the reverberation is highly variable and depends critically on the ocean and seabed properties, which are typically poorly known. This has motivated interest in techniques that are invariant to the environment. In passive sonar, a scalar parameter termed the waveguide invariant, has been introduced to describe the slope of striations observed in lofargrams. In this work, an invariant for active sonar is introduced. This active invariant is shown to be present in the time-frequency structure observed in sonar data from the Malta Plateau, and the structure agrees with results produced from normal mode simulations. The application of this feature in active tracking algorithms is discussed.
The Journal of the Acoustical Society of America 04/2008; 123(3):1329-37. · 1.55 Impact Factor
