Focusing of a sound beam in an underwater waveguide using a vertical antenna

We propose a method for calculating the amplitude-phase distributions of signals on the elements of an emitting antenna for generating narrow wave beams and field focusing at given points of the waveguide. The efficiency of the method is confirmed by the data of a field experiment conducted in Lake Ladoga.     

双扩展水声信道的时延多普勒域稀疏估计

利用双扩展水声信道在时延-多普勒域存在的稀疏结构,将信道估计转化为压缩感知框架下的稀疏恢复问题可改善估计性能。但是,稀疏恢复经典方法如l_1范数、近似l₀范数无法适应水声信道时延-多普勒域稀疏度的动态变化,而匹配追踪(Matching pursuit,MP)、正交匹配追踪(Orthogonal Matching Pursuit,OMP)等贪婪类算法则存在着易进入局部最优解、二维搜索导致运算复杂度高等问题。提出在时延-多普勒域稀疏恢复的目标函数中引入非均匀范数约束(Non-uniform Norm Constraint,NNC),即在时延-多普勒域信道响应中根据每个时延-多普勒域位置的幅值分别分配为l₀或l₁范数约束,因而可通过不同范数约束组合的方式适应不同的时延-多普勒域稀疏度;同时,通过对非均匀范数代价函数进行梯度下降迭代求解并将梯度解投影至解空间推导了非均匀范数稀疏恢复的迭代求解方法,从而实现双扩展水声信道时延-多普勒估计。数值仿真和实验数据处理表明该算法相对经典方法有较明显的性能改善。通过仿真、海上水声通信实验结果可获取结论,利用时延-多普勒域稀疏特性的信道估计方法结合均衡器可有效提高双扩展信道条件下的水声通信性能。     

Formation of a narrow sound beam in the underwater sound channel by a vertical array

The possibility of using a vertical array for the generation of a narrow wave beam that propagates in the underwater sound channel along a given reference ray is discussed. The variational problem of choosing the initial field at the array aperture to provide the minimal possible average beam width along the propagation path of a fixed length is solved.     

Testing underwater bottom-moored antenna arrays in the sea and in a man-made lake

Measurements of response, gain, and noise immunity are carried out for an underwater compensated additive receiving array with randomly spaced hydrophones that is moored at the bottom of a man-made lake with multimode sound propagation. The in-sea locating ability of a similar array is demonstrated with the sources of noiselike signals at frequencies of 5–100 Hz. A dedicated numerical processor is developed and tested for processing the signals received by a random underwater array.     

Diffraction focusing of the sound field of a vertical antenna array in a range-dependent shallow sea

Using the adiabatic approximation for the mode representation of the sound field, it is shown that the efficiency of the diffraction focusing grows when the thickness of the isovelocity water layer increases with distance.     

Surface reverberation spectrum of underwater sound

A general formulation is derived for the power spectrum of fluctuations in the signal received by way of reflection from a rough sea surface, in terms of an arbitrary wavenumber-frequency spectrum of the surface waves and an arbitrary source directivity function. The formulation is then applied to the specific case of backscatter for which the extreme cases of acoustically slightly rough and very rough sea surfaces are examined in detail. The results of the analysis are compared with experiments, and with the somewhat more restricted results of other theoretical work.     

Transient fields of linear antenna arrays

An approximate analytical method is presented, which allows calculation of the transient field strength of linear antenna arrays. The detailed investigation of a logarithmicperiodic dipole antenna on the base of different degrees of approximation shows the separate influence of the element radii and the transient mutual coupling between neighbouring elements. For a better physical understanding of its transient radiation process the cylindrical linear antenna when excited by a step-function with finite rise time is discussed in detail presenting an extension of a recently published approximate version of the Singularity Expansion Method to nonzero center-loading. Dedicated to Professor H. Lueg (RWTH Aachen) on the occasion of his 65th birthday (30 April 1979)     

Statistics of caustics in an underwater sound channel

The numerical-analytical phase screen method is used to analyze the statistics of the density of caustics in an underwater sound channel with large-scale random inhomogeneities. Different cases of wave propagation direction with respect to the channel axis are considered, and the influence of the inhomogeneity correlation radius is investigated.     

Statistical characteristics of adaptive antenna arrays

Using the method of successive approximations, we obtain expressions for the spectral and correlation characteristics of the output signal of adaptive antenna arrays under the stationary operation regime in the first-order approximation, which is also called Born’s approximation. We show that the fluctuations of the weight coefficients result in distortions of the output-signal spectrum, such as its broadening and the appearance of a “trough” in a band that is somewhat narrower than the input-signal spectrum width. N. I. Lobachevsky State University, Nizhniy Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 10, pp. 1013–1024, October 1999.     

Formation of Brillouin waves in the underwater sound channel

The mechanism of the vertical sound field structure formation in the underwater sound channel is considered. The calculations are performed by the ray method for the rays that have upper turning points at the ocean surface. It is shown that the vertical field structure is formed by the ray pairs producing opposing waves in the vertical. The rays belonging to one pair have the same sign of their departure angles at the source. The pairs are formed because of the presence of a minimum in the ray cycle length as a function of the departure angle. The resulting ray pairs are analogs of Brillouin waves.     

Effect of Mesoscale eddies on underwater sound propagation

The effects of sound speed variables induced by an anticyclonic eddy and a field of three cyclonic eddies on long-range sound propagation are investigated respectively. A deep-sea analytical eddy model [Henrick RF, Siegmann WL, Jacobson MJ. General analysis of ocean eddy effects for sound transmission applications. J Acoust Soc Am 1977;62:860-870] is used to determined sound speed distributions produced by warm-core ring in the southwest of South China Sea. Furthermore, the above analytical eddy model is generalize to include the azimuth angle variation and is used to determined sound speed distributions produced by Gulf Stream rings with different strengths. The theoretical temperature fluctuations induced by the warm eddy and a eddy field including three cyclonic eddies agree qualitatively with the in situ investigation data. The transmission loss of acoustic energy through the cross section of the warm-core ring center and three cyclonic eddy centers is simulated using 2-D parabolic-equation (PE) numerical modeling. It is found that the acoustic field has a significant change with variation of the location of SOFAR axis in the presence of the warm eddy and three cyclonic eddies comparing with the scenario of no eddies. When the source is located in the outside of the warm-core eddy and three cyclonic eddies respectively, and the receiver is located in outside of the eddy, the transmission loss as a function of range is investigated at different receive depth. It is shown that the changes of transmission loss caused by the warm-core eddy and three cyclonic eddies are as much as 20 dB than that of no-eddy situation. In the case of three cyclonic eddies, the largest discrepancy of transmission loss is about 40 dB near the range of 45 km for a 25-Hz source being located at a depth of 1500 m.     

可变参数水声发射系统

本文给出了一种完整的供水声通信、数传、遥测、遥控以及水声传播研究用的水声发射系统，包括可编程信号源，高效功率放大器和宽带高效发射换能器等部分，具有良好的功能和实用价值。     

一种改进的水下爆炸声混响模型

为了研究水下爆炸声脉冲引起的混响,基于射线理论,利用椭球的几何性质,建立了改进的三维近程双基地混响平均强度模型,推导出爆炸深度任意的双基混响平均强度预报公式。计算分析了爆炸深度和脉冲宽度对混响强度的影响,并与实验结果进行了比较,结果表明:爆炸深度对混响强度的影响相对较小,脉冲宽度越大,混响强度越大。