水声信道高速率数据传输的换能器研制

在复杂的水声信道中 ,实现高速率的数据传输 ,水声换能器的带宽是一个重要的技术问题。实验中 ,为了拓宽换能器的带宽 ,我们利用圆柱形压电换能器的径向振动与液腔振动相耦合原理研制自由溢流式换能器和利用纵向振动与前盖板弯曲振动相耦合原理研制纵向振动复合棒换能器。本文介绍适应于复杂的水声信道中 ,进行高速率数据传输的两种宽频带换能器的研制     

Spatial and temporal pulse design considerations for a marinesediment classification sonar

A linear FM sonar system was developed to support the objective of remote acoustic classification of seafloor sediments. It is a calibrated, wideband, digital, frequency modulated sonar that provides quantitative, high-resolution, low-noise sub-bottom data. Since the linear sonar system can precisely transmit a specified waveform, the calibrated digitally recorded reflection data can be processed to estimate the acoustic impulse response of the seabed and sediment attenuation. An acoustic pulse with special frequency domain weighting characteristics is designed to provide low temporal sidelobe levels and a nearly constant resolution with depth even after passing through a sediment with high losses such as sand. After correlation processing, the wideband acoustic pulse yields an effective beam pattern with high spatial resolution and insignificant sidelobe levels. Data sets generated with the FM profiler indicate that the required temporal and spatial characteristics of the sonar are realized in practice     

多波束声呐图像条带中央和边缘残差处理方法

多波束声呐图像是进行海底底质分类的主要数据源之一,由于受海洋噪声、声波散射和混响、仪器设备等因素影响,其经各项常规改正后仍存在明显残差,突出表现在中央波束区和条带重叠区,难以形成高质量的声呐图像。文中分析了多波束声呐图像残差的成因及影响,提出了一种基于多条带最小二乘拟合的多波束声呐图像残差处理方法。首先,得到相邻声脉冲(ping)信号中央区域、重叠区域以及整体趋势的拟合函数;然后,通过拟合函数计算得到中央和重叠区域的残差改正系数;最后,通过改正系数进行残差改正。实验分析表明,该方法在保留原始细节的基础上,有效削弱了残差对声呐图像的影响,对多波束声呐图像处理具有参考和应用价值。     

水下爆炸信号的定位研究

被动声纳中,目标定位的测定只能利用目标声源发出的信号或噪声,爆炸声源与常用的电声式脉冲声纳发射器比较起来,具有某些明显的优点,可代替发射换能器来实现距离和方位的测定.针对水下爆炸声源,采用三元标量阵进行研究,通过确定信号到达各个阵元的时延信息来测定其目标方位和距离,并利用内插方法提高定位精度.实验结果表明,不同位置处爆...     

声学方法进行海底沉积物遥测分类:综述

本文讨论利用声学遥测方法进行海底沉积物分类的目的、用途及该项技术的发展背景和现状。重点介绍目前主要的用于浅、表层沉积物分类的声学遥测方法，包括选通等幅脉冲、宽频带脉冲、连续线性扫频脉冲等正入射方法和侧扫声呐、多波束测深等斜入射方法。回顾它们的发展历史，评述它们的技术现状，并讨论它们的进一步发展     

Experimental study of the ballast in situ sediment acoustic measurement system in South China Sea

The mechanical structure, the function modules, the working principles, and a sea trial of the newly developed ballast in situ sediment acoustic measurement system are reported in this study. The system relies on its own weight to insert transducers into seafloor sediments and can accurately measure the penetration depth using a specially designed mechanism. The system comprises of an underwater position monitoring and working status judgment module and has two operation modes: self-contained measurement and real-time visualization. The designed maximum working water depth of the system is 3,000?m, and the maximum measured depth of seafloor sediment is 0.8?m. The system has one transmitting transducer with the transmitting frequency band of 20–35?kHz and three receiving transducers. The in situ acoustic measurement system was tested at 15 stations in the northern South China Sea, and repeated measurements in seawater demonstrated good working performance. Comparison with predictions from empirical equations indicated that the measured speed of sound and attenuation fell within the predicted range and that the in situ measured data were reliable.     

Experimental investigation of the combustive sound source

In this paper, we describe a unique low frequency underwater sound source called the combustive sound source (CSS). In this device, a combustible gas mixture is captured in a combustion chamber and ignited with a spark. The ensuing combustion produces expanding gases which in turn produce high intensity, low frequency acoustic pulses. With high-speed motion pictures of the CSS event, we relate the motion of the bubble to the acoustic waveform. We also compare the measured first bubble period in the CSS pressure signature with the predictions of the Rayleigh-Willis equation, including the dependence of the radiated acoustic waveform on the volume and depth of the bubble. Measurements of the first bubble period agree with Rayleigh-Willis theory in trend, but not in absolute value. In addition, we discuss the variation of the acoustic output with the fuel/oxygen mixture. Finally, several other factors that affect the acoustic output of CSS are discussed. These include the shape of the CSS combustion chamber, the type of oxidizer and fuel, and the ignition source     

Analysis of finite-duration wide-band frequency sweep signals forocean tomography

A group of amplitude and frequency modulated signals which generate narrow synthesized pulses are described. The pulse-compression properties of these signals should approach those of maximal (M) sequence phase-modulated signals now commonly used in ocean experiments. These amplitude-tapered linear frequency-sweep (chirp) type signals should be accurately reproducible with most acoustic sources since they have controllable limited-bandwidth frequency content and differentiable phase. The Doppler response of the signals is calculated using a wideband approach, where the frequency shift from relative motion is not constant throughout the waveform. The resultant Doppler effect on the matched-filter output is a function of the signal duration. The signals are suitable for use with tunable resonant transducers, and have adequate Doppler response for use with Lagrangian ocean drifters     

Biomimetic sonar locates and recognizes objects

An active sonar is described that adaptively changes its location and configuration in response to the echoes it observes in order to locate an object, position it at a known location, and identify it using features extracted from the echoes. The sonar consists of a center transmitter flanked by two receivers that can rotate and is positioned at the end of a robot arm that has five degree-of-freedom mobility. The sonar operates in air using Polaroid transducers that are resonant at 60 kHz with a nominal wavelength equal to 6 mm. The emitted pulse has a short duration with a useful bandwidth extending from 20 to 130 kHz. Using binaural information, the transmitter rotates to position an echo-producing object on its axis to maximize the acoustic intensity incident on the nearest echo-producing feature. The receivers rotate to maximize the echo amplitude and bandwidth. These optimizations are useful for differentiating objects. The system recognizes a collection of ball bearings, machine washers, and rubber O-rings of different sizes ranging from 0.45 to 2.54 cm, some differing by less than 1 mm in diameter. Learning is accomplished by extracting vectors of 32 echo envelope values acquired during a scan in elevation and forming a data base. Recognition is accomplished by comparing a single observed echo vector with the data base to find the least squared error match. A bent-wire paper clip illustrates the recognition of an asymmetric pose-dependent object     

Self-monitoring sonar transducer array with internal accelerometers

To achieve the maximum detection range for active sonars, it is desirable to maximize their acoustic power. In attempting to do so, there are two limitations which are caused by cavitation on the face of the transducer and an interaction effect between transducer elements of the array. To avoid these limitations, a method is proposed for driving the array elements in phase with a uniform velocity to yield the desired radiation. For this purpose, a sonar transducer array which has internal accelerometers for monitoring the motion of the transducer element is being developed. This paper presents an outline of this sonar array and a method of high-speed numerical calculation which was developed for the prediction of the radiation characteristics of the array using the fractional Fourier transform algorithm     

Pressure Dependence of Cymbal Transducers

The hydrostatic pressure limit that a receiver can withstand without failure is of major importance in underwater sonar systems. In this paper, the hydrostatic pressure tolerance and sensitivity of cymbal receivers were investigated. The failure mode in cymbal transducers under hydrostatic pressure is described. Effects of cavity geometry and material selection on hydrostatic piezoelectric coefficients and pressure limits were evaluated using both experimental data and finite-element analysis (FEA). It was found that cavity depth has a very strong effect on the stability of underwater sensitivity and pressure tolerance of these devices. Cymbals made with soft piezoelectric transducers (PZTs) possess higher figures of merit and better pressure tolerance than those made with hard PZTs. Alternatively, the cymbal sensitivity and pressure tolerance can be improved by changing the cap material.     

Potential performance of parametric communications

From a designer viewpoint, a parametric array should not appear different from any other type of acoustic transducer and should be described by a limited set of design equations together with their range of validity. In this paper, these design equations are stated and discussed. They are used to optimize the acoustic parameters of an underwater communication system using parametric transduction and to evaluate its performance in terms of signal-to-noise ratio and data-rate limits as a function of transmission range. It turns out that, for a maximum data transmission rate at a given range, there is a set of optimum design parameters which is a function of the array size only. This means that, once given an operational range, the primary frequency, the electrical power, the maximum acoustic source level, and the directionality of the transducer can be deduced directly from the array diameter     

Theoretical accuracy of Doppler navigation sonars and acoustic Doppler current profilers

This paper addresses the problem of Doppler shift estimation in Doppler sonar systems. The analysis focuses on the single-beam geometry formed by a circular planar array and considers both narrow-band (or so-called incoherent) and wide-band (or coherent) Doppler sonars, transmitting, respectively, one long continuous-wave pulse and a train of short continuous-wave pulses. The correlation function of the reverberation signal at the beam output is derived for volume reverberation. Directive transmission or reception and a combination of both is considered. Estimation theory is applied to derive the Cramer-Rao bound of the Doppler parameter estimate. The effect of pulse duration, sonar geometry, beamwidth, and signal-to-noise ratio are discussed. The accuracy of coherent and incoherent systems is compared for a specific case.     

Active sonar detection in shallow water using the Page test

The use of active sonar in shallow water results in received echoes that may be considerably spread in time compared to the resolution of the transmitted waveform. The duration and structure of the spreading and the time of occurrence of the received echo are unknown without accurate knowledge of the environment and a priori information on the location and reflection properties of the target. A sequential detector based on the Page test is proposed for the detection of time-spread active sonar echoes. The detector also provides estimates of the starting and stopping times of the received echo. This signal segmentation is crucial to allow further processing such as more accurate range and bearing localization, depth localization, or classification. The detector is designed to exploit the time spreading of the received echo and is tuned as a function of range to the expected signal-to-noise ratio (SNR) as determined by the transmitted signal power, transmission loss, approximate target strength, and the estimated noise background level. The theoretical false alarm and detection performance of the proposed detector, the standard Page test, and the conventional thresholded matched filter detector are compared as a function of range, echo duration, SNR, and the mismatch between the actual and assumed SNR. The proposed detector and the standard Page test are seen to perform better than the conventional thresholded matched filter detector as soon as the received echo is minimally spread in time. The use of the proposed detector and the standard Page test in active sonar is illustrated with reverberation data containing target-like echoes from geological features, where it was seen that the proposed detector was able to suppress reverberation generated false alarms that were detected by the standard Page test     

Pulse-length-tolerant features and detectors for sector-scan sonar imagery

This paper presents a neural-network-based system to detect small man-made objects in sequences of sector-scan sonar images created using signals of various pulse lengths. The detection of such objects is considered out to ranges of 150 m by using an experimental sector-scan sonar system mounted on a vessel. The sonar system considered in this investigation has three modes of operation to create images over ranges of 200, 400, and 800 m from the vessel using acoustic pulses of a different duration for each mode. After an initial cleaning operation performed by compensating for the motion of the vessel, the imagery is segmented to extract objects for analysis. A set of 31 features extracted from each object is examined. These features consist of basic object size and contrast features, shape moment-based features, moment invariants, and features extracted from the second-order histogram of each object. Optimal sets of 15 features are then selected for each mode and over all modes using sequential forward selection (SFS) and sequential backward selection (SBS). These features are then used to train neural networks to detect man-made objects in each sonar mode. By the addition of a feature describing the sonar's mode of operation, a neural network is trained to detect man-made objects in any of the three sonar modes. The multimode detector is shown to perform very well when compared with detectors trained specifically for each sonar mode setting. The proposed detector is also shown to perform well when compared to a number of statistical detectors based on the same set of features. The proposed detector achieves a 92.4% probability of detection at a mean false-alarm rate of 10 per image, averaged over all sonar mode settings.     

Design constraints and error analysis of the temporal correlationlog

The basic principles of spatial and of temporal correlation sonar for velocity measurement of both surface vessels and autonomous underwear vehicles are considered, showing the advantages of using a pulsed temporal correlation log. To achieve correlation, the conditions for waveform invariance must be met, and there is a velocity-depth limitation to be observed; these are described in detail. Practical design constraints such as the effects of different transducer sizes/geometry and carrier frequencies are also considered     

多波束与侧扫声纳海底目标探测的比较分析

侧扫声纳是目前常用的海底目标(如沉船、水雷、管线等)探测工具,在测深领域,多波束以全覆盖和高效率证明了它的优越性。由于多波束具有很高的分辨率,目前在工程上已经开始应用多波束进行海底目标物的探测。对多波束和侧扫声纳进行了比较分析,并着重探讨了影响多波束分辨率的各种因素。结果表明:多波束的最大优点在于定位精度高,但其适用范围不如侧扫声纳广泛,尤其受到水深和波束角的限制,多波束和侧扫声纳在探测海底目标时具有很好的互补性,同时应用可以提高目标解译的准确性。     

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.     

Temporal resolutions of time-reversal and passive-phase conjugation for underwater acoustic communications

In this paper, we study the temporal resolution of a time-reversal or passive-phase conjugation process as applied to underwater acoustic communications. Specifically, we address 1) the time resolution or the pulse width of a back-propagated time-compressed pulse as compared with the original transmitted pulse; 2) the effectiveness of temporal focusing as measured by the peak-to-sidelobe ratio of the back-propagated or phase-conjugated pulse (both pulse elongation and sidelobe leakages are causes of intersymbol interference and bit errors for communications); 3) the duration of temporal focusing or the temporal coherence time of the underwater acoustic channel; and 4) the stability of temporal focusing as measured by the phase fluctuations of successive pulses (symbols). Binary phase-shift keying signals collected at sea from a fixed source to a fixed receiver are used to extract the above four parameters and are compared with simulated results. Mid-frequency (3-4-kHz) data were collected in a dynamic shallow-water environment, exhibiting high temporal fluctuations over a scale of minutes. Despite this, the channel is found to be highly coherent over a length of 17 s. As a result, only one probe signal is used for 17 s of data. The bit error rate and variance of the symbol phase fluctuations are measured as a function of the number of receivers. They are of the same order as that calculated from the simulated data. The agreement suggests that these two quantities could be modeled for a communication channel with high coherence time. The phase variance can be used to determine the maximum data rate for a phase-shift keying signal for a given signal bandwidth and a given number of receivers.     

高分辨图象声纳目标模拟器的设计

高分辨图象声纳在小目标的探测和识别中扮演重要角色，目标模拟器是图象声纳设备调试的主要仪器之一。文章讨论了模拟器信号发生的机理，分析了近场和远场声基阵输出的信号模型。给出了利用该仪器测得的某型图象声纳的波束图。