Evaluation of adaptive phased array antenna, far-field nullingperformance in the near-field region

A theory for analyzing the behavior of adaptive phased array antennas illuminated by a near-field interference test source is presented. Conventional phased array near-field focusing is used to produce an equivalent far-field antenna pattern at a range distance of one to two aperture diameters from the adaptive antenna under test. The antenna is assumed to be a linear array of isotropic receive elements. The interferer is assumed to be a bandlimited noise source radiating from an isotropic antenna. The theory is developed for both partially and fully adaptive arrays. Results are presented for the fully adaptive array case with single and multiple interferers. The results indicate that near-field and far-field adaptive nulling can be equivalent. The adaptive nulling characteristics studied in detail are the array radiation patterns, adaptive cancellation, covariance matrix eigenvalues, and adaptive array weights     

The constant modulus array for cochannel signal copy and directionfinding

The constant modulus (CM) array is a blind adaptive beamformer capable of recovering a narrowband signal among several cochannel sources without using a pilot or training signal. It is a conventional weight-and-sum adaptive beamformer whose weights are updated by the constant modulus algorithm. An adaptive signal canceller follows the beamformer to remove the captured signal from the array input and to provide an estimate of its direction vector. Based on a Wiener model, we investigate the steady-state properties of the CM array and the signal canceller. For mutually uncorrelated sources and noise, it is shown that the signal canceller exactly removes the source captured by the array. Thus, identical stages of the CM array and signal canceller may be used in a multistage system to recover several cochannel sources. Computer simulations are presented to verify the analytical results and to illustrate the transient behavior of the system     

米波稀布阵雷达自适应旁瓣对消性能分析

详述了米波稀布阵雷达自适应旁瓣对消工作原理,米波稀布阵综合脉冲孔径雷达(SIAR)是一种新型米波分布阵体制雷达,采用稀布阵列天线,通过各个阵元全向发射正交编码频率信号以使各向同性照射,在接收端通过DBF和发射脉冲综合形成接收和发射波束。采用最小均方准则(LMS)对米波稀布阵雷达的抗干扰性能作了仿真,根据试验实测数据进行分析,取得了良好的干扰抑制效果,并分析比较了点频单个干扰源和多个干扰源对消处理效果,具有较强的工程实用价值。     

Analysis of phase-focused near-field testing for multiphase-centeradaptive radar systems

Airborne or spaceborne radar systems often require tests before deployment to verify how well the system detects targets and suppresses clutter and jammer signals. The radar antenna diameter can be large and thus the conventional far-field test distance is impractical to implement. The theory and simulations of phase-focused near-field testing for adaptive phased array antennas is discussed. With near-field source deployment, standard phased-array near-field phase focusing provides far-field adaptive nulling equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. Both main beam clutter sources and sidelobe jammer sources are addressed. The phased array antenna elements analyzed are one-half wavelength dipoles over the ground plane. Bandwidth, polarization, array mutual coupling, and finite array edge effects are taken into account. Numerical simulations of an adaptive antenna that has multiple displaced phase centers indicate that near-field and far-field testing can be equivalent     

单元方向可控自适应圆阵的理论研究

本文采用一种振子型合成元作为阵元组成圆阵天线,通过一组加权,可控制阵中所有的元方向图型都指向预先任意规定的相控方向。这在全向范围内,任何方向都能形成强定向波束,以有效地获取有用信号。在此基础上,本文通过引入等效干扰源的概念,把Gabriel的各向同性元自适应直线阵理论扩展应用到有向元圆阵,对单元方向可控自适应圆阵专题进行了研究。它包括:阵远区辐射图型;单元方向可控圆阵系统输入协方差矩阵的导出;单元方向可控自适应圆阵辐射图型正交分解;阵系统输出噪声瞬态特性公式。     

An adaptive tracking algorithm for direction finding and array shape estimation in a nonstationary environment

This paper presents an adaptive tracking algorithm for the subspace-based direction of arrival estimation of multiple sources in a nonstationary, environment. The nonstationarities are due to moving sources or to timevarying distortions of the sensor array shape. The proposed algorithm relies on the properties of a linear operator, referred to as the Propagator, which only exploits the linear independency of the source steering vectors. The Propagator allows not only the calibration of the array shape, but also the determination of the source and the noise subspaces without any eigendecomposition of the cross-spectral matrix of the received signals. A gradientbased adaptive algorithm is here proposed for the on-line estimation of the Propagator. A theoretical analysis of the behaviour of this algorithm in a nonstationary environment is given. Simulations are carried out in the case of moving sources and in the case of a time-varying array shape. They exhibit the good performances of the proposed algorithm.     

Maximizing jammer effectiveness for evaluating the performance of adaptive nulling array antennas

A theory is developed which is used to find interference source distributions which maximize consumption of the degrees of freedom forN-channel adaptive nulling arrays with arbitrary element positions. For a given number of interference sources, after proper positioning, these sources represent a maximally stressed environment for the adaptive array degrees of freedom. The interference covariance matrix eigenvalues are shown to have a direct bearing on the number of degrees of freedom consumed as well on the adaptive cancellation. Numerical examples are given showing that certain source geometries produce the situation where little or no adaptive cancellation is possible due to the available degrees of freedom being severely taxed.     

Optimal radiation pattern of an array in the presence of two directional interferences

In the presence of two directional interferences, the radiation pattern of an adaptive array can be expressed in terms of the powers of the sources and their directions of arrival. This is used to illustrate the performance and operation of adaptive arrays in the case of multiple jammers. In particular, the case of main-beam jamming is examined.     

Robust adaptive beamforming for general-rank signal models

The performance of adaptive beamforming methods is known to degrade severely in the presence of even small mismatches between the actual and presumed array responses to the desired signal. Such mismatches may frequently occur in practical situations because of violation of underlying assumptions on the environment, sources, or sensor array. This is especially true when the desired signal components are present in the beamformer "training" data snapshots because in this case, the adaptive array performance is very sensitive to array and model imperfections. The similar phenomenon of performance degradation can occur even when the array response to the desired signal is known exactly, but the training sample size is small. We propose a new powerful approach to robust adaptive beamforming in the presence of unknown arbitrary-type mismatches of the desired signal array response. Our approach is developed for the most general case of an arbitrary dimension of the desired signal subspace and is applicable to both the rank-one (point source) and higher rank (scattered source/fluctuating wavefront) desired signal models. The proposed robust adaptive beamformers are based on explicit modeling of uncertainties in the desired signal array response and data covariance matrix as well as worst-case performance optimization. Simple closed-form solutions to the considered robust adaptive beamforming problems are derived. Our new beamformers have a computational complexity comparable with that of the traditional adaptive beamforming algorithms, while, at the same time, offer a significantly improved robustness and faster convergence rates.     

An adaptive algorithm for separating and tracking multipledirectional sources in linear arrays

An algorithm for spatially filtering out, enhancing, and tracking individual directional sources in an adaptive array is proposed and investigated. In this algorithm, the sources are separated by using an adaptive beamformer whose outputs are processed by using the LMS algorithm to track distinct sources individually. From the LMS weights used, the source locations can be estimated. Whenever significant changes in these are detected, the beamformer is updated so that its outputs will be due to different sources in the steady state. With this algorithm, the problems of look-direction errors in look-direction constrained arrays and of large signal power in power inversion arrays are eliminated, and the enhancement of multiple moving sources becomes a natural process. Furthermore, because the sources are individually tracked and the beamformer is only updated occasionally, the algorithm possesses fast tracking behavior, and its implementation complexity is comparable to that of beamformer-based adaptive arrays using the LMS algorithm     

Steerable ELF/VLF radiation produced by an array of ionospheric dipoles generated from HF heating

A very low frequency (VLF) or extremely low frequency (ELF) dipole source has been created within the lower ionosphere by modulating the atmospheric dynamo currents with a ground-based high power HF source from the Arecibo Observatory. The authors and their colleagues have demonstrated that ELF or VLF generated in this way and injected into the earth-ionosphere waveguide could be received a few thousand kilometers away. The injection properties due to an array of ionospheric dipoles as a function of array geometry and element currents that will allow steerable ELF/VLF radiation within the earth-ionosphere waveguide are investigated theoretically. The ionospheric array factors for a linear and a planar array of Hertzian dipole sources are developed and their properties examined. The principle of pattern multiplication is then applied to include the effect of the ionospheric array element. This provides a means for predicting the field strengths at a remote receiving site due to a steerable linear or planar array of ionospheric sources generated by high power HF periodic plasma heating.     

Spectral analysis and adaptive array superresolution techniques

Recent nonlinear "superresolution" techniques reported in the field of spectral analysis are of great interest in other fields as well, including radio-frequency (RF) adaptive array antenna systems. This paper is primarily a "cross-fertilization" treatise which takes the two most popular nonlinear techniques, the Burg maximum entropy method and the maximum likelihood method, and relates them to their similar nonlinear adaptive array antenna counterparts, which consist of the generic sidelobe canceller and directional gain constraint techniques. The comparison analysis permits an examination of their principles of operation from the antenna spatial pattern viewpoint, and helps to qualify, their actual superresolution performance. A summary of the resolution performance of several adaptive algorithms against multiple-incoherent sources is provided, including a universal graph of signal-to-noise ratio (SNR) versus source separation in beamwidths for the case of two equal-strength sources. Also, a significant dividend in the easy resolution of unequal-strength sources is reported. The superresolution of coherent spatial sources or radar targets is more difficult for these techniques, but successful results have been obtained whenever sufficient relative motion or "Doppler cycles" are available. Two alternate adaptive spatial spectrum estimators are suggested, consisting of a circular array predicting to its center point, and a new "thermal noise" algorithm.     

Steady-state analysis of the multistage constant modulus array

The multistage constant modulus (CM) array is a cascade adaptive beamforming system that can recover several narrowband cochannel signals without training. We examine its steady-state properties at convergence using a stochastic analysis and computer simulations. Based on a Wiener model of convergence for the gradient adaptive algorithms, closed-form expressions are derived for the CM array and canceller weight vectors, as well as the effective source direction vectors at all stages along the cascade system. The signal-capture and direction-finding capabilities of the system are also discussed. Computer simulations for stationary and fading sources are presented to confirm the theoretical results and to illustrate the rapid convergence behavior of the adaptive algorithms     

An algorithm for multisource beamforming and multitarget tracking

A new algorithm for simultaneous robust multisource beamforming and adaptive multitarget tracking is proposed. Self-robustness to locations errors or variations is introduced by a source-subspace-based tracking procedure of steering vectors in the array manifold. This LMS-type procedure is generalized from a former work we developed in the single source case. Two beamforming structures are actually proposed. The first is adaptive and optimal for uncorrelated sources and correlated noise. The second is conventional and optimal for correlated sources and uncorrelated white noise. The proposed algorithm and MUSIC show an identical asymptotic variance in localization for immobile sources, whereas for the mobile case, the proposed algorithm is highly advantageous. Then, it is shown that the additional use of some kinematic parameters (i.e., speed, acceleration, etc.) inferred from the reconstructed trajectories improves the tracking performance and overcomes some of the problems of crossing targets. The efficiency of multitarget tracking and the robustness of multisource beamforming are proved and then confirmed by simulation. The number of sources can be initialized and tracked by a marginal proposed procedure. The beamforming performance is shown to be optimal as the single source case. Finally, the algorithm has a very low order of arithmetic complexity     

Noise emissions sources direction-finding in the process of their background air threats detection in radars with phased antenna array

We analyze two options of combined systems for spatial signal processing in radars with phased antenna array, where the detection of noiseless point targets with background noise emissions is accompanied by the direction finding of their sources. In the first option the direction finding of noise emissions is based on the shape analysis of the adaptive radiation pattern of the phased antenna array, formed during the adaptive target finding with background noise emissions. In the second option the bearing angles are determined on the basis of maxima of “spectral functions” of different evaluations of correlation matrix, formed on the basis of input readings. An example how to build such a combined system for spatial signal processing based on general adaptive grid filter is presented. It is shown that the effect of simultaneous target finding in external noise background and direction finding of their sources is achieved by single utilization of the most complex tuning operation of the adaptive grid filter. This operation is same for both of these tasks, and it is easier comparing to solving these two problems separately.     

Spatial spectral estimation using multiple beam antennas

The processing simplifications which result in using a multiple beam antenna (MBA) as a spatial sensor for performing spectral estimation are considered. Sources are presumed to be located over a two-dimensional field of view characterized by the two angular coordinatesthetaandphi. The MBA configuration consists of an aperture (usually either a reflector or lens) illuminated by a collection of feeds located in its focal plane (see Fig. 1), followed by a switch network for selecting the outputs of any desired feed port. Using the MBA as the spatial sensor for performing spectral estimation, as contrasted to the array antenna configuration, has a distinct advantage: for a given collection of source wavefronts incident on the aperture, a crude estimate of each source position is obtained simply by monitoring the power output of each feed port. This is to be contrasted to the array configuration, where the average output power of each element port is the same, so long as the wavefronts incident on the aperture emanate from uncorrelated sources. As shall be developed further, this initial crude estimate of source location can be used to develop refined estimates using processing algorithms which significantly reduce processing requirements when compared to those required using a comparable array when the number of anticipated sources existing over the field of view (FOV) is large. Finally, since the spectral estimate of the source location is essentially an "open-loop" estimate, involving a priori measured quantities such as the antenna port radiation patterns, we consider the effects of measurement errors on the estimate. The results are normalized so as to be generally applicable to both the array antenna configuration as well as for the MBA.     

Superresolution of uncorrelated interference sources by using adaptive array techniques

By using adaptive techniques an antenna array can achieve an angular resolution of uncorrelated interference sources substantially greater than the aperture of diffraction limit. The antenna beam is scanned over the interference source locations, and for each scan condition the adaptive processor is allowed to reach its steady state. It is shown that for realistic ratios of interference strengths to system thermal noise, resolutions in excess of 0.25 times the Rayleigh limit can be achieved.     

Experimental performance of adaptive beamforming in a sonarenvironment with a towed array and moving interfering sources

The performances of adaptive array algorithms are known to suffer from a strong degradation in scenarios with moving interfering sources. In this article, basic adaptive beamforming techniques are compared using shallow sea sonar data recorded in a towed horizontal array environment with moving interfering sources originated from shipping noise. Our experimental results show the relationship between the practical performances of adaptive and conventional beamforming techniques compared in terms of output SINR or a related measure given by the noncompensated postbeamforming interference power. These results demonstrate noticeable performance improvements that can be achieved using several robust algorithms relative to traditional adaptive beamforming schemes     

Constrained estimates of multipath covariance

When multipath propagation occurs, the covariance among signals traveling along rays emanating from a common source is expected to be larger than the covariance between signals generated by independent sources. Several data adaptive constrained estimates of the covariance are derived by the author as bilinear forms and some simulations are presented. The ability of a bilinear form to distinguish a 0-dB (relative to uncorrelated noise) correlated arrival pair from a 0-dB independent source is studied using an expected narrowband cross-spectral matrix corresponding to a simulated acoustic field with a 32-element line array at Nyquist spacing. An adaptive set of filter vectors obtained from the classical minimum variance problem are found to minimize sidelobe interference to 2 dB above the background noise level at the cost of reduced peaks having an 18-dB output above the uncorrelated background