AFIT neural network research

A brief summary of research done at the Air Force Institute of Technology (AFIT) in the area of neural networks is provided. It has been shown that backpropagation, used for feedforward artificial neural networks, is just a degenerate version of an extended Kalman filter, and that networks can do about as well as the optimum statistical classification technique. A method of finding the importance of features for use by a neural network classifier has been determined. Techniques for using neural networks for image segmentation have been developed. In optical pattern recognition, techniques that allow the processing of real FLIR (forward-looking infrared) images with existing binary spatial light modulators have been devised. An optical direction of arrival detector applicable to laser illumination direction determination has been designed and tested; the design is similar to a fly's eye. Coated mirrors for the optical confocal Fabry-Perot interferometer have been designed, specified, fabricated, and installed. Significant progress has been made in the use of neural networks for processing multiple-feature sets for speech recognition     

谱估计测向的MUSIC算法研究

由天线阵对被测信号的响应矩阵的特征值,可得到对应于信号和噪声的特征向量,以此扩展成信号子空间和噪声子空间。利用信号子空间与噪声子空间的正交关系,即可求得信号方位角的估计值。这种空间谱估计测向的MUSIC算法,用于圆形天线阵对非相干信号进行测向,可以克服一般干涉仪测向体制中测向模糊和测向盲区的问题。     

A Survey of Electronic Attitude Measurement Systems

Introducing rectangular coordinates for the reference coordinate system and the space-vehicle fixed-coordinate system, it will be shown that the transformation matrixes between both coordinate systems which describe the attitude of the space vehicle can be easily computed as functions of the rectangular coordinates of two precise directions known in both coordinate systems. In detail, an electronic attitude measurement will be described based on two precise propagation directions of RF waves. In order to measure the propagation direction in the space-vehicle fixed-coordinate system, a very general interferometer is described. This interferometer is characterized by the fact that the antennas of the interferometer are arbitrarily distributed on the surface of the space vehicle and that the electronic computation of the coordinates of the propagation direction of an RF wave is extremely simple. The design and some modifications of this system are discussed     

Polarization Mismatch Errors in Radio Phase Interferometers

An analysis is presented which deals with the effects of polarization mismatch errors on the accuracy of a phase interferometer used for position location of unknown emitters relative to known calibration emitters. Closed-form expressions for the induced phase difference between interferometer antennas are derived for several combinations of receiving and transmitting antenna polarizations. Errors contributed by mechanical misalignment between antennas, as well as effects of power loss attributable to polarization mismatch, are also considered. The analysis leads to the conclusion that circularly polarized interferometer and transmitter antennas are best suited for the position location application, if it is assumed that polarization tracking of the interferometer antennas is not available. It is shown that a reasonable amount of ellipticity can be tolerated before the phase error becomes significant.     

Bearing Ambiguity and Resolution in Interference Direction Finders

Radio direction finders based on the principles of interference measurement provide increased angular resolution as the number of bearing ambiguities increase. In spite of the diversity of interference direction finder design, the resolution of bearing ambiguities reduces to a simple relative phase measurement on crossed baselines less than ?/2 long. This principle is applied in a review of current interference direction finder (DF) designs from the Adcock to the multiwavelength interferometer including both analog and digital bearing computation and display. Analysis shows that simple digital logic circuits may be used to resolve bearing ambiguities in multichannel DF rather than the "sense" displays of conventional analog design. This technique has the advantage of keeping direction finding and sense finding functions simultaneous but separate so that the DF information is not degraded by the injection of the ambiguity resolving sense signal.     

Novel wideband multimode hybrid interferometer system

In this paper, a novel hybrid of a three-element interferometer comprised of multimode antennas is analyzed. The phase ambiguities associated with the long baselines of the interferometer are resolved using the "coarse" angle estimates provided by the multimode antenna. This results in the elimination of the short baseline interferometers of the conventional five-element interferometer. It is shown here that the signal-to-noise ratio (SNR) must be above a threshold value to resolve the phase ambiguities with a high degree of probability. An expression that shows the dependence of this threshold SNR on the interferometer spacing and the variance of the angle estimates provided by the multimode antenna is derived. A single three-element wideband multimode antenna interferometer can replace several five-element conventional interferometers, each covering a separate frequency band.     

Interferometer Design for Elevation Angle Estimation

Radars that are developed for the purpose of monitoring aircraft landings in the terminal air traffic control system can be designed to exploit the relatively high signal-to-noise ratio that characterizes the power budgets calculated for such a link. An interferometer using a pair of low gain antennas can be used to obtain passive coverage over a targe azimuth and elevation sector. A large baseline can be used to obtain the desired elevation angle estimation accuracy. In this paper an optimal tradeoff between the width of the subarray aperture and the width of the interferometer baseline is performed that achieves a specified elevation angle estimation error while minimizing the overall height of the interferometer configuration. The algorithm searches through the class of antenna patterns that can be synthesized from so-called finite impulse response, linear phase digital filters. For the specific problem of designing an elevation sensor for monitoring landing aircraft on final approach, the elevation angle can be estimated with no more than 1-mrad rms error when the aircraft is within ± 60° azimuth, 2.5° to 40° elevation, using two 7-wavelength subarray antennas spaced 8 wave-lengths apart. The design of a separate sensor for resolving the interferometer ambiguities is formulated as a hypothesis testing problem and solved using statistical decision theory. A bound on the probability of an ambiguity error is derived that accounts for the effects of ground reflection multipath and receiver noise.     

ACCURACY ANALYSIS OF PASSIVE LOCATION SYSTEM WITH PHASE DIFFERENCE RATE MEASUREMENT

The conventional mono- station passive loca-tion techniques of direction finding are low in speedand accuracy.It may notmeet the requirements ofmodern targeting or accuracy attacking actions.For a moving observer,by employing some passivemeasurements such as Doppler or Phase DifferenceRate(PDR) ,the location error can be reduced tonearly one- fifth of the conventional error[1～ 4 ] .In this paper,the location method using PDRis introduced in section one.Location accuracyanalysis is given i…     

冷原子干涉仪发展现状与应用分析

基于冷原子干涉仪的量子传感器具有极高的理论精度,在自然科学的各个领域特别是重力测量和惯性传感领域有着广泛的应用。首先介绍了原子干涉仪的基本原理,在此基础上给出了基于双光子受激Raman跃迁的脉冲式Mach-Zehnder原子干涉仪的干涉相位,描述了原子干涉仪在重力仪和陀螺上的应用及其发展历程,着重分析了当前发展最成熟的原子重力仪的发展历程和趋势,指出了原子干涉仪应用的发展方向。     

An Analysis of Wide-Band Microwave Monopulse Direction-Finding Techniques

This paper presents the concept, theory of operation, characteris tic equations, and error analysis of four wide-band monopulse techniques. The basic techniques described, which include pure amplitude monopulse, phase and amplitude monopulse (two-and three-channel configurations), and pure phase monopulse interferometer, are particularly applicable to monopulse direction finding systems that require frequency coverage over several octaves and open-loop angle bearing of several degrees. Sufficient detail and working formulas are included to permit a trade-off analysis to be made between the direction-finding techniques for selection in specific hardware applications.     

A Quasi-Optic Imaging Resonator for Plasma Diagnostics

A new resonant quasi-optic interferometer has been developed which has a unique capability to deduce spatial information about perturbing plasma media. The fundamental property of the resonator is that it is multiply degenerate. Its geometry consists of four spherical mirrors arranged in a confocal ring configuration such that the transverse modes are simultaneously resonant. Energy is coupled into and out of the resonator by thin dielectric film beam splitters. When perturbed, it is found that the resonant frequency of the system depends on the point at which the field is measured and this dependence is directly related to the geometry of the perturbation. The results of a theoretical analysis of the perturbation of the resonator are presented and preliminary experimental measurements of a 4-mm system are discussed.     

ATS-6 Interferometer

The Applications Technology Satellite-6 (ATS-6) RF interferometer is utilized primarily as a precision 3-axis attitude sensor having an unambiguous field of view of 350°. This function requires two separated ground transmitters, each using one of the two available frequency channels or sharing a single channel by time multiplexing. For 3-axis control, one uplink transmitter can provide 2-axis attitude (pitch and roll) with other sensors (e.g., a Polaris tracker) providing yaw attitude. By utilizing two uplink transmitters and the Earth sensor or three time multiplexed uplink transmitters, the interferometer can also provide measurements of ATS-6 spacecraft orbit position. Uplink frequencies are 6.150 and 6.155 GHz. The receiving antennas are spaced at 19.95 wavelengths (?) for the vernier baseline and 1.66 ? for the coarse baseline. Spacecraft system weight is 8.39 kg (18.5 lb) and power requirement is 15.5 W. Flight evaluation results are given for the interferometer including R F link budgets, modulation of uplink carrier, signal-to-noise ratio, and dropout behavior. A hardware calibration model is described, containing major biases in the phase measurements. Techniques for flight calibration as both an attitude and spacecraft position sensor are outlined . Flight testing has shown that on-line calibration of receiver/converter biases must be performed on a short term routine basis. Interferometer resolution was found to be 0.00140 space angle with negligible noise (jitter) at transmitted power levels above 72 dBW. As an attitude sensor, the interferometer has demonstrated the ability to provide stabilization to better than 0.     

一种基于多基线载波相位差测量的欺骗干扰检测方法

由于卫星导航信号到达地面的信号功率较低,且民用部分信息公开,极易受到欺骗干扰信号的影响,从而导致接收机解算出错误的位置、速度或时间.针对传统的欺骗干扰检测与抑制方法不能对欺骗干扰进行测向的问题,提出了一种基于阵列多天线接收信号的载波相位差进行欺骗信号检测与抑制的方法.该方法能够在缺少先验知识的条件下,借鉴相关干涉仪测向...     

仅用相位差变化率的机载单站无源定位方法及其误差分析

针对以往单站无源定位系统中长短基线干涉仪系统复杂、角度测量易受干涉仪通道幅度/相位不一致的影响等问题,提出了一种仅仅利用长基线干涉仪(LBI)的相位差变化率实现单站无源定位的新方法,并对该定位方法误差的克拉美-罗下限(CRLB)进行了推导和分析,计算并分析了不同的干涉仪基线安装方向、不同的观测器运动方式下的定位误差几何分布(GDOP)。结果表明,仅用相位差变化率定位时,为了达到侧翼最好精度,干涉仪最好平行机身安装;为了达到侧前方较好精度,干涉仪需垂直安装;观测器的运动姿态最好发生某种机动变化。     

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     

可移动柔性工装对接稳定性研究

随着航空工业的快速发展,传统飞机工装已经不足以满足实际生产过程中的需要。因此,为提高生产效率,实现智能化装配过程,数字化柔性工装成为飞机工装制造的主流发展方向。通过对工装本身设计特点所需体现的关键测量特性进行控制,结合数字化测量设备坐标系建立与拟合的原理,完成可移动柔性工装在移动过程中的对接稳定性研究,进而提高飞机的测量精度和装配效率。     

空间大型星载抛物面天线研究进展

抛物面天线作为星载天线的重要组成部分,在深空探测、移动通信、国防事业、气象监测等方面均具有广泛应用,近年来随着上述学科的快速发展,抛物面天线的研究也越来越受到人们的重视。针对空间大型星载抛物面天线的发展与需求,首先系统地概括了国外星载抛物球面天线的发展现状,综述了刚性、网状以及充气式星载抛物球面天线,对各类星载抛物球面天线的结构、性能进行了较为详细的描述和分析,对国内在该领域的部分研究成果进行了简述。然后梳理了星载抛物柱面天线的发展,对国内外具有代表性的抛物柱面天线进行了介绍,对抛物球面天线和抛物柱面天线进行了参数对比与分析。接下来对近年来针对星载抛物面天线的相关技术进行了介绍。最后对星载抛物面天线的发展趋势做了简要分析与预测。     

Performance of phased-array antennas with mechanical errors

The performance of planar phase-array antennas with mechanical errors is investigated. Errors in array element positions as a result of structural distortions are considered as deterministic and predictable. Detailed calculations for two assumed modes of distortion reveal that their effects on antenna performance are the loss of peak response in the scan direction and the broadening of the mainlobe, while the far-out sidelobe structure remains relatively intact. For large antennas, performance improvement can be expected by suitable phase compensation. Performance of antennas with random errors in their element positions must be treated statistically. Expressions of average directivity and sidelobe level corresponding to arbitrary error magnitudes in element position, amplitude and phase of excitation as well as finite rate of failure of element modules were derived and verified by direct numerical calculations from the antenna directivity patterns. For a planar phased-array antenna typical for space-based radars, the standard deviation of element position errors must not exceed 1% of the operating wavelength in order to maintain a -10 dBi sidelobe level     

Polarization Mismatch Errors in Amplitude Comparison Direction Finders

The polarization unbalance between the two antennas of an amplitude direction finder causes measurement errors for incident waves of variable polarization. These errors are analyzable qualitatively and quantitatively by means of Poincaré's sphere.     

表面不连续特征双站散射特性的试验研究

 采用一种新颖的不连续特征双站雷达散射截面（RCS）测试方法,通过系列化测试,研究了单直缝隙板、单直台阶板的双站散射特性,提出了不连续特征双站散射具有相似性、对称性和平移性的特点,分析了不同不连续特征具有的异同点。试验结果表明：对不连续特征,不同双站下的RCS散射曲线均具有和对应入射频率下的单站散射曲线分布形式相似的特点,同时不同的不连续特征对幅值的影响不同;如果测试方式和试验件对称,则双站和单站散射曲线均具有关于峰值所在方位角对称的特点,单直缝隙板由于试验件本身的对称性表现为较好的曲线对称性,而单直台阶板则表现为稍弱的对称性;散射曲线随双站角的增大,具有明显的平移特性,平移的幅度为双站角的1/2,平移方向为从发射天线指向接收天线,平移后的曲线峰值处于发射天线和接收天线夹角平分线上。