混杂组元低频吸波阵列层板复合材料的吸波性能

采用电磁仿真运算的方法设计了方形和十字形混杂组元低频吸波阵列,采用模压工艺制备了基于混杂组元低频吸波阵列的吸波层板复合材料。研究了各单组元阵列本征吸波特性随单元结构变化的规律和混杂组元低频阵列本征吸收带的叠加效应。吸波性能测试结果表明,不同形状阵列单元的混杂可以有效拓宽阵列的本征吸收带宽,方形和十字形混杂组元阵列为双峰吸收阵列,吸收峰频率分别为3.1与4.5 GHz。混杂组元低频阵列的引入可以有效改善层板低频吸波性能,5 mm厚吸波阵列层板的反射率在2~6 GHz范围内-4.7 d B,6~16 GHz范围内-7 d B,阵列吸波层板的宽频吸波性能显著优于传统的阻抗渐变型吸波层板,阵列吸波层板力学性能与树脂基复合材料层板相当。     

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     

A Microwave Hologram Radar System

An airborne microwave hologram radar system has been developed which is a two-dimensional analog to optical holography. The field of view of the radar is directly below and to either side of the aircraft. Resolution is realized in the along-track direction by utilizing the synthetic aperture technique, and in the cross-track direction by means of a phased receiving array. The theory of operation is summarized, the demonstration system is described, and results for both the normal and contouring modes of operation are presented.     

Small Aperture Angle Measurement for Active Echo Location Systems

The problem of bearing estimation for active systems is examined from the point of view of the generalized wideband ambiguity function (GAF). The maximum likelihood (ML) estimator is derived and its local and global properties are discussed. A structure is proposed which searches the three-dimensional ambiguity surface in two stages first, in range-Doppler, and then, in bearing with the goal of reducing search complexity when utilizing highly resolvent waveforms. Comparisons are made between the ML estimators and structures utilizing phase information which generate closed form estimators. The beneficial results of full bandwidth utilization are discussed in terms of both local and global properties of the GAF.     

Advances in non-linear apodization

Selected new methods and applications of non-linear apodization for irregularly-shaped and parse coherent apertures and arrays are presented. The benefits include unproved impulse response performance, i.e., reduced peak sidelobes and integrated sidelobe power, along with improved mainlobe resolution, compared to classic windowing techniques. Nonlinear apodization (NLA) techniques can also serve as powerful engines for effective superresolution and bandwidth extrapolation of coherent data for filling sparse apertures. The sparse aperture filling property of superresolution algorithms for radar data forms the basis for a new concept which is introduced here: synthetic multiple aperture radar technology (SMART). Increased swath and/or reduced antenna size are some of the benefits postulated for SMART applied to synthetic aperture radar (SAR) systems. The benefits of these new methods and applications for nonlinear apodization are then demonstrated for two specific applications: 1) sidelobe control for Y-type synthetic aperture radiometers, such as the European Soil Moisture and Ocean Salinity (SMOS) system (Kerr et al.) and JPL's proposed GeoSTAR (Lambrigsten) concept; and, 2) filling of sparse synthetic aperture radar data by exploiting the bandwidth extrapolation properties of nonlinear apodization based superresolution techniques. The methods that have been developed and demonstrated herein have potential application to a wide range of passive and active microwave remote sensing and radar systems.     

A fiber-optic current sensor for aerospace applications

A robust, accurate, broadband, AC sensor using fiber optics that is being developed for space applications at power frequencies as high as 20 kHz is described. It can also be used in low- and high-voltage 60-Hz terrestrial power systems and in 400-Hz aircraft systems. It is intrinsically immune to electromagnetic interference (EMI) and has the added benefit of excellent isolation. The sensor uses the Faraday effect in optical fiber and standard polarimetric measurements to sense electrical current. The primary component of the sensor is a specially treated coil of single-mode optical fiber, through which the current-carrying conductor passes. Improved precision is accomplished by temperature compensation by means of signals from a novel fiber-optic temperature sensor embedded in the sensing head. The technology used in the sensor is discussed, and the results of precision tests conducted at various temperatures within the wide operating range as well as the results of early EMI tests are reported     

Optical Simulation of Radar Ambiguities

Coherent optical systems, because of their basic similarity to coherent radar systems, can be used to simulate many of the characteristics of the latter. This paper discusses the use of a coherent optical system for the simulation of the range and azimuth ambiguities that sometimes occur in radar systems. The optical configurations for implementing these simulations are described in detail, and extensive experimental results are presented.     

The Susceptivity of MTI Systems to White Noise

The performance of various coherent MTI systems in the presence of white noise is investigated. The single-pulse signal-to-noise ratios at the output of the filters are presented for the case of large system bandwidth. Generalized results, calculated as a function of the system's bandwidth, are given. Finally, those results are compared to the optimum signal-to-noise acheivable by a matched filter.     

The Effect of Spectral Shape on the Probability of Error in Laser Binary Communication Systems

The effect of spectral bandwidth and spectral shape upon laser binary communication systems is investigated. The probability of error is plotted as a function of bandwidth for Lorentzian and Gaussian shaped spectra. The results show that even when the average noise power and the signal-to-noise ratio are held constant, the probability of error varies considerably as a function of spectral bandwidth.     

Quadrature Sampling with High Dynamic Range

Many radio and sonar systems require signal outputs in complex low-pass form. To achieve this, it is possible to use uniform sampling of the bandpass signal, together with computation of the quadrature component by way of a Hilbert transform. The bandpass to low-pass translation is accomplished by undersampling. A hardware implementation is described which achieves 70 dB spurious-free dynamic range and a bandwidth of 30 kHz.     

MLS system error model identification and synthesis

Signal-in-space modeling of approach and landing guidance systems such as the microwave landing system (MLS), which is necessary for aircraft certification, is addressed. A technique which can, in principle, identify the error sources in navigation systems and can also perform the inverse operation of synthesizing an error generator whose output is statistically equivalent to the original error data is described. The technique can also be used to validate equipment error budget assumptions. The identification and synthesis technique applies not only to MLS but to all navigation systems     

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.     

Efficient wideband jammer ing when using stretch processing

Techniques are described for performing adaptive jammer ing over extremely wide bandwidths on radar systems which use linear frequency modulated (LFM) waveforms and stretch processing. It is assumed that the range uncertainty of the target is a small percentage of the equivalent range extent of the uncompressed pulse. The assumption allows the cancellation to take place following stretch processing in either the time domain using a narrowband sliding filter that keeps up with the chirp fate or in the frequency domain. The new approach supports ing performance over gigahertz of bandwidth comparable to that previously achieved over a few megahertz using approximately the same number of spatial degrees of freedom.     

重力测量稳定平台控制系统设计与实现

介绍了重力测量惯性稳定平台的系统工作原理.设计了一种基于DSP加FPGA架构的嵌入式控制系统.详细描述了控制系统的功能划分以及硬件实现.提出了电流、速度和位置三环回路控制方案.车载测试结果显示稳定平台能实现地垂线跟踪,验证了所设计控制系统的有效性.     

脉冲超宽带信号测量性能分析

超宽带技术是一项新兴的无线通信技术,具有极其广阔的发展前景,但目前仅用于室内短距离通信,少见用于航天测控系统。为了将超宽带技术应用于测控系统中,以模糊函数为工具,对脉冲超宽带信号的测量性能进行分析。首先推导矩形脉冲串信号和载波调制矩形脉冲串信号的模糊函数,并对其模糊特性进行仿真分析。在此基础上,主要针对用于测控系统的伪码调制脉冲超宽带信号,利用其模糊函数分析其测距测速性能。结果表明:该超宽带信号具有良好的测距测速性能,其最大无模糊距离为1个伪码周期,最大无模糊多普勒频率为脉冲重复频率的倒数;单脉冲宽度越窄,其测距性能越好而测速性能越差。     

Electronically steerable plasma mirror based radar-concept andcharacteristics

An alternative to using a phased array to steer a radar beam is to electronically control the orientation of an inertialess broadband microwave reflector. Recent experiments have demonstrated that a planar plasma mirror immersed in a magnetic field can be formed with electron densities high enough to reflect X-band microwave beams. A plasma mirror performs like a metal mirror, but it is inertialess. Compared to high performance phased array systems, a plasma mirror based radar system is much simpler and is therefore more affordable. Electronic steering of microwave beams using a plasma mirror permits the use of wide instantaneous bandwidth waveforms. Potential areas of application for a plasma mirror based antenna system include ship self-defense, high-resolution radar imaging, target identification, electronic countermeasures, high data rate communications, spread spectrum links and remote sensing. As a reflector, the plasma mirror exhibits extremely low loss and the reflectivity is very nearly 100%. Since a perfectly reflecting object cannot radiate, the noise temperature contribution of the plasma mirror to the antenna temperature is likely to be small. The plasma sheet can be steered in elevation by tilting the magnetic field, and steering in azimuth may be accomplished by designating cathode initiation sites. Switching times between successive mirror orientations may be less than 20 μs     

Spread-Spectrum Signaling for Distributed Fiber Optic Systems

One of the stringent requirements on the receivers of distributed industrial fiber optic systems is fast and accurate adaptive thresholding and a wide dynamic range. This is due to the existence of variable link lengths and geometry, especially in multipleaccess situations, coupled with unipolar optical signaling. This paper analyzes the application of a simple spread-spectrum system in such fiber-optic data links. It proposes a detector model that yields a tractable expression for the probability of error. The optimum threshold and the probability of error are then related to the received optical power, bit rate, and spectrum-spreading factor. A procedure is indicated by which, based on the minimum expected received power, a spreading facor and a threshold are chosen to insure a given maximum error rate at all the receivers.     

Atmospheric Attenuation Models for Satellite-To-Earth Links

In an earlier paper by the authors1 it was mentioned that atmospheric attenuation effects could be included by assuming that the attenuation is directly proportional to number of air masses. While this assumption is valid for the optical region, it is not true in the microwave region (1 to 40 GHz). For a clear atmosphere, the two constituents which absorb microwaves are oxygen and water vapor. An analysis of the atmospheric attenuation data for the microwave region has been made and some rather simple expressions for computing the attenuation as a function of zenith angle are developed with an accuracy suitable for satellite antenna footprint calculations.     

民用飞机航电数据网络带宽资源分配优化设计

民用飞机航电数据网络在设计过程中,通过对带宽资源的合理分配,来满足网络成员系统的数据传输需求。而在设计过程中,存在因网络带宽需求分布的非均匀性等因素造成的局部带宽资源不足的问题,因此需要在初步带宽资源分配结果上进行局部优化设计。提出一种通过网络带宽资源使用率最高的链路和端系统进行识别,以及进行针对性物理链路连接关系调整,从而实现降低交换机之间链路带宽资源使用率的方法。通过网络仿真对该方法的优化效果进行了分析,发现这种方法能够降低网络局部链路的较高带宽资源使用率,单轮优化的有效降幅超过15%。该网络带宽资源分配优化设计方法,一方面能够有效解决因飞机网络局部带宽不足问题,降低由此导致的网络拥堵、排队、丢包等风险,确保网络确定性;另一方面还能尽量保持当前分配结果,减少设计优化变更对网络时延性能和网络布线方面的影响。