A Weighted Inner Product Estimator in the Geometric Algebra of Euclidean 3-Space for Source Localization Using an EM Vector-sensor

In this paper, the source localization by utilizing the measurements of a single electromagnetic (EM) vector-sensor is investigated in the framework of the geometric algebra of Euclidean 3-space. In order to describe the orthogonality among the electric and magnetic measurements, two multivectors of the geometric algebra of Euclidean 3-space (G3) are used to model the outputs of a spatially collocated EM vector-sensor. Two estimators for the wave propagation vector estimation are then formulated by the inner product between a vector and a bivector in the G3. Since the information used by the two estimators is different, a weighted inner product estimator is then proposed to fuse the two estimators together in the sense of the minimum mean square error (MMSE). Analytical results show that the statistical performances of the weighted inner product estimator are always better than its traditional cross product counterpart. The efficacy of the weighted inner product estimator and the correctness of the analytical predictions are demonstrated by simulation results.     

DOA estimation and mutual coupling calibration with the SAGE algorithm

In this paper, a novel algorithm is presented for direction of arrival（DOA） estimation and array self-calibration in the presence of unknown mutual coupling. In order to highlight the relationship between the array output and mutual coupling coefficients, we present a novel model of the array output with the unknown mutual coupling coefficients. Based on this model, we use the space alternating generalized expectation-maximization（SAGE） algorithm to jointly estimate the DOA parameters and the mutual coupling coefficients. Unlike many existing counterparts, our method requires neither calibration sources nor initial calibration information. At the same time,our proposed method inherits the characteristics of good convergence and high estimation precision of the SAGE algorithm. By numerical experiments we demonstrate that our proposed method outperforms the existing method for DOA estimation and mutual coupling calibration.     

A nonlinear equivalent circuit method for analysis of passive intermodulation of mesh reflectors

Passive intermodulation(PIM) has gradually become a serious electromagnetic interference due to the development of high-power and high-sensitivity RF/microwave communication systems, especially large deployable mesh reflector antennas. This paper proposes a field-circuit coupling method to analyze the PIM level of mesh reflectors. With the existence of many metal–metal(MM) contacts in mesh reflectors, the contact nonlinearity becomes the main reason for PIM generation. To analyze these potential PIM sources, an equivalent circuit model including nonlinear components is constructed to model a single MM contact so that the transient current through the MM contact point induced by incident electromagnetic waves can be calculated. Taking the electric current as a new electromagnetic wave source, the far-field scattering can be obtained by the use of electromagnetic numerical methods or the communication link method. Finally, a comparison between simulation and experimental results is illustrated to verify the validity of the proposed method.     

基于几何特征的飞行器RCS高效计算方法(英文)

Taking into account the influences of scatterer geometrical shapes on induced currents, an algorithm, termed the sparse-matrix method （SMM）, is proposed to calculate radar cross section （RCS） of aircraft configuration. Based on the geometrical characteristics and the method of moment （MOM）, the SMM points out that the strong current coupling zone could be predefined according to the shape of scatterers. Two geometrical parameters, the surface curvature and the electrical space between the field position and source position, are deducted to distinguish the dominant current coupling. Then the strong current coupling is computed to construct an impedance matrix having sparse nature, which is solved to compute RCS. The efficiency and feasibility of the SMM are demonstrated by computing electromagnetic scattering of some kinds of shapes such as a cone-sphere with a gap, a bi-arc column and a stealth aircraft configuration. The numerical results show that： （1） the accuracy of SMM is satisfied, as compared with MOM, and the computational time it spends is only about 8% of the MOM; （2） with the electrical space considered, making another allowance for the surface curvature can reduce the computation time by 9.5%.     

A closed-form solution for moving source localization using LBI changing rate of phase difference only

Due to the deficiencies in the conventional multiple-receiver localization systems based on direction of arrival(DOA) such as system complexity of interferometer or array and amplitude/phase unbalance between multiple receiving channels and constraint on antenna configuration,a new radiated source localization method using the changing rate of phase difference(CRPD)measured by a long baseline interferometer(LBI) only is studied. To solve the strictly nonlinear problem, a two-stage closed-form solution is proposed. In the first stage, the DOA and its changing rate are estimated from the CRPD of each observer by the pseudolinear least square(PLS) method,and then in the second stage, the source position and velocity are found by another PLS minimization. The bias of the algorithm caused by the correlation between the measurement matrix and the noise in the second stage is analyzed. To reduce this bias, an instrumental variable(IV) method is derived. A weighted IV estimator is given in order to reduce the estimation variance. The proposed method does not need any initial guess and the computation is small. The Cramer–Rao lower bound(CRLB) and mean square error(MSE) are also analyzed. Simulation results show that the proposed method can be close to the CRLB with moderate Gaussian measurement noise.     

Nonuniform Three-dimensional Configuration Distributed SAR Signal Reconstruction Clutter Suppression

This paper deals with the problem of clutter suppression in spaceborne distributed synthetic aperture radar (D-SAR) with nonuniform three-dimensional (3D) configuration geometry. In order to make a breakthrough of the configuration limitation of the traditional space time adaptive processing (STAP) based on uniform array and improve the inhomogeneous clutter suppression performance, this paper considers signal reconstruction technique using array interpolation to process the D-SAR signal. An array interpolation signal reconstruction method based on pitching-partition is derived then a signal reconstruction 3D-STAP clutter suppression method applied to nonuniform 3D configuration is proposed. In particular, the proposed method is compared with conventional methods and the performance analysis is carried out based on simulations. The improvement factor (IF) for clutter suppression is imported and reported as a benchmark on the clutter suppression effect.     

Singular formalism and admissible control of spacecraft with rotating flexible solar array

This paper is concerned with the attitude control of a three-axis-stabilized spacecraft which consists of a central rigid body and a flexible sun-tracking solar array driven by a solar array drive assembly. Based on the linearization of the dynamics of the spacecraft and the modal identi- ties about the flexible and rigid coupling matrices, the spacecraft attitude dynamics is reduced to a formally singular system with periodically varying parameters, which is quite different from a space- craft with fixed appendages. In the framework of the singular control theory, the regularity and impulse-freeness of the singular system is analyzed and then admissible attitude controllers are designed by Lyapunov＇s method. To improve the robustness against system uncertainties, an H∞ optimal control is designed by optimizing the H∞ norm of the system transfer function matrix. Comparative numerical experiments are performed to verify the theoretical results.     

A Linear-correction Least-squares Approach for Geolocation Using FDOA Measurements Only

  A linear-correction least-squares(LCLS) estimation procedure is proposed for geolocation using frequency difference of arrival (FDOA) measurements only. We first analyze the measurements of FDOA, and further derive the Cram閞-Rao lower bound (CRLB) of geolocation using FDOA measurements. For the localization model is a nonlinear least squares(LS) estimator with a nonlinear constrained, a linearizing method is used to convert the model to a linear least squares estimator with a nonlinear constrained. The Gauss-Newton iteration method is developed to conquer the source localization problem. From the analysis of solving Lagrange multiplier, the algorithm is a generalization of linear-correction least squares estimation procedure under the condition of geolocation using FDOA measurements only. The algorithm is compared with common least squares estimation. Comparisons of their estimation accuracy and the CRLB are made, and the proposed method attains the CRLB. Simulation results are included to corroborate the theoretical development.     

Bernoulli particle flter with observer altitude for maritime radiation source tracking in the presence of measurement uncertainty

For maritime radiation source target tracking in particular electronic counter measures（ECM）environment,there exists two main problems which can deteriorate the tracking performance of traditional approaches.The frst problem is the poor observability of the radiation source.The second one is the measurement uncertainty which includes the uncertainty of the target appearing/disappearing and the detection uncertainty（false and missed detections）.A novel approach is proposed in this paper for tracking maritime radiation source in the presence of measurement uncertainty.To solve the poor observability of maritime radiation source target,using the radiation source motion restriction,the observer altitude information is incorporated into the bearings-only tracking（BOT）method to obtain the unique target localization.Then the two uncertainties in the ECM environment are modeled by the random fnite set（RFS）theory and the Bernoulli fltering method with the observer altitude is adopted to solve the tracking problem of maritime radiation source in such context.Simulation experiments verify the validity of the proposed approach for tracking maritime radiation source,and also demonstrate the superiority of the method compared with the traditional integrated probabilistic data association（IPDA）method.The tracking performance under different conditions,particularly those involving different duration of radiation source opening and switching-off,indicates that the method to solve our problem is robust and effective.     

A novel method for multi-angle SAR image matching

Multi-angle synthetic aperture radar(SAR) image matching is very challenging, because the same object may cause different backscattering patterns, heavily depending on the radar incident angle. A technique based on the relations between the invariant positions of ground targets among the reference and sensed images is proposed to accommodate the nonmatching patterns. It involves a target extraction using wavelet coefficient fusion, as well as a geometric voting matching routine for searching the matched control points(CPs) in the reference and sensed images, respectively. To accelerate the speed of the search, a robust, rapidly corresponding CPs determination strategy is exploited by utilizing the global spatial transformation model, as well as a procedure of outlier removal to ensure the desired accuracy. Meanwhile, the positions of the matched point pairs are relocated using mutual information. The final warping of the images according to the CPs is performed by using a polynomial function. The results show the possibility of matching multi-angle SAR images in general cases.     

CRB: Sinusoid-Sources' Estimation using Collocated Dipoles/Loops

This work derives new asymptotic Cram?r-Rao lower bounds (CRB) for the estimation of multiple pure-tone incident signals' azimuth-elevation arrival-angles, polarization parameters, frequencies, amplitudes, and temporal phases—based on data collected by spatially collocated but orthogonally oriented dipoles and/or loops. The incident sources are pure-tones at distinct, deterministic but unknown frequencies, in contrast to the case of all incident sources at one common known frequency, as has been investigated in the existing research literature on the CRB for diversely-polarized direction-finding. The derived CRBs are closed-form expressions, explicitly in terms of the signal parameters. The new CRBs presented here reveal how a constituent dipole and/or loop's presence and orientation may affect estimation precision, thereby offering guidelines to the system engineer on what dipole(s) and/or loop(s) to include or to omit in constructing the electromagnetic vector-sensor.     

Wideband array antenna concept

This presents a unique wideband array concept that includes a wideband array aperture incorporating radiating elements of different sizes and inter-element spacings. The array is a fully-filled aperture consisting of a core region surrounded by several layers of radiating elements which increase in size. By proper selection of the core region and the number of layers, one can form a beam with specified beamwidth over a five to one frequency range. The proposed concept considerably reduces the cost and complexity by decreasing the effective number of radiating elements and the corresponding receive or transmit modules. Numerical simulation results, ignoring the mutual coupling effects, are presented for array patterns at several frequencies to prove the concept. A possible beamforming concept is also included. Additional work is planned to prove the viability of the concept by taking the mutual coupling into account.     

线性极化敏感阵列的极化平滑算法及相干源参数估计

 研究了基于极化敏感阵列的相干信号参数估计问题。首先,提出一种利用均匀线性极化敏感阵列的新型极化平滑算法(ULAPSA),其能够处理更多相干信号的参数估计,且具有更小的计算量。然后,基于ULAPSA给出了波达方向（DOA）和极化参数估计的两种方法：第1种方法利用角度搜索得到信号的DOA估计,适应任何多分量的电磁矢量传感器阵列;第2种方法结合传播算子算法,无需奇异值分解和角度搜索,能够同时估计信号的DOA和极化参数。该方法适应完备的电磁矢量传感器、三偶极子或三磁环构成的矢量传感器。最后,通过仿真实验比较和分析了所提算法的性能。     

Experiments with a Low Polarization Error Loop Array Direction Finder

A previously reported HF loop antenna array for multipolarization direction finding consisting of two spaced loops is described. The method is compatible with twin channel Watson-Watt operation as an Adcock so that the direction finding process is independent of polarization characteristics of the element pattern. Experimentally measured direction finder (DF) bearings were compared with near simultaneous polarimeter data and conventional crossed loops bearings. A major reduction in polarization error was obtained at the expense of full 360° azimuth response. It is theoretically possible to cover 360° with two similar arrays of the type investigated or with crossed loops elements with quadrature phase shifters.     

口径耦合微带天线互耦时域有限差分法分析

在阵列天线中,互耦是阵列性能下降的一个重要因素,对互耦进行正确的评估、计算,有助于深入了解阵列特性并指导设计工作.采用时域有限差分法计算了口径耦合微带天线互耦系数,天线建模中引入非均匀网格和电阻性电压源,微带线与馈源用阶梯过渡来连接,在保证计算精度的前提下,计算空间和计算机时间大大减少,计算结果与矩量法及测量结果基本一致.     

活性碳毡对称振子阵列／树脂复合吸波材料的研究

研究了含活性碳毡对称振子阵列吸波复合材料的微波吸收特性.结果表明:含振子阵列吸波材料的吸波性能与入射电场方向、振子两臂间的电阻、振子间的距离和阵列的位置密切相关.对称振子阵列对电磁波的吸收呈各向异性,振子与入射电场平行时,吸波性能较好;随振子两臂间电阻或振子间距的增大,材料的吸波性能先提高后降低,电阻和振子间距均存在最佳值;随振子阵列和试样下表面距离的减小,材料的吸波性能提高,振子与入射电场平行时,本实验条件下可获得有效带宽13GHz,最大吸收峰值-30.3dB的反射衰减.     

Mutual coupling effects and their reduction in wideband directionof arrival estimation

The performance of multiple signal classification (MUSIC) algorithm using three different inputs over a wideband of frequencies is considered. These inputs are: 1) ideal voltages, 2) actual voltages which include coupling effects and are obtained with the method or moments, and 3) corrected voltages which are obtained from the actual voltages so that the mutual coupling effects are removed. Linear arrays of dipoles, sleeve dipoles, and spiral antennas are considered over 200 MHz to 400 MHz band     

Resolution and synthetic aperture characterization of sparse radar arrays

The concept of radar satellite constellations, or clusters, for synthetic aperture radar (SAR), moving target indicator (MTI), and other radar modes has been proposed and is currently under research. These constellations form an array that is sparsely populated and irregularly spaced; therefore, traditional matched filtering is inadequate for dealing with the constellation's radiation pattern. To aid in the design, analysis, and signal processing of radar satellite constellations and sparse arrays in general, the characterization of the resolution and ambiguity functions of such systems is investigated. We project the radar's received phase history versus five sensor parameters: time, frequency, and three-dimensional position, into a phase history in terms of two eigensensors that can be interpreted as the dimensions of a two-dimensional synthetic aperture. Then, the synthetic aperture expression is used to derive resolution and the ambiguity function. Simulations are presented to verify the theory.