一种宽带阵列幅相与互耦误差联合校正算法

宽带数字阵列雷达接收通道中存在随频率变化的幅相误差和互耦误差,会严重影响雷达性能。针对这一问题,提出了一种宽带数字阵列幅相与互耦误差联合校正算法。首先选取通带内多个离散频点,对于每个频点,将幅相误差和互耦误差作为一个整体,采用基于子空间原理的窄带校正算法估计其阵列失真参数,并计算校正矩阵;然后将其组合起来,构成频域离散校正矩阵;最后基于最小二乘准则,设计了宽带有限长脉冲响应(FIR)校正滤波器矩阵。利用该矩阵,可实现通带范围内任意带宽入射信号的校正。计算机仿真实验验证了该算法的有效性。对实测数据的处理结果表明该算法在宽带数字阵列雷达系统中具有实用价值。     

ANALYSIS OF THE RETURNED SIGNAL MODEL IN BISTATIC RADAR SYSTEMS

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Synthetic aperture beamformer for imaging through a dielectric wall

A coarray-based aperture synthesis scheme using subarrays and postdata acquisition beamforming is presented for through-the-wall wideband microwave imaging applications. The wall causes wave refraction and a change in the propagation speed, both effects alter the travel time between the transmitter, the target, and the receiver. Coherent combining of the pulse waveforms emitted by the different transmitters and incident at the receivers through reflections from targets and clutter requires incorporation of wall effects into the beamformer design. Simulation results verifying the proposed synthetic aperture technique for a through-the-wall imaging (TWI) system are presented. The impact of the wall ambiguities or incorrect estimates of the wall parameters, such as thickness and dielectric constant, on performance is considered.     

Fiber-optic radar calibration

A simple fiber-optic radar calibration target is described. Its operation is based on a wideband fiber, a laser transmitter that is directly modulated by the down-converted radar signal and an optical diode receiver recovering said signal. Further up-conversion having a common local oscillator with the first mixer ensures fidelity of the calibration return. Measured useful bandwidth exceeds 200 MHz and practically any radar RF frequency can be handled when suitable mixers are employed. Amplifiers can be added to the down-converted path as desired to compensate for the fiber loss. Modulation and LO sweep provide easy ways of introducing artificial fluctuations and Doppler frequencies. Particularly pulsed radars are readily tested with the proposed scheme as no restrictions are posed by the radar's TR-switch delays.     

Computer Simulation of VLF Power Amplifiers under Reactive Loading

Recent efforts have been made to achieve substantially increased baud rates with the U. S. Navy's very low frequency (VLF) communication system. Because of the constraints imposed by existing transmitter and antenna structures it was necessary to reexamine the operation of every major system component. Each particular VLF transmitter was subjected to an extensive theoretical analysis followed by a detailed on-site measurement program. Following a review of three major systems, Cutler, Lualualei, and North West Cape, a sufficient body of data was available to substantiate the validity of the computer-simulated transmitter analysis presented here. With this method it is possibe to predict frequency-shift keying (FSK) and minimum shift keying (MSK) performance limitations for particular VLF transmitting systems based primarily on the 0 of the antenna at the operating frequency.     

The Galileo Plasma wave investigation

The purpose of the Galileo plasma wave investigation is to study plasma waves and radio emissions in the magnetosphere of Jupiter. The plasma wave instrument uses an electric dipole antenna to detect electric fields, and two search coil magnetic antennas to detect magnetic fields. The frequency range covered is 5 Hz to 5.6 MHz for electric fields and 5 Hz to 160 kHz for magnetic fields. Low time-resolution survey spectrums are provided by three on-board spectrum analyzers. In the normal mode of operation the frequency resolution is about 10%, and the time resolution for a complete set of electric and magnetic field measurements is 37.33 s. High time-resolution spectrums are provided by a wideband receiver. The wideband receiver provides waveform measurements over bandwidths of 1, 10, and 80 kHz. These measurements can be either transmitted to the ground in real time, or stored on the spacecraft tape recorder. On the ground the waveforms are Fourier transformed and displayed as frequency-time spectrogams. Compared to previous measurements at Jupiter this instrument has several new capabilities. These new capabilities include (1) both electric and magnetic field measurements to distinguish electrostatic and electromagnetic waves, (2) direction finding measurements to determine source locations, and (3) increased bandwidth for the wideband measurements.Deceased     

Distortion Generated in Angle-Modulation Systems by Phased Arrays

The case of linear, uniformly weighted phased arrays is examined via time and frequency domain analyses. Bounds that must be placed on array length, modulation frequency, modulation index ?, and scan angle relationships if excessive distortion is to be avoided in wideband angle-modulated communication systems are established. Distortion is shown to consist of odd harmonic terms. It is also shown that, for one class of equivalent RF signals, phased arrays produce approximately three times as much distortion in frequency-modulation (FM) systems as in phase-modulation (PM) systems. Graphs of distortion plotted as functions of signal and array parameters show that, for practical array sizes, distortion is a monotonically increasing function of the product of L and sin ? where L is the length of the array expressed in modulation wavelengths ?m and ? is the scan angle. In PM systems, distortion also increases monotonically with the modulation index ?. Plots of distortion versus L sin ? show that even relatively small arrays can produce intolerable distortion levels in wideband systems; e.g., an FM system having ? = 3,L = 0.35?m, and ? = 60 degrees exhibits approximately 20 percent distortion.     

宽带数据链路中的帧同步技术研究

文中对宽带数据链路中的帧同步技术进行了研究。采用QPSK调制解调时,数据帧中同步字采用两个重复的符号长度为16的恒幅零自相关(CAZAC)复序列,通过检测两个CAZAC同步字来确认系统处于失步或同步状态。分析结果表明,基于本文给出的同步字的数据帧格式在进行帧同步检测时,在引入帧同步校核系数(α=2)和帧同步保护系数(β=8)后,平均同步捕获时间很短;且一旦系统处于同步状态,其发生失步的概率极小。     

Ultrawideband radar special features & terminology

UWB radar has evolved in the fifteen years since DARPA proposed the present frequency-based definition. We suggest the UWB radar definition should reflect the physical features and practical applications. We propose a revised definition: an ultra wideband radar is a radar set which has a range resolution (/spl Delta/r) much smaller than the target length L along the radiated direction.     

A (design-oriented) reliability model for active phased arrays insatellite communication systems

This paper proposes a modeling approach to relate the power section reliability of satellite payloads to the on-board antenna complexity. The aim of the model is to support the development of design tools for satellite systems adopting active phased arrays. The achieved results are rendered parametric with respect to a key system requirement, the effective isotropic radiated power, due to its direct impact on transmitter configuration and technological features as well as on the critical compromise between transmitter and antenna specifications     

基于空域稀疏性的宽带DOA估计

利用宽带阵列接收信号的空域稀疏性,将宽带信号的波达方向(DOA)估计转化为一个稀疏信号重构的问题,提出了一种新的宽带信号DOA估计算法。该算法将宽带信号分解为多个子带信号,联合利用多个子带信号的空域稀疏性进行重构。它是对用于稀疏重构的标准的稀疏贝叶斯学习算法的推广,可适用于多冗余字典的信号模型。另外,通过对多快拍的阵列接收信号进行奇异值分解(SVD),提取信号子空间作为算法的输入数据,可以在有效减少运算复杂度的同时,提高对噪声的稳健性。与传统的宽带阵列DOA估计方法相比,该算法能够用于低信噪比、快拍有限和信源相关性较高的场合,同时算法的性能对信源个数的估计值不太敏感。仿真实验表明,该算法相对现有的基于子空间类的方法,具有更好的DOA估计性能。     

Automatic target classification of slow moving ground targets in clutter

A new approach is proposed which will allow air-to-ground target classification of slow moving vehicles in clutter. A wideband space-time adaptive (STAP) filter architecture, based on subbanding, is developed and coupled with a one dimensional template-based minimum mean squared error (MMSE) classifier. The performance of this STAP/ATC (automatic target classification) algorithm is quantified using an extensive simulation. The level of residual clutter afforded by various filter configurations and the associated incremental improvement in ATC performance is quantified, revealing the potential for realizable hardware and software implementations to achieve acceptable ATC performance.     

Bandwidth maximization for satellite laser communication

Free space optical communication between satellites networked together can make possible high speed communication between different places on Earth. The basic free space optical communication network includes at least two satellites. In order to communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. The pointing systems for laser satellite communication suffer during tracking from vibration due to electronic noise, background radiation from interstellar objects such as Sun, Moon, Earth, and Stars in the tracking field of view, and mechanical impact from satellite internal and external sources. Due to vibrations the receiver receives less power. This effect limits the system bandwidth for given bit error rate (BER). In this research we derive an algorithm to maximize the communication system bandwidth using the transmitter telescope gain as a free variable based on the vibration statistics model and the system parameters. Our model makes it possible to adapt the bandwidth and transmitter gain to change of vibration amplitude. We also present an example of a practical satellite network which includes a direct detection receiver with an optical amplifier. A bandwidth improvement of three orders of magnitude is achieved in this example for certain conditions, as compared with an unoptimized system     

Optical Ranging System Employing a High Power Injection Laser Diode

For many military applications small, lightweight range finders are required that operate at a high pulse repetition rate. It will be shown that a ranging system with an injection laser in the transmitter can fulfill these requirements. The performance of an optical ranging system employing a pulsed gallium arsenide laser diode transmitter, with peak powers up to 100 watts at room temperature is described. After a brief review of fundamental system parameters, calculations of the range capability of the system are given, based upon the Poisson distribution of photoelectron statistics. A discussion is presented on the experimental support of theoretical analysis.     

基于常规多普勒滤波器组结构的合成宽带距离像性能分析

为了消除散布效应对宽带信号的不利影响,可以利用多个窄带信号合成宽带信号;同时,通过在子带内处理相干脉冲串,可进行杂波抑制、目标速度估计和区分不同速度的目标。然而,在子带内分别使用常规多普勒滤波器组,会引进多普勒散布效应所造成的输出失配误差,从而造成合成距离像的失真。分析了多普勒散布效应与子带常规多普勒滤波器组输出失配之间的关系,推导了运动目标通过滤波器组后所成高分辨距离像的距离走动公式,给出了适合子带常规多普勒滤波器组的目标速度临界值。仿真实验验证了以上结论。     

Radar/sonar acceleration estimation with linear-period modulatedwaveforms

Doppler and acceleration tolerance of wideband LPM/HFM (linear period-modulated, hyperbolic frequency-modulated) and linear frequency-modulated (FM) signals are compared. A bank of filters matched to frequency-shifted versions of a wideband LPM/HFM transmission system yields a joint maximum likelihood estimate of range and acceleration and avoids acceleration-induced degradation in detection performance. Analytical and neurophysiological results suggest that such processing can be used in bat echolocation for detection and classification of insect wing motion since wideband LFM waveforms are much less Doppler-tolerant than HFM waveforms but have greater acceleration tolerance     

一种可齐平安装的机载复合结构宽带天线的设计

设计了一种可齐平安装的宽带超高频(UHF)天线.该天线由含有背腔结构的盘锥天线、盘锥顶盘与地板之间的短路结构、以及多板天线3个部分复合而成.这3个部分分别在频率的中端、低端和高端产生3个邻近的独立谐振点,从而有效地展宽了天线的工作带宽.分析了天线各主要结构参数对谐振频点和阻抗带宽的影响,给出了这些参数的最优值,并据此制...     

Range, radial velocity, and acceleration MLE using radar LFM pulsetrain

An efficient implementation of the maximum likelihood estimator (MLE) is presented for the estimation of target range, radial velocity, and acceleration when the radar waveform consists of a wideband linear frequency modulated (LFM) pulse train. Analytic properties of the associated wideband ambiguity function are derived; in particular the ambiguity function, with acceleration set to zero, is derived in closed form. Convexity and symmetry properties of the ambiguity function over range, velocity, and acceleration are presented; these are useful for determining region and speed of convergence for recursive algorithms used to compute the MLE. In addition, the Cramer-Rao bound (CRB) is computed in closed form which shows that the velocity bound is decoupled from the corresponding bounds in range and acceleration. A fast MLE is then proposed which uses the Hough transform (HT) to initialize the MLE algorithm. Monte Carlo simulations show that the MLE attains the CRB for low to moderate signal-to-noise depending on the a priori estimates of range, velocity, and acceleration