水下目标弹性声散射信号分离

水下目标弹性声散射与其他声散射成分在时域和频域上均存在混叠, 现有信号处理方法受分辨力限制无法在混叠状态下识别目标弹性声散射特征. 针对这个问题, 提出了一种目标弹性声散射信号分离方法. 以目标回波亮点模型为基础, 分析了线性调频信号入射时目标声散射成分的信号特性, 提出了一种目标声散射成分向单频信号的映射方法, 并理论推导出了目标声散射结构与映射结果之间的线性对应关系, 实现了通过窄带滤波分离出目标弹性声散射成分. 仿真与消声水池实验数据处理结果表明, 该方法基本可以完全分离出目标回波信号中的弹性声散射成分, 分离出的弹性声散射具有与理论一致的信号特征, 验证了该分离方法的有效性.     

水下掩埋目标的散射声场计算与实验

水下掩埋目标声散射问题是识别和探测掩埋目标的理论基础, 是声散射研究领域的热点问题. 本文基于射线声学推导了掩埋情况下目标声散射计算的格林函数近似式, 并在此基础上进一步给出了相应的远场积分公式. 在有限元方法的基础上, 将推导得到的公式写入有限元仿真软件, 对软件功能进行拓展, 构建二维轴对称目标的声散射模型, 并计算掩埋情况下弹性实心球在不同条件下的目标强度, 获得了其散射声场随频率、掩埋深度、沙层吸收系数等参数的变化规律. 开展实心球的自由空间和浅掩埋条件下水池声散射实验, 利用共振隔离技术处理实验数据, 提取目标声散射的纯弹性共振特征进行分析, 结果表明可将其用于掩埋目标识别和探测. 最后利用总散射声场与理论计算结果进行对比, 验证了理论仿真的正确性.     

An improvement in the range resolution of ultrasonic pulse echo systems by deconvolution

Ultrasonic pulse echo systems are often limited in range resolution by the bandwidth of the piezoelectric transducer. Significant improvements in the range resolution of such systems can be obtained by minimizing the effects of the transducer's dynamic response on the overall pulse echo process. An approach to minimize the effects of the transducer is developed from linear system and impulse response techniques. In essence, the pulse echo voltage of interest is deconvolved with a pulse echo reference voltage which is obtained from an air/water interface in the nearfield of the transducer . A computer study of the pulse echo process and the deconvolution process is presented to illustrate the nature of the improvement in range resolution for several cases of interest. Finally, experimental results are presented to illustrate the improvement using commercially available transducers.     

角度调谐的多普勒激光雷达硬目标转速测量

利用角度调谐方法确定直接探测多普勒测风激光雷达的初始工作点,角度调节精度可达10 μrad.在此工作点处使用频率稳定性为10-7,单纵模出现概率大于98%的Nd:YAG激光器,通过偏振分光光路测得参考信号与回波信号.配合目标准确放置到准直镜的焦距上,分别测量了配合目标静止,正转和反转时的参考信号与回波信号.利用配合目标静止时测得参考信号与回波信号的关系可得参考信号的角度矫正因子为1.1949,使用该矫正因子测得硬目标在正转和反转时转速与真实转速之间的误差小于0.5 m/s.     

Microwave Doppler spectra of sea return at small incidence angles: specular point scattering contribution

It is well known that the sea return echo contains contributions from at least two scattering mechanisms. In addition to the resonant Bragg scattering, the specular point scattering plays an important role as the incidence angle becomes smaller (≤20o). Here, in combination with the Kirchhoff integral equation of scattering field and the stationary phase approximation, analytical expressions for Doppler shift and spectral bandwidth of specular point scattering, which are insensitive to the polarization state, are derived theoretically. For comparison, the simulated results related to the two-scale method (TSM) and the method of moment (MOM) are also presented. It is found that the Doppler shift and the spectral bandwidth given by TSM are insufficient at small incidence angles. However, a comparison between the analytical results and the numerical simulations by MOM in the backscatter configuration shows that our proposed formulas are valid for the specular point scattering case. In this work, the dependences of the predicted results on incidence angle, radar frequency, and wind speed are also discussed. The obtained conclusions seem promising for a better understanding of the Doppler spectra of the specular point scattering fields from time-varying sea surfaces.     

Highlight model of echoes from sonar targets

I.IntroductionTheecho-rangingapproachistheonlycffectiveapproachfordetectionandidentifi-cationofunderwatertargetifitisasilcntobkct.Echoformationisaphysicalprocessinwhichthetargetisexcitcdbyincidentwavc.Byechothecharactcristicinformationsoftargethavebcencarricd.Suchinformationsarejustfundamentalsofdetectionandiden-hficationappliedbyactivesonars.Classificationandidentificationareimportantdevelopmentsofreccntsonarsin-c1udingthehomersoftorpedocs.Thisneedsadecpunderstandingrcferredtoechobe-haviors…     

利用正弦调频信号的宽带速度敏感特性抑制混响

有源声呐探测水中低速目标时,混响与目标回波的频率接近,目标回波与混响分离困难.根据运动目标与产生混响的散射体在速度域的差异,利用正弦调频信号(SFM)的速度敏感特性实现目标回波与混响的有效分离.推导了 SFM信号宽带混响抑制能力与速度分辨模糊之间的约束关系,建立了波形优化设计的搜索模型.通过多普勒滤波处理将数据从频率域...     

目标粗糙对合成孔径激光雷达回波的退相干效应

本文研究了目标表面粗糙对回波信号探测的影响,通过蒙特卡罗方法建立一维和二维目标高斯随机粗糙面模型,并对目标回波信号的探测过程进行仿真,研究目标表面粗糙对中频信号的影响.还进行了粗糙面和光滑面两组光外差探测实验,验证目标粗糙对合成孔径激光雷达回波信号严重的“退相干”效应.同时,采用数字波前分析仪对探测器处本振光和信号光波前分别进行检测,检测结果与仿真一致,证明粗糙面回波相位畸变严重.文中研究结果对设计光源参数、接收系统参数以及评估系统作用距离奠定基础,为合成孔径激光雷达系统设计提供定量参考.     

水下目标回波盲分离性能的瞬时频率特征评价方法

在水下主动声呐目标回波与混响盲分离中,针对分离结果顺序不确定性以及缺乏分离有效性衡量手段的问题,提出了以信号瞬时频率特征为指标的盲分离性能评价方法。推导了目标回波与混响的时频分布特性,理论表明目标回波在瞬时频率序列的中心偏离程度以及整体随机程度上低于混响,据此提取信号的瞬时频率方差与瞬时频率熵两种信号特征,并将二者作为从盲分离结果中识别目标回波的依据。海试数据结果表明,在盲分离得到的所有分离信号中,目标回波具有最小的瞬时频率特征值,并且该特征值越小,目标回波与混响的盲分离程度就越高。      

Using a grating analyser for SEMSANS investigations in the very small angle range

Spin-echo modulation small-angle neutron scattering (SEMSANS) is based on the detection of spatial beam modulation, which is induced by triangular spin echo precession regions and subsequent spin analyses. In order to detect such signal and exploit it for small angle scattering investigations neutron detection with sub-millimeter spatial resolution is required. Here an approach is reported where instead of a position sensitive detector an absorption grating is used to analyze the beam modulation stepwise. The spin-echo length scan in this case is performed by varying the sample-to-detector distance. The real space correlation functions of reference sample structures in the range 10² nm, i.e. giving rise to small-angle scattering in the very small-angle range, are recorded and analyzed successfully.     

水下目标几何声散射回波在分数阶傅里叶变换域中的特性

水下目标散射回波在时域、频域混叠在一起, 而且受声波入射角度的影响严重, 在不同的入射角度下表现出很大的差异, 需要建立全方位入射角度下回波分量的理论分析模型. 本文推导了目标几何声散射分量在分数阶傅里叶变换域中随入射角度变化的解析表达式; 确定了目标几何声散射回波分量在最佳分数阶傅里叶变换域中的全方位模型, 从理论上证明了目标回波的几何特征形式; 给出了离散分数阶傅里叶变换对声散射分量的分辨能力和计算精度与发射信号带宽和观测时间之间的关系. 实验数据处理表明, 建立的分数阶傅里叶变换域的全方位模型与目标几何特征是一致的, 对未知入射角度下的目标识别提供了理论依据.     

Reflectivity function reconstruction using the frequency deconvolution technique

Deconvolution of ultrasonic echo signals improves resolution and quality of ultrasonic images. A frequency deconvolution algorithm depends on the Fast Fourier transform is proposed for ultrasonic data. The stability of the algorithm and the influence of the truncation effect on the deconvoluted results were investigated with respect to the duration time of reflectivity function reconstruction and the signal to noise ratio. Reliability of the separation of reconstructing the reflectivity of a biological tissue is estimated by frequency deconvolution of the echo ultrasound signals.     

扩展性微动目标回波模拟与特征参数提取研究

研究了微动目标的多普勒回波模拟及特征参数提取技术. 提出了一种基于物理光学法和等效电磁流法的扩展性微动目标回波模拟方法. 将在目标坐标系下计算得到的后向散射场通过坐标转换, 成为雷达坐标系下的目标回波, 通过与解析信号模型对比验证方法的正确性. 分析了圆锥与带翼弹头的进动特性, 为获得较好的时频聚集性同时避免交叉项采用S-method方法对获取的回波信号进行时频分析, 分析了不同雷达波入射角度, 不同运动状态及不同几何外形的时频分布特点. 对时频分布图进行逆Radon变换, 将正弦曲线映射到参数空间, 从而获取目标的微动参数. 该研究结合电磁散射与信号处理技术, 通过对典型弹道目标的仿真, 获得一些不同于传统微动模型的结果, 结合电磁散射理论, 对这些现象进行了解释分析. 该研究成果在弹道目标的探测识别领域具有重要的理论与应用价值.     

Time-frequency analysis and modeling of the backscatter of categorized dolphin echolocation clicks for target discrimination

A set of dolphin echolocation clicks collected from an Atlantic bottlenose dolphin in Kaneohe Bay, Hawai'i from a previous experiment is examined in terms of their time and frequency characteristics. The center frequency and rms bandwidth are calculated for the clicks and these are clustered into four classes by using a model based on the Bayesian information criterion. The echo signatures are attained from a solid, elastic homogeneous sphere for each class of clicks from an acoustic scattering model. The results from the scattering model are compared to experimental values. The joint time-frequency content of the resulting echo signals is obtained by the reduced interference distribution (RID). The RIDs are plotted and examined for each signal class for four spherical targets of different material compositions. RID correlation values are obtained for a standard target versus comparison targets by using a time-frequency correlator. The results suggest that dolphins may discriminate by auditory inspection of the time-frequency information returned by the targets. The modification of the outgoing clicks and examination of time-frequency target information may be fundamental to a dolphin's ability to identify and discriminate targets.     

Acceleration target detection based on LFM radar

In radar systems, the echo signal caused by an accelerated target can be similarly considered as linear frequency modulation (LFM) signal. In high signal-to-noise ratio (SNR), discrete polynomial-phase transform (DPT) algorithm can be used to detect the echo signal, as it has low computation complexity and high real-time performance. However, in low SNR, the DPT algorithm has a large mean square error of the rate of frequency modulation and a low detection probability. In order to detect LFM signal in low SNR, this paper proposes a detection method, segment discrete polynomial-phase transform (SDPT), which means, at first, dividing the whole echo pulses into several segments with same duration in time domain, and then, using coherent accumulation method of DFT to segments, at last, processing this signal with DPT in intra-segment. In the case of a large number of segments, the SDPT can improve the output SNR. In addition, in a certain SNR, to the target signal with big sampling interval, large acceleration and less segments, this paper proposes an algorithm to detect the LFM signal generated from the combination of an improved DPT (IDPT) and fractional Fourier transform (FRFT). The output SNR of this algorithm is connected with the length of time delay. In the simulation, when the length of the time delay is 0.2 N, the output SNR is 2.5 dB more than that which results from directly using DPT. Finally, the detection performance and algorithm complexity of the proposed algorithm were analyzed, and the simulated and measured data verify the effectiveness of the algorithm.     

Reference Deconvolution in the Frequency Domain

Reference deconvolution, i.e., using the lineshape distortions of a reference signal with known ideal shape to deduce a correction function for the whole spectrum, is normally performed in the time domain. As a disadvantage, reference signals of higher multiplicity cannot be employed because of mathematical instabilities. In this work we show that these difficulties can be circumvented by carrying out reference deconvolution in the frequency domain. The computational demands of this approach are higher, but not prohibitive, because the width of the correction function is only a fraction of that of the whole spectrum. An iterative algorithm was implemented that yields the optimum widths of the correction function and of the ideal reference signal. Singular value decomposition was found to produce better results thanLUdecomposition of the design matrix. The feasibility of the deconvolution method and of the algorithm are demonstrated using both synthetic and experimental data.     

High resolution time-delay estimation of underwater target geometric scattering

The geometric scatterings carry the information of the shape of an underwater target. While the time-delay of the weak geometric scattering exists in the received signal cannot be obtained accurately by the conventional time-delay estimation methods because of the limit of the main-lobe width and the interferences from the side-lobe. In this paper, we propose a high resolution time-delay estimation (HRTDE) scheme consisting of two steps. Firstly, when a linear-frequency-modulated (LFM) pulse is transmitted by sonar, the dechirping method transforms the geometric scatterings with different time-delays into multiple single-frequency components respectively, in which the frequency of the dechirped signal shows a linear relationship with the time-delay of the geometric scattering. Then the multiple signal classification (MUSIC) algorithm is adopted to increase the spectrum resolution when multiple single-frequency signals exist in the dechirped signal and the frequency interval is smaller than the frequency resolution limit of the Fourier transform. Simulation results show that the main lobe of the proposed scheme is sharper and with less interference from the side-lobe, compared with the conventional time-delay estimation methods. The results from the anechoic pool experiment demonstrate that the proposed scheme achieves a better time-delay estimation performance for the weak geometric scattering generated by the bottom edge of the underwater target model than match filter based methods.     

Calibration of broadband active acoustic systems using a single standard spherical target

When calibrating a broadband active acoustic system with a single standard target such as a sphere, the inherent resonances associated with the scattering by the sphere pose a significant challenge. In this paper, a method is developed which completely eliminates the source of resonances through isolating and exploiting the echo from the front interface of a sphere. This echo is relatively insensitive to frequency over a wide range of frequencies, lacking resonances, and is relatively insensitive to small changes in material properties and, in the case of spherical shells, shell thickness. The research builds upon the concept of using this echo for calibration in the work of Dragonette et al. [J. Acoust. Soc. Am. 69, 1186-1189 (1981)]. This current work generalizes that of Dragonette by (1) incorporating a pulse compression technique to significantly improve the ability to resolve the echo, and (2) rigorously accounting for the scattering physics of the echo so that the technique is applicable over a wide range of frequencies and material properties of the sphere. The utility of the new approach is illustrated through application to data collected at sea with an air-filled aluminum spherical shell and long broadband chirp signals (30-105 kHz).     

Chirp excitation of ultrasonic guided waves

Most ultrasonic guided wave methods require tone burst excitations to achieve some degree of mode purity while maintaining temporal resolution. In addition, it is often desirable to acquire data using multiple frequencies, particularly during method development when the best frequency for a specific application is not known. However, this process is inconvenient and time-consuming, particularly if extensive signal averaging at each excitation frequency is required to achieve a satisfactory signal-to-noise ratio. Both acquisition time and data storage requirements may be prohibitive if responses from many narrowband tone burst excitations are measured. Here chirp excitations are utilized to address the need to both test at multiple frequencies and achieve a high signal-to-noise ratio to minimize acquisition time. A broadband chirp is used to acquire data at a wide range of frequencies, and deconvolution is applied to extract multiple narrowband responses. After optimizing the frequency and duration of the desired tone burst excitation, a long-time narrowband chirp is used as the actual excitation, and the desired tone burst response is similarly extracted during post-processing. Results are shown that demonstrate the efficacy of both broadband and narrowband chirp excitations.     

频域稀疏采样和激光成像方法

激光的单色性和自然图像频谱稀疏且集中在低频区间的特点,使图像频谱稀疏采样成像成为可能.基于小规模激光探测器,引入参考激光,本文提出了频域稀疏采样激光成像方法.介绍了频域稀疏采样激光成像的原理和成像系统结构,推导了激光回波重构复频谱的表达式,给出了重构频谱和复图像的仿真结果并分析了信号参数对重构效果的影响,同时采用相干系数、均方误差和结构相似度来评价其重构效果.规模为256×256的激光回波复图像仿真表明, 5个拼接1/4×1/4规模频域探测器组成的近似十字型稀疏采样结构,在约31.25%(5/16)的频域稀疏采样条件下,仍可获得较好的重构频谱和重构复图像.