THE EFFECTS OF CORRELATED SENSOR SIGNAL FLUCTUATION ON THE STATISTICAL PERFORMANCE OF AN AR HIGH RESOLUTION ARRAY PROCESSOR

The statistical performance of AR high resolution array processor in presence of correlated sensor sig-nal fluctuation is studied.Mean square inverse beam pattern and pointing error are examined.Speial atten-tion is paid to the effects of reference sensor and correlation between sensors.It is shown that fluctuationcauses broadening or even distortion of the mean square inverse beam pattern.Phase fluctuation causespointing error.Its standard variance is proportional to that of fluctuation ahd is related to the number ofsensors of the array.Correlation betwecn sensors has important effects on pointing error.     

Frequency invariant beamforming via optimal arraypattern synthesis and FIR filters design

An approach to designing time domain broadband frequency invariant beamformer via optimal array pattern synthesis and optimal FIR filters design is proposed. First, the working frequency band is decomposed into a number of narrow band frequency bins.The array weights at each frequency bin are designed via optimal array pattern synthesis methods to insure that the synthesized pattern approximates the desired one within the mainlobe area. Then, a bank of FIR filters corresponding to the input channels are designed to provide the frequency responses that approximate the array weights in the working frequency band for each sensor. Finally, each sensor feeds a FIR filter and the filter outputs are summed to produce the beam output time series. Both array pattern synthesis and FIR filters design problems are formulated as the second-order cone programming (SOCP), which can be easily solved using well-developed interior-point methods. Results of computer simulations and lake-experiment for a twelve-element semicircular array demonstrate satisfactory performance of the proposed approach.     

Range estimation by Doppler of multi-linein radiated noise spectrum

A method by applying Doppler frequency shift of multi-line in radiated noise spectrum to estimate the vertical range from a receiver to a moving vessel, which is supposed to move along a straight line at a constant velocity, is developed. This method is based on passive ranging by a single sensor and the depth of the sea and other environment parameters are not necessarily known. First the Wigner-Ville distribution is used as the instantaneous frequency estimator to find out the instantaneous frequencies of the muti-lines as a signal. Then define Doppler frequency shift basis functions, based on an algorithm called matching pursuit, by a searching strategy of variable span, and explore the minimum spatial distance between the signal and the Doppler frequency shift basis functions in a five-dimension space. The basis function of the obtained minimum spatial distance corresponds to the estimation of range and speed of the moving vessel. Computer simulations yield statistics errors in the range and speed estimates with differing intensities of noise. If the white noise deviation is less than 10% of the maximum Doppler frequency shift and time-window width is 1.47 times of reference-duration, relative error of range estimate is less than 5.4% and relative error of speed estimate is less than 1.4%. This estimation method has been tested and the result conforms to data collected during an experiment on the sea, the estimated speed is 52 knots and the estimated range is 42 m. The single point passive ranging method can be used for ranging purposes in sonar-buoys, mines, movement analysis of an underwater object in underwater acoustics experiment, and sound source level measurements.     

Single-row laser beam with energy strengthened ends for continuous scanning laser surface hardening of large metal components

For laser surface hardening of metal components with large superficies,a binary grating is proposed to generate single-row laser beam with proportional-intensity diffractive orders.To obtain a uniform hardened band distribution and improve the wear resistance of the sample surface,the binary grating is designed to produce single-row laser beam with energy strengthened at the two ends.The profile of the laser beam spot was designed to be strip with high length-width ratio to improve the machining efficiency of the hardening of large surfaces.A new advantage is suggested to obtain proportional intensity spots with evenness.The design results show that the diffractive efficiency of the binary grating is more than 70%,and the uniformity is less than 3%.The surface profile of the grating fabricated was measured,which shows that the fabrication error is less than 2%.The application of the binary grating in the laser surface hardening of metal components with large superficies is experimentally investigated,and the results show that the hardness distribution of the modified layer is more uniform than that hardened by Gaussian laser beam or array spots with equal intensity distribution.     

Investigation of long-range sound propagation in surface ducts

Understanding the effect of source-receiver geometry on sound propagation in surface ducts can improve the performance of near-surface sonar in deep water. The Lloyd-mirror and normal mode theories are used to analyze the features of surface-duct propagation in this paper. Firstly, according to the Lloyd-mirror theory, a shallow point source generates directional lobes, whose grazing angles are determined by the source depth and frequency. By assuming a part of the first lobe to be just trapped in the surface duct, a method to calculate the minimum cutoff frequency （MCF） is obtained. The presented method is source depth dependent and thus is helpful for determining the working depth for sonar. Secondly, it is found that under certain environments there exists a layer of low transmission loss （TL） in the surface duct, whose thickness is related to the source geometry and can be calculated by the Lloyd-mirror method. The receiver should be placed in this layer to minimize the TL. Finally, the arrival angle on a vertical linear array （VLA） in the surface duct is analyzed based on normal mode theory, which provides a priori knowledge of the beam direction of passive sonar.     

Research on a port and starboard ambiguity conjugate cancelling method for twin-line array

It is always an important problem in sonar signal processing for towed array to distinguish the target between port and starboard.It is studied in this paper the normalized beam output function for the parallel twin-line array and found a method that can enhance its port and starboard ambiguity suppression by equalizing the amplitude and phase of the beam output from the conventional beam forming.This method gets the weight value by analyzing the normalized beam output function,and then uses the amplitude weight and phase compensation for beam outputs of the twin arrays to cancel the conjugate（mirror） output. Therefore,the port and starboard restrain ratio is maximized.Both the computer simulation and sea trial data showed that this is a simple and practical method,can better enhance the port and starboard restrain ratio,especially when the band is wide.     

The time-delay estimation method of resolving left-right target ambiguity for twin-line array and its realization

The directivity function of twin-line array sonar can be used to identify left/right target. But the performance is limited by the distance between two lines and also exists the blind area for target identification. A method, which is based on the time-delay estimation between two line arrays, is proposed to identify left/right target. The method is robust with the variation of distance between two line array, the blind area is considerable decreased. The theoretical analysis results and realization method is described. An algorithm base on interpolation method for precisely estimating tune-delay of two line array are derived, therefore the left-right ambiguity problem is solved.     

Matched wavelength and incident angle for the diagnostic beam to achieve coherent grating tiling

Design and operation of a practical,accurate alignment diagnostic system is important for the grating tiling technology,which is supposed to be applied in a chirped-pulse amplification system to increase the output power.A diagnostic method is proposed and demonstrated for grating tiling.Provided that the wavelength and incident angle of the diagnostic beam are properly set,the far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings.Therefore,rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam.The real-time monitoring and alignment can be achieved without disturbing the main beam.     

Matched wavelength and incident angle for the diagnostic beam to achieve coherent grating tiling

Design and operation of a practical, accurate alignment diagnostic system is important for the grating tiling technology, which is supposed to be applied in a chirped-pulse amplification system to increase the output power. A diagnostic method is proposed and demonstrated for grating tiling. Provided that the wavelength and incident angle of the diagnostic beam are properly set, the far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings. Therefore, rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam. The real-time monitoring and alignment can be achieved without disturbing the main beam.     

Propagation of phase-locked truncated Gaussian beam array in turbulent atmosphere

Truncation manipulation is a simple but effective way to improve the intensity distribution properties of the phase-locked Gaussian beam array at the receiving plane. In this paper, the analytical expression for the propagation of the phase-locked truncated Gaussian beam array in a turbulent atmosphere is obtained based on the extended Huygens-Fresnel principle. Power in the diffraction-limited bucket is introduced as the beam quality factor to evaluate the influence of different truncation parameters. The dependence of optimal truncation ratio on the number of beamlets, the intensity of turbulence, propagation distance and laser wavelength is calculated and discussed. It is revealed that the optimal truncation ratio is larger for the laser array that contains more lasers, and the optimal truncation ratio will shift to a larger value with an increase in propagation distance and decrease in intensity of atmosphere turbulence. The optimal truncation ratio is independent of laser wavelength.     

基于超指向性多极子矢量阵的水下低频声源方位估计方法研究

针对水下低频声源的方位估计问题,基于基元紧密排列的二维矢量阵,建立了一种超指向性波束形成方法.根据矢量基元差分运算构建各阶多极子模型,获得了几乎与频率无关的模态函数,并经加权计算可在低频条件下实现超指向性波束,以解决阵列孔径对波束性能的限制.同时,结合输出信噪比最大准则所得波束,分析了超指向性波束形成算法的稳定性与波导的影响程度,探索模态阶数与阵列参数的选取原则.通过阵列性能的仿真计算与实际阵列的测量数据表明,该算法可在小尺寸阵列孔径下获得良好的阵列波束,兼具了水下线型超指向性阵和环形超指向性阵的优点,可有效实现水下低频声源的水平方位估计;且波束性能可通过调节模态阶数与基元间距以达到最佳,并受水下声波导多途与频散效应影响有限.     

声呐员听音移频操作客观规律研究*

声呐员在听音过程中，可以对收听到的声音进行移频操作，通过这一操作，声呐员可以将信号调制到自己认为合适的频段以便检测识别信号，而移频的具体频段没有定量的参考依据，这使声呐员移频操作较为盲目。本文从心理声学的角度对声呐员的移频操作进行研究，给出了移频前后信号的响度差值的变化规律和信号尖锐度的变化规律。研究结果表明，经过移频操作，响度移频前后在2000-5000 Hz左右存在最大值，而尖锐度随着移频频率和信号的频率的升高而升高，本文所得出的结论可为声呐员对舰船辐射噪声被动听音移频操作提供参考依据。     

微弱信号源的和波束定向方法与分裂波束定向方法的性能比较

本文讨论水下辐射噪声源的精确定向问题，给出被动声纳和波束定向与分裂波束定向方法的性能比较。指出在一定信噪比下，分裂波束精确测向技术比和波束定向技术具有较高的定向精度。但是，随着信噪比的下降，两者趋于一致。推导了估计定向精度的分析表达式，给出在直线阵和圆弧阵情况下，延时估计和声源入射角偏差之间的换算公式和数值模拟结果。同时给出数字式声纳用以计算入射信号左波束和右波束数据的互谱来实现分裂波束定向的方法。     

前视声呐圆弧接收阵列结构优化设计

针对前视声呐线阵波束形成各向同性较差及半圆阵有效孔径受限的问题,提出了一种优化的圆弧接收阵列结构设计方法。根据不同的成像开角,设计圆弧接收阵圆心角使全部阵元参与所有来波方向的波束形成。建立圆弧接收阵的远近场聚焦模型,并推导圆弧接收阵的指向性函数,根据边界波束栅瓣阈值条件计算具有最佳波束性能的圆弧接收阵半径。对圆弧阵、线阵及半圆阵在不同扇区开角下的波束性能及扫查开角120°、纵深100 m的仿真图像对比表明,在成像开角为90°到130°时,圆弧接收阵主瓣性能提升了8%,边界波束栅瓣级优化了4%,交叉向分辨深度相对线阵与半圆阵分别增加了20 m和45 m,同时参数存储量仅为线阵的2.7%。该阵列兼具有效孔径大、波束各向同性的优点,能有效提高前视声呐系统探测性能,并降低算法实现难度。      

Extraction of small boat harmonic signatures from passive sonar

This paper investigates the extraction of acoustic signatures from small boats using a passive sonar system. Noise radiated from a small boats consists of broadband noise and harmonically related tones that correspond to engine and propeller specifications. A signal processing method to automatically extract the harmonic structure of noise radiated from small boats is developed. The Harmonic Extraction and Analysis Tool (HEAT) estimates the instantaneous fundamental frequency of the harmonic tones, refines the fundamental frequency estimate using a Kalman filter, and automatically extracts the amplitudes of the harmonic tonals to generate a harmonic signature for the boat. Results are presented that show the HEAT algorithms ability to extract these signatures.     

水声信号处理领域新进展

本文介绍近30年来水声信号处理领域理论研究的新进展和在声纳设计中的应用。包括水声信号建模、声场匹配、海洋波导和内波现象的探索和研究、声矢量场信息获取和处理,低频水声信道的时/空相关特性,水下目标辐射噪声的不变特征量提取和检测技术,水下语音、图像传输和抗干扰技术。同时概述,声纳设计的前沿领域:大孔径拖曳线列阵声纳、高分辨力合成孔径声纳、深海传呼机等的发展情况。     

水下蛙人被动探测测向实验研究

为了对携带开式呼吸器的水下蛙人进行被动探测,本文采用四个水听器组成的十字阵在浅海水域对水下开式蛙人进行了被动探测、测向实验。结果表明:蛙人辐射噪声具有周期性特征,其周期与蛙人的呼吸频率有关;另外,蛙人呼吸辐射噪声具有显著频带,并且其频带与蛙人携带的呼吸器有关。根据蛙人辐射噪声的显著频带和周期性特征,设计信号处理算法实现了对蛙人的被动声学探测识别。使用互相关方法处理一对水听器的接收信号,可以确定蛙人的方位,但是根据理论分析会出现一个假方位。对比十字阵中两对水听器的测向结果可确定蛙人的准确方位。     

基于遗传算法的不等间距阵优化方法*

阵元数一定情况下,针对不等间距拖线阵高频信号处理存在的空间增益和无栅瓣兼容问题,提出一种基于遗传算法的不等间距拖线阵阵型优化方法。该方法以频带交互下最大旁瓣级最小作为约束因子,通过模拟生物自然进化过程搜索全局最优解,得到优化后的阵元位置。优化后阵型在保证低频信号处理空间增益不变情况下,实现对高频信号高空间增益、无栅瓣处理。验证分析结果表明：相比未优化阵型,在高频信号处理方面,优化后阵型在无栅瓣情况下,空间增益提升值近似理论值提升值；在低频信号处理方面,优化后阵型具有相同的空间增益。为实际不等间距拖线阵阵型优化和应用提供了一种思路。     

水声被动目标识别技术挑战与展望

低频水声探测和船舶减振降噪技术发展，使得传统水声目标识别技术性能逐渐下降。该文分析了声呐工作带宽、探测频率、船舶减振降噪给识别技术带来的挑战。针对低频声呐广泛使用的低频线谱识别，研究了低频线谱的识别能力问题；针对智能识别技术发展，研究了深度学习技术在船舶辐射噪声识别中的应用问题，并给出了数据试验结果，文章最后指出了水声被动目标识别技术亟需开展的研究内容和方向。