Pointing Accuracy and Dynamic Range in a Steered Beam Adaptive Array

The performance of a steered beam adaptive array as a function of the beam pointing error is examined. The purpose is to determine how close the steered beam has to be to the actual desired signal arrival angle for good performance. It is shown that the beam pointing error that can be tolerated is essentially a question of dynamic range. The greater the desired signal dynamic range that must be accommodated by the array, the more accurate the beam pointing angle must be.     

The Effects of Gaussian Interference on Communication Systemswith Adaptive Arrays

The performance of a bandlimited binary phase-shift-keyed (BPSK) communication system is examined when the received BPSK signal is corrupted by both thermal noise and a directional Gaussian noise interfering signal. The system uses an LMS adaptive array to suppress this interference. The effects of signal power levels, arrival angles, bandwidths, and the array bandwidth are examined. The performance of a system that uses tapped delay lines for the array weights is also examined. It is shown that the performance of a system with tapped delay lines is not affected by the interference bandwidth for a single interferer.     

The Steady-State Antenna Patterns of Adaptive Arrays

The antenna pattern of a receiving adaptive array of arbitrary three-dimensional geometry operating in an environment of K sources, one desired signal and (K - 1) jammers, is considered. It is shown that the adapted (voltage) antenna pattern of the array is a linear combination of K (or less) basis patterns, each of which is a function of one source only. We find that these basis patterns have a simple physical meaning, namely, the kth basis pattern is the pattern realized by the array when the transmission of source k is considered a desired signal and all other sources are turned off. When the array elements are isotropic, these basis patterns are retrodirective (that is, the mainlobe of the kth basis pattern points at source k). It had been shown that this property is also exhibited by a different decomposition of the adapted pattern in the special case of a single jammer (K = 2). In contrast, our decomposition which is simpler than the earlier one, yields retrodirective beams for all K. The simple, physically meaningful, pattern decomposition developed here is quite significant in the insight it provides regarding the basic underlying principles of adaptive arrays. It is also instrumental in elucidating their capabilities and limitations.     

On combining adaptive nullsteering with high resolution angleestimation under main lobe interference conditions

When adaptive nullsteering is applied to attenuate interfering signals which are received via the main lobe, two limitations manifest themselves: the sidelobe level increases and the peak of the main beam shifts away from the desired look direction (squinted main beam). A two-step method is described which maintains angular accuracy even under main lobe interference conditions. In the first step, adaptive nullsteering is performed to enable the detection of targets; the distortion of the main beam is allowed. In the second step, high-resolution angle estimation techniques are used to obtain the angles of the target and the interference sources. The procedure requires prior knowledge of the number of interfering signals; an interference map can be used to provide this information     

反向交叉眼对单脉冲雷达干扰效果分析及仿真验证

交叉眼干扰被认为是对单脉冲雷达干扰最有效的方式之一。基于雷达方程建立了隔离平台回波下的两点源反向交叉眼干扰模型，推导了交叉眼干扰欺骗角一般性公式，研究了干扰机发射天线间距、干扰平台旋转角和干扰机相对雷达之间距离等参数变化对角度欺骗效果的影响，并依据单脉冲雷达接收机获取角度的信息处理流程，建立了单脉冲雷达接收机仿真模型，对交叉眼数学模型的正确性和局限性进行了分析。研究结果表明：单脉冲雷达越靠近两点源交叉眼干扰机中心线、干扰机两发射天线间距越大、与干扰机距离越近时，角度欺骗效果越好；单脉冲雷达的欺骗角度随着与干扰机距离的接近呈指数式增大；数学模型和仿真模型计算的单脉冲雷达角度误差最大值随干扰机天线与雷达天线中心连线的夹角的增大呈指数化增长。研究可为交叉眼干扰工程设计作参考。     

Angle Estimation with Adaptive Arrays in External Noise Fields

A method of estimating the angle of arrival of a signal at an array of sensors in an external noise environment is outlined. The development is based on a maximum likelihood estimator and leads naturally to adaptive sum and difference beams which null the external noise sources. An algorithm for estimating angle of arrival, based on the outputs of adaptively distorted sum and differnce beams, is shown to perform well in the presence of sidelobe and/or main beam interference.     

Angle measurement in the presence of mainbeam interference

The problem of reducing interference impinging on an antenna array when the sources lie in the main beam is addressed. Adaptive antenna arrays are incorporated to form adapted sum and difference beams in which the interference signals are suppressed. Monopulse error curves are then obtained, providing the necessary distortion correction curves across the entire mainbeam tracking angle region. New Cramer-Rao (C-R) bounds on the angle estimation error are derived with generalized assumptions on the signal amplitude and phase. The bounds previously derived by others are valid under different conditions. With these generalized assumptions on the signal characteristics, a Monte Carlo simulation is performed, based on the estimation procedure presented, to determine the angle estimation error. These errors are compared with the C-R bounds. Good performance is shown for sufficient S/N₀ and angular separation between the target and the interference sources     

Steered Beam and LMS Interference Canceler Comparison

The performance of two kinds of interference cancelers is compared, namely, a constrained steered beam interference canceler (CSBIC) and a least mean square interference canceler (LMSIC). For simplicity, this is done for a two-element array. In our comparison we use the array output desired signal-to-interference power ratio (SIR) and the desired signal-to-noise ratio (SNR). These power ratios are obtained for the CSBIC and LMSIC under two sets of conditions: 1) The error in the assumed angle of incidence for the CSBIC is small, and the LMSIC operates in a codetracking mode. 2) The error in the assumed angle of incidence for the CSBIC is large, and the LMSIC operates in a code-acquisition mode. Comparison of the corresponding power ratios obtained under these two sets of conditions then establishes the condition under which it is more desirable to use a CBSIC as compared with an LMSIC.     

一种雷达模拟器天线系统设计

该系统采用角锥喇叭天线组成比幅测角系统,介绍了一种用于雷达模拟器的天线系统设计,解决了方位波束宽度大和天线增益高之间的矛盾,具有结构简单、易于实现、天线增益和测角精度高等特点。经测角误差分析和电磁仿真,证明天线系统的增益系数、测角范围和测角精度等主要技术指标满足雷达模拟器设计要求,具有推广应用价值。     

Directivity-Bandwidth Limitations on Retrodirective Data Links

In a retrodirective array, frequencies differing from the pilot signal retrodirect at different angles from that of the pilot. For the retrodirected beamwidth to cover the reception point over the desired bandwidth, the product of directivity and fractional bandwidth squared cannot exceed a maximum value. This value is dependent only upon the angular retrodirection range, with smaller angular ranges allowing a higher product. For example, a ±30 degree angular range and 5 percent bandwidth allows 36-dB directivity.     

MLS: a total system approach

A typical approach and landing operation is described. The microwave landing system (MLS) is then examined, its design characteristics and how it works are shown, and how the MLS design fulfils the user's operational requirements by protecting the guidance signal from reflected signal interference is highlighted. MLS angle system accuracy is discussed in great detail, and its reliability, integrity, and coverage volume are briefly considered. MLS availability at any runway to all aircraft types and their landing scenarios, which is accomplished using narrow scanning beam antennas, is examined     

Cancellation of specular and diffuse jammer multipath using ahybrid adaptive array

It has been demonstrated that specular or diffuse jammer multipath can be canceled to a desired level by using an adaptive array that combines bandwidth partitioning with tapped delay lines. Such hybrid systems are studied. In particular, the author studies an ideal two-element array that uses bandwidth partitioning in both the main and auxiliary channels, with an Mth-order adaptive finite impulse response filter in each subband of the auxiliary. The ability of this system to cancel specular moderately diffuse and diffuse multipath is studied. The combinations of bandwidth partitioning and filter order that can achieve a specified jammer cancellation level are discussed     

Agile-beam synthetic aperture radar opportunities

The application of agile-beam (electronically steered) antennas to synthetic aperture radars (SARs) is examined. Agile-beam antennas are shown to provide significant advantages for SAR applications in which rapid search of wide areas is desired at high resolution. The analysis provides some useful insights into the operation of SARs and dispels a number of common misconceptions about the limitations of SARs     

低速大迎角张线尾撑系统支架干扰影响研究

为了进一步提高低速大迎角试验数据的质量，掌握支架干扰规律，对Ф3．2m风洞张线尾撑系统进行了支架干扰试验研究。研究结果表明：张线尾撑装置的横梁对飞机纵向的远场干扰量较小，大迎角区域内尾支杆对飞机纵向的近场干扰量较大；迎角小于15°范围内，支架使飞机偏航力矩系数减小、滚转力矩系数增大，随侧滑角增大支架干扰量增大；去掉立尾后尾支杆对俯仰力矩的干扰明显减小。     

Phase-only LMS and perturbation adaptive algorithms

The author defines a phase-only gradient-based adaptive algorithm analogous to the least mean square (LMS) algorithm. Two phase-only perturbation algorithms are then defined. It is shown that the gradient for the perturbation algorithms can be obtained by cross-correlating binary perturbation sequences, that are applied to the adaptive phases, with the resulting instantaneous output power or voltage. It is also shown that a single set of phase-only adaptive weights can be used to simultaneously null interference in multiple output beams. Simulation results are presented for all of the new algorithms. The phase-only perturbation techniques eliminate the need for element level receivers and support low cost retrofitting of adaptive nulling on phased arrays by using conventional beamsteering circuits to apply the adaptive weights     

An Application of Radiometric Techniques to Precision Angle Tracking Radar Systems

A new technique is described which provides for precision angle tracking of celestial radio sources with a conventional monopulse antenna receiving system. It is shown that this technique is readily adapted to angle tracking radars. The features of conventional monopulse operation are preserved while permitting precise angle tracking of noise sources when signal to noise ratios are much less than unity. Measurements, using a four-horn monopulse feed with a 28-foot parabolic reflector and a "monopulse radiometer" produced the characteristic monopulse angle detection functions when using the sun, the moon, and Cassiopeia A as boresight reference sources. Precision measurements were made to 8 arc second under varying weather conditions using 28-foot radio astronomy antennas. The accuracy of the measurements were limited by the antenna angle encoders, consequently no conclusions are drawn with regard to the absolute accuracy of the measurements. The celestial coordinates of four discrete radio sources and the equations for coordinate transformation to local elevation and azimuth are contained in the Appendixes.     

High-efficiency Loran-C interference identification by synchronous sampling

A new technique is proposed for estimating the frequencies within carrier-wave interference (CWI) affecting Loran-C receivers. Its novelty lies in that interference samples are taken synchronously with the Loran-C pulses and ensemble-averaged in the time domain prior to conventional spectrum analysis. Each interferer is thus automatically weighted according to its effect on the phase tracking operation of the receiver. The new method substantially increases the efficiency with which the most insidious interferers may be pinpointed. It requires little additional computational power or memory in the receiver.     

Adaptive array transient sidelobe levels and remedies

The transient sidelobe level of a sidelobe canceler (SLC) is a function of the external noise environment, the number of adaptive auxiliary antennas, the adaptive algorithm used, auxiliary antenna gain margins, and the number of samples used to calculate the adaptive weights. An analytical result for the adaptive sidelobe level is formulated for the case when the adaptive algorithm is the open-loop, sampled matrix inversion (SMI) algorithm. The result is independent of whether concurrent or nonconcurrent data processing is used in the SMI algorithm's implementation. It is shown that the transient sidelobe level is eigenvalue dependent and increases proportionally to the gain margin of the auxiliary antenna elements with respect to the quiescent main antenna sidelobe level. Techniques that reduce this transient sidelobe level are discussed, and it is theoretically shown that injection independent noise into the auxiliary channels significantly reduces the transient sidelobe level. It is demonstrated that using this same technique reduces the SMI noise power residue settling time     

Digital Simulations of Effects of Two-Antenna Interference on Space Vehicle Guidance

This study determines through digital simulations the effects of two-antenna interference, defined as interference occurring at the ground guidance system antenna between the signals from two antennas located on, opposite sides of a space vehicle. It is demonstrated that errors due to interference during early booster flight are recovered from at booster cutoff; interference produced errors occurring shortly before cutoff are not recovered; and modulating the gain of one spaceborne antenna is beneficial to guidance in the interference region.     

头部和后体对钝头体侧向力的影响

钝头体大迎角飞行时会出现随机的非对称流动现象,引起不确定的较大侧向力,进而使其偏离运行轨道。通过在钝头体头部施加人工扰动块可以固定其大迎角下的非对称流场结构,得到确定的侧向力,以利于改善钝头体的大迎角飞行特性及机动性。本文讨论了在头部人工扰动块主控流场结构的基础上,模型后体对侧向力影响的存在性问题,在迎角为50°、雷诺数为1.54×105的条件下,利用实验对周向角为90°和270°、子午角为10°的扰动位置的球形扰动主控下的侧向力影响因素进行了研究。发现钝头体大迎角下的非对称流动结构在头部主控的基础上,后体对非对称流动的影响不会消失,且其为影响头部扰动主控作用的重要因素。尽管模型后体的影响不会改变钝头体头部对于流场结构的主控地位,但会影响头部扰动控制的精准程度。所以在通过钝头体头部施加扰动进而得到确定的侧向力的同时,还需要减小模型后体对流场的影响,对其结构和加工质量进行优化,以更好地通过人工扰动主控流场结构。