一种用于分布式阵列的球谐波域声源定位方法

本文提出一种适用于任意阵型和阵元指向性的球谐波域声源定位方法,能够在较宽的频域范围内,尤其是低频,提供较高的空间分辨率。水下噪声源的高分辨识别具有重大意义。传统阵列信号处理方法对低频噪声源的精确定位要求阵列具有较大孔径和较多的阵元数,导致系统过于庞大且成本较高。我们基于声场的球谐波表达和变换,采用分布在一定空间区域内的多个阵列估计该区域的球谐波系数矢量,对系数矢量进行信号处理实现声源定位。理论证明了该方法具有理想的空间选择特性。在一种特定的阵元分布下,仿真研究了该算法的方位谱估计性能以及阵元不一致性和位置误差对声源定位性能的影响。仿真结果显示,该算法在低频具有较高的空间分辨率且误差对算法定位性能的影响有限。      

The Design of Triode Harmonic Generators

EXISTING analytical methods for predetermining the anode and grid currents of a triode harmonic generator are examined. These methods are found to give large unpredictable errors which may be of the order of 25 per cent. A new analytical method is devised, based on a power series representation of the tube characteristics, whereby the anode current components can be forecast with a maximum error of less than 6 per cent for harmonic generation up to fifth order. This method is also applied to the prediction of grid currents and is found to be more accurate than previous analytical methods.     

Generalized Sommerfeld integrals and field expansions in two-medium half-spaces

Time harmonic modal electromagnetic fields in two-medium half-spaces are investigated. For practical and numerical considerations, the primary sources of the modal fields are chosen to be the spherical multipoles, and the potential vectors are z-directed. It is shown that modal fields of such combination are not able to represent a conventional spherical modal field. The horizontal rotating potentials are added to ensure proper representation and fast convergence. The recurrence relations which transform the spherical Hankel-Legendre functions into the Fourier-Bessel integrals are derived. The secondary fields of the Sommerfeld type are obtained for all spherical multipole sources and the added horizontally rotating potentials. The combination of the modal fields are capable of representing arbitrary electromagnetic fields resulting from radiation and scattering problems.     

An Effective NF-FF Transformation Technique With Planar Spiral Scanning Tailored For Quasi-Planar Antennas

An effective probe compensated near-field—far-field transformation technique with planar spiral scanning is developed in this paper. It makes use of an ellipsoidal modelling of the source, instead of the previously adopted spherical one. Such a modelling, tailored for quasi-planar antennas, allows one to consider measurement planes at a distance smaller than one half the antenna maximum size, thus reducing the error related to the truncation of the scanning surface. Moreover, it is quite general, containing the spherical modelling as particular case, and reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.      

Study of Mutual Coupling Between Circular Stacked-Patch Antennas on a Sphere

A rigorous mathematical analysis is given of spherical stacked-patch arrays with emphasis on the physical interpretation of mutual coupling mechanisms present in doubly- curved convex structures. The analysis method is based on electromagnetic field representation in terms of spherical harmonics where each harmonic has the same angular variation as the spectral source component. To obtain the spectral representation the vector-Legendre transformation is applied to currents and fields. A novel approach to the mutual coupling calculation within the method of moments analysis of spherical arrays is applied. By expressing the patch current in terms of two suitable potential-like auxiliary functions, it is possible to avoid the use of Euler's formulas for coordinate system rotation and the related lengthy integrations. Instead, the rotation of antenna elements and corresponding current distributions can be done in closed form with the help of Vilenkin's addition theorem for associated Legendre functions. It is shown that the new approach results in significant acceleration and improved accuracy of the analysis of spherical patch antenna arrays. The algorithm is successfully tested against a commercially available electromagnetic software and measurements performed on the developed laboratory model, confirming its accuracy for both input impedance and mutual coupling calculation and with only a small difference between the predicted and measured resonant frequencies, due to limitations in the experimental model. The influence of the structure parameters on mutual coupling level is extensively investigated, including all coupling mechanisms and leakage of energy due to curvature of the structure. It is shown that stacked-patch antennas can have reduced coupling level comparing to single patch antennas with possible deep nulls above the antenna resonant frequency.      

时谐场和复数场的对应研究

电磁时谐场的复数表示理论,其基础在于对应,即电磁一次量的实部(或虚部)对应时谐场量.这种方法简单明了,因此获得了广泛的应用.然而必须提出,电磁二次量的复数表示并不能从能量定理的普遍形式导出[6],因此不少文献[3-5]均避免给出复Poynting定理.本文提出了时谐场二次量的复数表示的对应定理,并探讨其中各项的相关意义,从而使复Poynting定理,Foster定理和Lorentz定理有了坚实的基础.     

3D model representation using spherical harmonics

The authors introduce a method for decomposing a 3D model into spherical harmonic functions for later reconstruction. It is shown that spherical harmonic representations of models are efficient as the amount of data is reduced significantly. This method can be incorporated within model based coding (MBC) applications resulting in a performance increase     

Spherical harmonics and Earth-rotation synthesis in radio astronomy

That spherical harmonic functionsY_{nm}(theta,phi)can be used to advantage in the Earth-rotation synthesis of radio astronomy maps is shown in this paper. As Earth rotates the baseline of a radio interferometer generates a cone whose angletheta, measured from theN-Spolar axis, can be varied by changing the baseline's azimuthal direction on the surface of Earth. A series of Earth-rotation measurements, at different cone angles but with baselines of equal lengthb, can be regarded as being made on a baseline sphere of radiusb, the analog of theuvplane in Fourier-type synthesis. The measured output distribution can be expanded as a spherical harmonic series on the baseline sphere. The coefficients of the series are related to the coefficients of the spherical harmonic series expansion of the source distribution on the celestial sphere by a matrix transformation. The matrix [B] is a function only of the baseline configuration (it does not vary with source declination). Inversion of the matrix leads to the solution for the source coefficients, from which a spherical harmonic map is formed of the source distribution.     

A high-resolution technique for ultrasound harmonic imaging using sparse representations in Gabor frames

Over the last few decades there were dramatic improvements in ultrasound imaging quality with the utilization of harmonic frequencies induced by both tissue and echo-contrast agents. The advantages of harmonic imaging cause rapid penetration of this modality to diverse clinical uses, among which myocardial perfusion determination seems to be the most important application. In order to effectively employ the information, comprised in the higher harmonics of the received signals, this information should be properly extracted. A commonly used method of harmonics separation is linear filtering. One of its main shortcomings is the inverse relationship between the detectability of the contrast agent and the axial resolution. In this paper, a novel, nonlinear technique is proposed for separating the harmonic components, contained in the received radio-frequency images. It is demonstrated that the harmonic separation can be efficiently performed by means of convex optimization. It performs the separation without affecting the image resolution. The procedure is based on the concepts of sparse signal representation in overcomplete signal bases. A special type of the sparse signal representation, that is especially suitable for the problem at hand, is explicitly described. The ability of the novel technique to acquire "un-masked," second (or higher) harmonic images is demonstrated in series of computer and phantom experiments.     

垂直电偶极子在地-电离层波导中场的球级数解

在非理想导电地面与电离层条件下,我们导出了ELF/SLF垂直电偶极子在球形地-电离层壳体中产生的电磁场的球谐级数表达式,采用一种加速收敛算法,算出了波导中的电磁场分布.根据计算结果,在SLF频段,地面与电离层之间的电磁场可理解为二个"行波"的叠加,且与SLF频段的球面二阶近似算法计算结果吻合很好.在ELF频段,壳体中的电磁场是驻波,其频率变化规律能正确反映出"舒曼"谐振现象.     

Weighted fourier series representation and its application to quantifying the amount of gray matter

We present a novel weighted Fourier series (WFS) representation for cortical surfaces. The WFS representation is a data smoothing technique that provides the explicit smooth functional estimation of unknown cortical boundary as a linear combination of basis functions. The basic properties of the representation are investigated in connection with a self-adjoint partial differential equation and the traditional spherical harmonic (SPHARM) representation. To reduce steep computational requirements, a new iterative residual fitting (IRF) algorithm is developed. Its computational and numerical implementation issues are discussed in detail. The computer codes are also available at http://www.stat.wisc.edu/-mchung/softwares/weighted.SPHARM/weighted-SPHARM.html. As an illustration, the WFS is applied i n quantifying the amount ofgray matter in a group of high functioning autistic subjects. Within the WFS framework, cortical thickness and gray matter density are computed and compared.     

A novel surface registration algorithm with biomedical modeling applications.

In this paper, we propose a novel surface matching algorithm for arbitrarily shaped but simply connected 3-D objects. The spherical harmonic (SPHARM) method is used to describe these 3-D objects, and a novel surface registration approach is presented. The proposed technique is applied to various applications of medical image analysis. The results are compared with those using the traditional method, in which the first-order ellipsoid is used for establishing surface correspondence and aligning objects. In these applications, our surface alignment method is demonstrated to be more accurate and flexible than the traditional approach. This is due in large part to the fact that a new surface parameterization is generated by a shortcut that employs a useful rotational property of spherical harmonic basis functions for a fast implementation. In order to achieve a suitable computational speed for practical applications, we propose a fast alignment algorithm that improves computational complexity of the new surface registration method from O(n3) to O(n2).     

Linearized optical modulator with fifth order correction

A linearized solution for a series cascade of three Mach Zehnder interferometers separated by directional couplers is obtained that eliminates both the third and fifth order coefficients of the Taylor series representation of the transfer function. The device is characterized in terms of harmonic distortion with a three tone signal. ~20-dB improvement in third harmonic distortion is obtained compared to a two section device     

超远距离目标的交叉定位算法研究

从球面几何的角度入手,提出了一种高精度球面三角定位算法。该算法运算量小,定位速度快,尤其适合于超远距离地面目标的定位。分析表明,目标距离比较远时,球面三角定位要比平面交叉定位的定位误差小得多。目标距离一定时,要使圆概率误差半径最小,侦察站和目标须成等边三角形配置。     

2D-TDOA passive location based on geodetic longitude and latitude

Because of the surface of the earth is not a plane,there may be some errors in the conventional ultra-short wave passive TDOA location method using plane coordinates.In order to use the geodetic longitude and latitude coordinates directly for TDOA location,a series of spherical distance TDOA location models based on spherical two-point distance calculation were proposed.The optimal location models were adopted,and a grid point search method was applied with model validation.The numerical results show that the spherical precise calculation model has the smallest positioning error,which is independent of latitude.Positioning error of the spherical approximation calculation model is worse than the first model,and increases with the increase of latitude.The equidistant orthographic cylindrical projection model has a small positioning error in the low latitudes.However,it has a large positioning error in the middle and high latitudes.The equiangular orthographic cylindrical projection model and the equiareal orthographic cylindrical projection model have a relatively large positioning error,which is around 26% at any latitude and is not applicable.     

Approximate Error Rate Performance of 8-Point AM-PM Signal Constellations on Voiceband Telephone Channels

The purpose of this paper is a detailed comparison of the error rate performance of six 8-point signal constellations under the conditions of Gaussian noise alone, or combined with either phase jitter, second order harmonic, or third order harmonic distortion for application to the voiceband telephone channel. An approximate evaluation technique is developed for inclusion of the effects of second and third order harmonic distortion which was found to be in agreement with laboratory performance measurements of several of the signal constellations reported here. A signal constellation, which consists of points in a circle surrounding a single point in the center (denoted the 1-7 constellation), was found to be superior to the other 5 constellations considered under the conditions of Gaussian noise combined with second or third order harmonic distortion. Under the condition of phase jitter not exceeding 1.5 degrees RMS, it is also superior to the other constellations. Thus, it appears that the 1-7 constellation in conjunction with a good phase locked loop would offer superior performance.     

Novel calibration method for camera array in spherical arrangement

A camera array placed in a spherical arrangement facilitates a new paradigm for future interactive visual applications, and the calibration of the cameras is a crucial task for such implementation. It is difficult to apply previous calibration methods to this special camera system because opposite features can commonly be found among the opposing cameras used in this system. In this case, a part-by-part calibration may fail in terms of error accumulation. In this paper, we propose an opposite feature based camera calibration method for a camera array in a spherical arrangement. An adaptive scheme is designed on a cube with opposite colour patterns and optimisations in handling the above problem of opposite features being invisible to cameras at opposite positions, based upon which, with our method, a common visible feature point is not required in advance. By applying a practical camera array in a spherical arrangement, and using 3D modelling, the superiority of the proposed method was verified through computer simulations. The results demonstrate that the proposed calibration method can contribute to the imaging system and future visual applications.     

谐波分析方法在提高精密转台回转精度中的应用

为了提高某精密转台的测角精度和改进结构设计,对其方位轴系角位置测量精度进行了检测,该误差包括轴系误差、编码器误差及其他误差项。在数据处理中,采用傅里叶谐波分析方法,用不同阶次的简谐波进行拟合,去掉低阶误差,可提高位置测量精度,同时分析了各误差的来源。建立的误差查询表格可用于对后期测量结果的修正。结果证明,角位置测量误差修正后的值是修正前的1/4左右,幅值不超过0.8,极大地提高了角位置测量精度。此实验研究了使用谐波分析来提高转台位置测量精度的方法;同时分析了各误差来源,便于采取相应措施对轴系进行改进。     

新型三相三线制并联有源滤波器的滑模控制方法

为了提高有源滤波器的谐波补偿效果,设计了一种新型滑模控制器,用于三相三线制并联有源滤波器的参考电流跟踪控制.谐波电流检测方法采用基于瞬时无功功率理论的谐波电流检测方法,能快速、准确的检测出负载电流中的谐波分量.直流侧电压控制方法采用PI控制方法实现.Simulink仿真结果显示,与传统的滞环比较控制方法相比,所设计的新型滑模控制方法能够有效的降低跟踪误差,提高有源滤波器的谐波补偿效果.