Array antennas for DGPS

Multipath interference limits the speed and accuracy of determining position by “differential” GPS techniques. A geodetic surveyor, for example, requires multipath interference rejection of about 36+20 log₁₀ sin ϵ dB, where ϵ is the elevation angle of the satellite being observed. Signal processing in a GPS receiver cannot satisfy this requirement. A receiving antenna is required that can sufficiently reject signals arriving from below the horizon. Available antennas have inadequate rejection, and brute-force methods of improving them, by enlarging their ground-planes, are impractical. A compact, ground-planeless, dual-band, GPS antenna with improved multipath rejection has been designed and field-tested. This antenna resembles a vertical post rather than a horizontal platter; within its 0.1-m diameter, 0.4-m tall radome is a vertical array of turnstile elements. In field tests, a three-element array antenna rejected multipath better than a 0.5-m diameter ground-plane antenna by an average of 5 dB. A five-element array antenna appears superior to a 0.9-m diameter ground-plane antenna     

基于单个长基线干涉仪的运动单站直接定位

 针对以往单站无源定位系统中采用的多通道干涉仪或阵列测向系统复杂、易受通道间幅相不一致性影响等缺点,提出了一种基于信号子空间分解的运动单站单个长基线干涉仪(LBI)直接定位(DPD)方法。该方法采用量子粒子群优化(QPSO)方法获得定位初值,再利用Newton迭代方法得到精估计值。仿真结果表明:增加观测器的机动性,可对辐射源实现快速无模糊的定位;定位性能在较高信噪比(SNR)下可接近定位误差的克拉美-罗下限(CRLB),在低信噪比下优于多通道干涉仪仅测角(BO)和长基线相位差变化率定位方法。     

基于单个长基线干涉仪的运动单站直接定位

针对以往单站无源定位系统中采用的多通道干涉仪或阵列测向系统复杂、易受通道间幅相不一致性影响等缺点,提出了一种基于信号子空间分解的运动单站单个长基线干涉仪(LBI)直接定位(DPD)方法。该方法采用量子粒子群优化(QPSO)方法获得定位初值,再利用Newton迭代方法得到精估计值。仿真结果表明:增加观测器的机动性,可对辐射源实现快速无模糊的定位;定位性能在较高信噪比(SNR)下可接近定位误差的克拉美-罗下限(CRLB),在低信噪比下优于多通道干涉仪仅测角(BO)和长基线相位差变化率定位方法。     

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.     

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.     

The Effect of Ground Reflections and Scattering on an Interferometer Direction Finder

The azimuth and elevation angles of an airborne beacon transmitter can be determined using a direction finder comprised of two orthogonal interferometers located near the earth's surface. In this paper we consider the reflection of the incident electromagnetic field by the earth's surface and the resultant effect on the direction-finding system. The analysis yields an upper bound or limit to the interferometer phase error and the corresponding error in bearing angle that can result because of specular reflection and diffuse scattering.     

移相干涉技术用于运动姿态精密测量

针对测量系统运动中姿态测量的需求,提出了一种利用移相干涉技术测量角度的方法来实现运动姿态的准确测量。建立了由组合式移相干涉仪为主的三轴转角实验装置,该装置测量角度的分辨力优于0.006″。采用实验装置对微位移工作台运动过程中的姿态变化进行了测量,得到了工作台移动中绕三轴转角变化的测量结果,并利用激光小角度干涉仪进行了相关测量点的对比实验,数据最大偏差0.03″。实验结果验证了利用移相干涉测角法可以实现运动中姿态的精确测量。     

Monopulse Resolution of Interferometric Ambiguities

Ambiguities in interferometers with high angular accuracy must be resolved to achieve a practical system design. A new technique for ambiguity resolution is described and is based on monopulse circuitry used with the interferometric elements. The overall angular accuracy of the system is achieved by the interferometer; the angular accuracy of the monopulse subsystem is used to resolve interferometric ambiguities. An expression for the probability of correct ambiguity resolution is derived as a function of element size and monopulse accuracy which indicates that high probability of ambiguity resolution results when the size of the interferometric elements are a fraction of the interferometric baseline. Finally, a comparison between conventional monopulse and interferometric system designs is made for the three principal parameters, signal sensitivity, angular accuracy, and field of view, that dictate the appropriate choice for a particular application. Interferometric systems are more appropriate than monopulse systems for those applications in which angular accuracy and field of view are more important than signal sensitivity.     

Pseudo-Noise Interferometer

This paper describes a radio interferometer that can be employed for beaming and attitude determination. It differs from conventional interferometers in that it employs a carrier modulated by a pseudonoise sequence. This enables it to provide an unambiguous angular resolution of the signal source.     

Omnidirectional Instantaneous Direction Finding System

A system for providing near-instantaneous determination of the azimuthal angle of arrival of a received signal has been designed and fabricated at L band. Direction is determined by phase measurement of the outputs of a multimode antenna, which has no moving parts. Effects of multipath interference are reduced to a minimum through a combination of signal processing techniques. The position of the transmitting source is presented on a PPI display.     

Multicomponent receiver architectures for GPS interference suppression

The global positioning system (GPS) is a one-way satellite-based navigation system employing spread-spectrum techniques that is widely used for commercial and military applications. Although the very low signal-to-noise ratio (SNR) is handled by the large spreading gain, GPS is susceptible to high-power interference signals and various types of jammers. We propose multicomponent receiver architectures for GPS interference suppression. A conventional antenna system is first considered which utilizes a minimum-variance distortionless-response (MVDR) beam former and assumes that the GPS signal angle of arrival (AOA) and the antenna model are known at the receiver. However, this receiver is sensitive to AOA estimation errors and can have a high computational complexity. This sensitivity problem is eliminated by a multicomponent system based on a multistage matched filter (MF). Since this MF receiver also has a high computational complexity because the jammer AOAs must be estimated, we introduce a blind interference canceler based on the constant modulus (CM) array that is insensitive to AOA estimation errors and has a low computational complexity. Computer simulations are provided to illustrate the performance of the various systems for interference suppression in example signal scenarios.     

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…     

Doppler Radar Tracking of Hostile Fire

Two fixed parameters define a two-dimensional projectile track relative to a radar: an arrival angle, and the expected distance of closest approach (miss distance). Both parameters can be obtained explicitly, the angle from the history of the phase difference between two elements of an interferometer antenna, and the miss distance from the history of the Doppler frequency.     

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。     

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.     

Characterizing radar emissions using a robust countermeasurereceiver

The author presents a high-sensitivity signal processing approach for detecting and estimating the angle of arrival (AOA), frequency and pulse repetition frequency (PRF) of multiple radar emitters using broadband interferometers. Two time series are generated using information embedded in the sampled cross correlation of the signals obtained from three antenna elements (i.e. two base legs). The phase and amplitude of a complex Fourier transform of these two time series with respect to the sampling clock are used to estimate the AOA and PRF of pulsed emitters     

Theory of angle estimation using a multiarm spiral antenna

An analysis of angle estimation using a single aperture, multimode spiral is presented. It is demonstrated that in addition to the classical Comparison method, modern parameter estimation techniques such as Multiple Signal Classification (MUSIC) and MLM (Maximum Likelihood Method) can be applied to multimode antenna technology for angle estimation. These techniques, coupled with the advent of digital receivers make possible the elimination of the mode-forming network. Results of computer simulations show that these new approaches give lower bias and variance, and also extend coverage toward boresight compared with the traditional Comparison method     

Radio Interferometry of Random Terrestrial Backgrounds

When relative motion exists between the antenna of a microwave radiometer and a random background, the variance of the radiometer output can, in many cases, be primarily determined by the spatial variance of the background. The result is a decrease in signal-to-noise ratio for small target detection. In this paper an interferometer is examined as a potential means of suppressing this background variance until the limiting variance imposed by the receiver is more nearly realized. A bivariate normal distribution in conjunction with three different correlation functions is assumed for the background apparent temperature field. The elements of the interferometer are assumed to possess a very narrow "pencil" power pattern. With relative motion between antenna and background, the audio power density spectrum at the output of the interferometer is exanmned. The background variance suppression achieved by an interferometer is shown to be a function of element spacing, wavelength, correlation distance for the background, range to the background, and form of the correlation function of the background. The result is that a radiometer may be designed which is receiver-limited rather than background-limited.     

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.     

浅析水平扭转M型下滑天线对入口高度及下滑角的影响

对于M型下滑天线,对天线作微调能改变近场DDM分布,从而对入口高度产生影响。本文阐述了扭动天线及调整辐射场形的原理。通过扭动上、下天线,定量分析入口高度和下滑角的改变量与扭动距离的依赖关系,并对飞行校验中的实例进行比较。对比扭动对近场(入口高度)和远场(下滑角)的影响。理论计算得出:上天线向跑道方向扭动一定角度,可以明显提高入口高度,向跑道外侧扭动后,入口高度减小;扭动下天线情况则相反,且两者基本都不影响下滑角。因此表明,扭动天线是一种有效改变近场的方法。