Source localization in shallow water in the presence of sensorlocation uncertainty

The problem of source localization in shallow water in the presence of sensor location uncertainty is considered. The Cramer-Rao Bound is used to carry out a feasibility study for the joint source and sensor location problem when the multipath propagation channel is modeled as a known, deterministic waveguide. Unlike the free-space propagation channel, the boundedness of the shallow-water waveguide along its vertical axis provides the key to joint determination of the source and sensor location parameters. It is seen that, when a set of intuitive identifiability conditions are satisfied, numerical examples indicate that, for the scenarios considered, the resulting loss in accuracy with which the source location can be estimated due to sensor location uncertainty may be tolerable     

Multichannel processing of broad-band multiuser communicationsignals in shallow water acoustic channels

High-throughout multiple-access communication networks are being considered for use in underwater acoustic channels. Bandwidth limitations of underwater acoustic channels require receivers to process broad-band communications signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operations. While greatly reducing intersymbol and multiple-access interference, spatial diversity implies high increase in adaptive multiuser receiver complexity. To reduce the complexity of the optimal multichannel combiner, spatial structure of multipath is exploited. The complexity of resulting adaptive decentralized multichannel multiuser receiver is reduced at almost no cost in performance. Comparison of proposed multichannel receivers in an experimental shallow water channel demonstrates superior performance of spatial signal combining. The use of multiple input channels is shown to provide high level of tolerance for the near-far effect in both centralized and decentralized receivers. Decentralized receiver with reduced-complexity combining is found to satisfy the performance/complexity trade-off required for practical receiver realization in shallow water networks     

基于小型三维阵列的瞬态声源快速被动定位方法

重物在落水和着底过程中都会产生瞬态声信号,这类信号可被运用于浅水区域水下目标定位。 针对浅水区域目标定位的问题,提出了一种基于小型立体五元基阵的瞬态声源快速被动定位算法。 在分析重物落水信号特征的基础上,选取合适的广义互相关加权函数求得传声器之间的声程差,运用快速最小二乘搜索算法进行声源定位。 结果表明:运用 5 传声器阵列可以同时兼顾定位精度和鲁棒性,且满足实时性要求,该方法可运用于浅水区域瞬态声源定位等领域。     

A New Method for Matched Field Localization Based on Two-Hydrophone

The conventional matched field processing (MFP) uses large vertical arrays to locate an underwater acoustic target. However, the use of large vertical arrays increases equipment and computational cost, and causes some problems such as element failures, and array tilting to degrade the localization performance. In this paper, the matched field localization method using two-hydrophone is proposed for underwater acoustic pulse signals with an unknown emitted signal waveform. Using the received signal of hydrophones and the ocean channel pulse response which can be calculated from an acoustic propagation model, the spectral matrix of the emitted signal for different source locations can be estimated by employing the method of frequency domain least squares. The resulting spectral matrix of the emitted signal for every grid region is then multiplied by the ocean channel frequency response matrix to generate the spectral matrix of replica signal. Finally, the matched field localization using two-hydrophone for underwater acoustic pulse signals of an unknown emitted signal waveform can be estimated by comparing the difference between the spectral matrixes of the received signal and the replica signal. The simulated results from a shallow water environment for broadband signals demonstrate the significant localization performance of the proposed method. In addition, the localization accuracy in five different cases are analyzed by the simulation trial, and the results show that the proposed method has a sharp peak and low sidelobes, overcoming the problem of high sidelobes in the conventional MFP due to lack of the number of elements.     

Performance analysis of digital acoustic communication in a shallowwater channel

The major obstacle to underwater acoustic communication is the interference of multi-path signals due to surface and bottom reflections. High speed acoustic transmission over a shallow water channel characterized by small grazing angles presents formidable difficulties. The reflection losses associated with such small angles are low, causing large amplitudes in multi-path signals. In this paper we propose a simple but effective model for multi-path interference, which is then used to assess the performance of a digital communication system operating in a shallow water channel. The results indicate that transmission rates in excess of 8 kbits/s are possible over a distance of 13 km and channel depth of only 20 meters. Such a system offers improved performance in applications such as data collection from underwater sensors     

Recent advances in high-speed underwater acoustic communications

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel     

Phase-coherent digital communications for underwater acousticchannels

High-speed phase coherent communications in the ocean channel are made difficult by the combined effects of large Doppler fluctuations and extended, time-varying multipath. In order to account for these effects, we consider a receiver which performs optimal phase synchronization and channel equalization jointly. Since the intersymbol interference in some underwater acoustic channels spans several tens of symbol intervals, making the optimal maximum-likelihood receiver unacceptably complex, we use a suboptimal, but low complexity, decision feedback equalizer. The mean squared error multiparameter optimization results in an adaptive algorithm which is a combination of recursive least squares and second-order digital phase and delay-locked loops. The use of a fractionally spaced equalizer eliminates the need for explicit symbol delay tracking. The proposed algorithm is applied to experimental data from three types of underwater acoustic channels: long-range deep water, long-range shallow water, and short-range shallow water channels. The modulation techniques used are 4- and 8-PSK. The results indicate the feasibility of achieving power-efficient communications in these channels and demonstrate the ability to coherently combine multiple arrivals, thus exploiting the diversity inherent in multipath propagation     

A Novel Method for Real-Time Monitoring of Channel Siltation Based on Bistatic Scattering Theory

Monitoring the thickness changes of channel siltation is paramount in safeguarding navigation and guiding dredging.This paper presents a novel method for realizing the field monitoring of channel siltation in real time.The method is based on the bistatic scattering theory and concerned more with the receiving and processing of multipath signal at high-frequency and small grazing angle.By use of the multipath propagation structure of underwater acoustic channel,the method obtains the silt thickness by calculating the relative time delay of acoustic signals between the direct and the shortest bottom reflected paths.Bistatic transducer pairs are employed to transmit and receive the acoustic signals,and the GPS time synchronization technology is introduced to synchronize the transmitter and receiver.The WRELAX (Weighted Fourier transform and RELAX) algorithm is used to obtain the high resolution estimation of multipath time delay.To examine the feasibility of the presented method and the accuracy and precision of the developed system,a series of sea trials are conducted in the southwest coast area of Dalian City,north of the Yellow Sea.The experimental results are compared with that using high-resolution dual echo sounder HydroBoxTM,and the uncertainty is smaller than ±0.06 m.Compared with the existing means for measuring the silt thickness,the present method is innovative,and the system is stable,efficient and provides a better real-time performance.It especially suits monitoring the narrow channel with rapid changes of siltation.     

Analysis of channel effects on direct-sequence and frequency-hoppedspread-spectrum acoustic communication

Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles, moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance     

ICESat-2激光卫星与光学遥感影像融合水深测量

针对浅海测深的数据特点和应用需求,以我国南海甘泉岛为例,研究了利用ICESat-2(Ice,Cloud,and Land Elevation Satellite-2)激光卫星数据和光学遥感影像开展主被动融合水深测量的方法。首先通过信号点提取、水面/水底识别、水下点折射改正等步骤处理ICESat-2数据,获得水深值,随后以激光点作为控制,计算光学水深反演模型参数,最后由点及面地获取大范围高精度水深。实验表明,甘泉岛区域主被动融合测深中误差优于1.30m,基于激光卫星数据的主被动融合测深方法能够为浅水水深测量提供新手段。     

Analysis of EMD algorithm for identification and extraction of an acoustic signal in underwater channel against wind driven ambient noise

Sonar generated acoustic signals transmitted in underwater channel for distant communications are affected by numerous factors like ambient noise, making them nonlinear and non-stationary in nature. In recent years, the application of Empirical Mode Decomposition （EMD） technique to analyze nonlinear and non-stationary signals has gained much attention. It is an empirical approach to decompose a signal into a set of oscillatory modes known as intrinsic mode functions （IMFs）. In general, Hilbert transform is used in EMD for the identification of oscillatory signals. In this paper anew EMD algorithm is proposed using FFT to identify and extract the acoustic signals available in the underwater channel that are corrupted due to various ambient noises over a range of 100 Hz to 10 kHz in a shallow water region.Data for analysis are collected at a depth of 5 m and 10 m offshore Chennai at the Bay of Bengal. The algorithm is validated for different sets of known and unknown reference signals. It is observed that the proposed EMD algorithmidentifies and extracts the reference signals against various ambient noises. Significant SNR improvement is alsoachieved for underwater acoustic signals.     

Acoustic wave propagation in horizontally variable media

An overview of the multipath expansion method of solving the Helmholtz wave equation to describe the underwater sound field for a fixed point source in a plane multilayered medium is presented. The approach is then extended to account for horizontal variations in bottom depth, bottom type, and sound speed in the stationary phase approximation. Comparisons of model results to a limited number of measured data sets and standard propagation codes are presented.     

Spatio-temporal processing of coherent acoustic communication datain shallow water

Achieving reliable underwater communication in shallow water is a difficult task because of the random time-varying nature of multipath propagation. When the product of Doppler-related signal bandwidth spread and multipath-related time spread of the channel is larger than one, some types of adaptive signal processing may not work very well. In this paper, various methods of coherent space-time processing are compared for a condition of a marginally overspread channel operating at 50 kHz. Various combinations of suboptimal spatially adaptive and time adaptive methods are considered. The coherent path beamformer (CPB) and recursive least squares (RLS) adaptive beamformer, both in combination with RLS time filtering, are analyzed. Also considered in the analysis is the combined RLS space-time optimal adaptive processor. Many experiments using broad-band phase-shift-keyed transmissions in shallow water have been conducted to provide data for testing these various processing methods. Because of the rapid time variation of the multipath, the product of bandwidth spread and time spread at this test site approached unity. In this environment, a suboptimal approach consisting of the adaptive beamformer followed by RLS equalization reduced reverberation and transmission errors     

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

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.     

两种基于贝叶斯点估计理论的多声源定位方法研究

海洋环境参数失配是制约匹配场定位性能的主要因素之一。为了克服环境失配，本文基于贝叶斯理论，将环境参数与声源的距离和深度一起作为未知量进行反演。然而在进行多声源定位时，反演参数的维数几何增长，极大地增加了反演问题的复杂性和计算量。为此本文将声源强度和噪声方差表示成其极大似然估计值，从而将这些参数进行隐式采样，大大降低了反演的维数和难度。文章比较了两种贝叶斯点估计方法，最大后验概率密度方法和最大边缘后验概率密度方法。最大后验概率密度方法的解是令后验概率密度取得最大值的参数组合，可以利用优化算法快速获得。最大边缘后验概率密度法将其他参数积分，得到目标参数的一维边缘概率分布，分布的最大值为反演结果。该方法得到最优估计值的同时可以获取参数估计的不确定信息。在环境参数和声源参数都未知的情况下，利用蒙特卡洛法在不同信噪比情况下对两种声源定位方法进行分析，实验结果表明：(1)对于敏感参数，如声源距离、水深和海水声速，最大边缘后验概率密度法比最大边缘后验概率密度方法的性能好。(2)对于较不敏感的参数，如海底声速、海底密度和海底声衰减，当信噪比较低时，最大边缘后验概率密度方法能较好地平滑噪声，从而比最大边缘后验概率密度法具有更好的性能。由于声源距离和深度是敏感参数，研究表明最大边缘后验概率密度法提供了一种在不确知环境下更可靠的多声源定位方法。     

Low complexity adaptive equalizers for underwater acoustic communications

Interference signals due to scattering from surface and reflecting from bottom is one of the most important problems of reliable communications in shallow water channels. To solve this problem, one of the best suggested ways is to use adaptive equalizers. Convergence rate and misadjustment error in adaptive algorithms play important roles in adaptive equalizer performance. In this paper, affine projection algorithm （APA）, selective regressor APA（SR-APA）, family of selective partial update （SPU） algorithms, family of set-membership （SM） algorithms and selective partial update selective regressor APA （SPU-SR-APA） are compared with conventional algorithms such as the least mean square （LMS） in underwater acoustic communications. We apply experimental data from the Strait of Hormuz for demonstrating the efficiency of the proposed methods over shallow water channel. We observe that the values of the steady-state mean square error （MSE） of SR-APA, SPU-APA0 SPU-normalized least mean square （SPU-NLMS）, SPU-SR-APA0 SM-APA and SM-NLMS algorithms decrease in comparison with the LMS algorithm. Also these algorithms have better convergence rates than LMS type algorithm.     

浅海声信道文本信息传输研究

论文首先介绍浅海声信道声传输的基本特性,指出在此类信道中进行多媒体信息传输的特殊困难.随后论述了文本信息传输所具有的抗噪声、抗起伏等优良性能,较能适应于水声信道的复杂性和多变性,但属于较高速率的文本信息传输,多途干扰仍然是文本信息正确检测的根本障碍.文中分析了频率跳变技术克服时域扩散较短的浅海多途的可行性和需解决的关键技术.海上获得的初步实验结果说明了文本信息传输所具有的优越性,值得今后继续深入的研究.     

Acoustic telemetry system for underwater control

Underwater acoustic communication in the multipath environment encountered in shallow water is restricted mostly by signal fading. It degrades the signal detection and time synchronization required for reliable acoustic communication. An approach to time synchronization and to the frequency diversity method is presented. A communication algorithm for obtaining a reliable acoustic underwater link, and offering an easy-to-implement decoding scheme is introduced, and system realization is described     

On the numerical modeling of tsunami wave generation and propagation

Abstract

In this article three main stages of tsunami wave evolution are investigated. At first, the development of disturbances from a given patched elevation of the bottom surface in an incompressible nonviscous fluid of the uniform depth is considered. Then, a tsunami wave diffraction by underwater bottom elevation or cavity is investigated. In this case the shallow water equations are already used, and it is supposed that a cylindrical wave is spread from patched water elevation over the epicentrum. Last, the tsunami propagation and transformation in a shallow water region and its run‐up on a beach are investigated on the basis of the improved shallow water theory, taking into consideration the nonlinear and dispersive terms of higher order. The proposed theory is tested in a problem of collisions of two solutions. Solutions of the first and the second problems are obtained by the method of integral Laplace's transformation with following numerical inversion of transformations. A finite difference method for a solution of the last problem is used.     

BDRM理论在双轴海洋声道中的应用

由于表面声道与深海声道间的耦合效应,声波在双轴海洋声道中的传播比较复杂,因此求解双轴海洋声道中的声场就比较困难。在 WKBZ 本征函数的基础上,推导出了参考界面相位修正的一致表达式,并将浅海声传播的波束位移射线简正波(BDRM)理论应用于计算双轴海洋声道中的声场,进行了数值模拟并与传统简正波方法进行比较,结果表明应用 BDRM 理论计算的传播损失具有很高的精度和速度,可以对双轴海洋声道内声传播问题进行分析和预报。