Wideband channel model for aeronautical telemetry

A multipath channel model for wideband aeronautical telemetry links is presented. Channel sounding data were collected at Edwards AFB, CA, at both L-band and lower S-band. Frequency domain analysis techniques were used to evaluate candidate channel models. The channel model is composed of three propagation paths: a line-of-sight direct path and two specular reflections. The first specular reflection is characterized by a relative amplitude of 70% to 96% of the line-of-sight amplitude and a delay of 10-80 ns. This path is the result of "ground bounces" off the dry lake bed at Edwards and is a typical terrain feature at DoD test ranges located in the western United States. The amplitude and delay of this path are defined completely by the flight path geometry. The second path has much lower amplitude and a longer delay. The gain of this path is well modeled as a zero-mean complex Gaussian random variable. The relative amplitude is approximately 2% to 8% of the line-of-sight amplitude. The mean excess delay is 155 ns with an rms delay spread of 74 ns.     

Modeling and performance of HF/OTH radar target classificationsystems

The effects of a class of multipath propagation channels on the performance of a over-the-horizon (OTH) radar target classification system are considered. A Rician frequency-selective fading channel model is employed to characterize the effects of the multipath propagation medium and evaluate the performance of radar target classification systems. The performance of classification algorithms that employ relative amplitude, relative phase, and absolute amplitude measurements as features is investigated. Performance estimates of the various classification algorithms for interesting sets of channel parameters are obtained by means of Monte-Carlo simulations     

PCM/FM aeronautical telemetry in frequency selective multipath interference

The performance of PCM/FM using a delay-line detector in multipath interference is analyzed. In aeronautical telemetry applications, the multipath interference causes a in the signal spectrum which is web modeled by a channel composed of a line-of-sight signal and a strong specular reflection with magnitude /spl Gamma/ and excess delay less than a bit time. Analysis shows that the bit error probability is strongly dependent on the location of the within the spectrum. The bit error probability averaged over the location is also analyzed. For the case of no predetection filtering, the average bit error probability is dependent on the depth of the but not on the width. The loss in performance (relative to the AWGN environment) is bounded by (1-/spl Gamma/)/sup -2/ which is the depth of the . For the case of predetection filtering, the average bit error probability is dependent on both the width and depth of the . The performance loss is bounded by (1-/spl Gamma/)/sup -2/spl Gamma// which is less than that for the case of no predetection filtering.     

Aircraft flight parameter estimation using acoustic multipath delays

The signal emitted by an airborne acoustic source arrives at a stationary sensor located above a flat ground via a direct path and a ground-reflected path. The difference in the times of arrival of the direct path and ground-reflected path signal components, referred to as the multipath delay, provides an instantaneous estimate of the elevation angle of the source. A model is developed to predict the variation with time of the multipath delay for a jet aircraft or other broadband acoustic source in level flight with constant velocity over a hard ground. Based on this model, two methods are formulated to estimate the speed and altitude of the aircraft Both methods require the estimation of the multipath delay as a function of time. The methods differ only in the way the multipath delay is estimated; the first method uses the autocorrelation function, and the second uses the cepstrum, of the sensor output over a short time interval. The performances of both methods are evaluated and compared using real acoustic data. The second method provides the most precise aircraft speed and altitude estimates as compared with the first and two other existing methods.     

Radar measurement extraction in the presence of sea-surface multipath

In the presence of sea-surface multipath monopulse radar signals from a low elevation target have three alternative paths in addition to the direct (radar-to-target) path due to reflections from the sea surface. The specular reflection causes significant signal fading. The diffuse reflection causes an approximately constant bias to the in-phase component of the monopulse ratio, which is the standard extractor of the direction of arrival (DOA) in the monopulse processing. The diffuse reflection also causes higher standard deviation to the in-phase component of the monopulse ratio. We propose a maximum likelihood (ML) angle extraction technique for low elevation targets of known average signal strength having a Rayleigh fluctuation. The results show that this method reduces the error of the estimated angle compared with the conventional monopulse ratio estimator. Subsequently, the ML angle extractor is modified for the unknown average signal strength case. This modified angle extractor has only a small performance degradation compared with the known average signal strength case, but it performs much better than the monopulse ratio based estimator. An algorithm to calculate the accuracy of the estimated angle (or height) is also presented. This angle extractor reduces the root-mean-square error (RMSE) by more than 50% in the signal processing stage when used in a low flying target tracking scenario. The same algorithm can be used to track sea skimmers.     

On Optimizing Array Reception of Multipath

This paper reviews system configuration requirements and analyzes detectability performance characteristics for maximum likelihood array reception of multipath. Performance is analyzed to determine the effects of channel multipath structure (multipath delay and signal power division among the paths), space-time correlation properties of the incident processes, and the array spacing. It is shown by a series of case studies, that for single element coupling, as well as array coupling, an increased multipath delay factor results in decreased system detectability for fixed signal and noise intensity levels. The performance capacity is degraded as the available signal power tends to distribute more uniformly between the paths. These effects are attributed to the loss of effective signal energy concentration, resulting in a lower effective pre-detection signal-to-noise ratio. An investigation of the effects upon system performance, due to array element spacing, shows that performance is enhanced by increasing the spacing relative to the multipath delay factor and the reciprocal signal bandwidth. The former is the result of a more directive detectability (beam) pattern arising from the increased spacing. In effect, with increased spacing, the main lobe of the pattern is narrowed, while the side lobes are optimally suppressed by the required noise related array element link, frequency filters (weights).     

Beamspace ML bearing estimation incorporating low-angle geometry

A problem in low-angle radar tracking, namely, bearing estimation in the presence of a strong specular multipath component that arrives within the beamwidth of the direct path signal, is studied. Three-dimensional beamspace domain maximum likelihood (3D-BDML) is a computationally simple ML bearing estimation algorithm applicable in this scenario which operates in a 3-D beamspace. A variation of 3D-BDML incorporating the multipath geometry as a priori information is presented. In symmetric 3D-BDML the pointing angle of the center beam is equal to the bisector angle between the direct path ray and the image ray, which may be estimated a priori given only the radar height and the target range. The effect of the inclusion of a priori information on the performance of 3D-BDML is analyzed in terms of the dependence on the relative phase difference between the direct and specular path signals, the sensitivity to error in the bisector angle estimate, and the results of operation when no specular multipath component is present in the data. In addition, computationally simple schemes for coherently incorporating multifrequency data into 3D-BDML are investigated     

Adaptive SNR-based carrier phase multipath mitigation technique

An improved technique that mitigates specular multipath in Global Positioning System (GPS) differential carrier phase measurements is described. It adaptively estimates the spectral parameters (frequency, amplitude, phase offset) of multipath in the associated signal-to-noise ratio (SNR), and then constructs a profile of the multipath error in the carrier phase. A multipath correction is subsequently made by subtracting the profile from the actual phase measurement data. The technique is demonstrated on ground based experimental data, as well as flight data from the atmospheric research satellite CRISTA-SPAS. Ground experiments were conducted on static platforms in severe multipath environments. Multipath was deliberately introduced by either strategic placement of reflectors or electronic injection. This allowed for some control over the strength and frequency of the multipath. Averaging the results from 43 ground and 18 flight data sets, the differential carrier phase multipath was reduced by 47%. The complete results for both ground and flight tests are presented and are accompanied by discussions of individual cases     

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     

Multipath Tracking Errors in Elevation-Scanning and Monopulse Radars

The tracking performance of elevation- scanning and monopulse radars in the presence of multipath propagation are compared. The key difference between these two generic types of radars is the way they respond to moving targets. There are no significant differences between their responses to pure specular multipath, nor to diffuse multipath for targets on radial courses. However, they are found to respond quite differently to the diffuse com ponent for low-altitude crossing targets. For these conditions the tracking errors for elevation-scanning radars may be several times those for monopulse radars.     

Multipath Fluctuation Effects in Track-While-Scan Radar

A theoretical model of diffuse multipath reflections from rough surfaces is applied to the prediction of multipath power distribu tions in radar coordinates: elevation angle, time delay, and Dop pler frequency. These distributions are used to predict radar tracking errors in elevation angle, for both monopulse and scan ning antenna systems, and typical results are presented. These show a small increase in tracking error for scanning systems, on radially approaching targets, caused by sensitivity of these trackers to amplitude scintillation of the composite direct-plus multipath signal. Effects of knife-edge diffraction and of vegetation ion are briefly considered.     

航空电子MB-OFDM-UWB无线互连信道分析与仿真

多频带正交频分复用超宽带（MB-OFDM-UWB）是航空电子内部无线互连（WAIC）的候选通信体制之一,不同于普通的室内无线信道,超宽带信号在金属和复合材料的机舱内部存在复杂的反射和多径干扰。给出一种适用于WAIC应用的MB-OFDM-UWB无线通信系统物理层方案,并参照IEEE802.15.4a标准提出一种适应航空电子机舱内部特征的多簇多径UWB信道模型;通过基于接收球的反射角误差法仿真得到无线信道的冲激响应,确定了机舱环境下信道模型的簇到达率、径到达率和混合因子;进而采用Simulink进行MB-OFDM-UWB信号系统仿真,表明在110 Mb/s码速率条件下,当误码率低于国际电信联盟（ITU）标准规定的10^-6,机舱环境下的信噪比应比普通室内环境高1 dB。另外,在研究过程中,为了确认电磁仿真中接收球半径的有效性,针对基准环境将仿真结果与文献报道的实测结果进行了对比。     

Performance Degradation Due to Specular Multipath Intersymbol Interference

Plots of performance degradation are used to compare the effects of fading and intersymbol interference in a two- component specular multipath digital communications channel. Similar plots are then used to compare two practical receivers designed to combat the interference. Degradation plots are shown to allow easy identification of each receiver's range of usefulness, as well as identification of variance bounds demanded of channel parameter estimates which the receivers require.     

基于导航卫星反射信号的空间飞行器探测定位方法

导航卫星信号经由空间飞行器后将发生反射与散射,此信号对于飞行器自身的定位通常作为多径信号被消除。利用双基无源雷达的概念,提出利用该信号实现空间飞行器的远程定位方法。该方法充分利用空间飞行器对于导航卫星信号的反射与散射,在地面/空中设置可以接收此反射信号的接收机,从而实现飞行器的探测。接收机利用码延迟锁定环路实现直射信号与反射信号的锁定,并通过求解直射信号与反射信号的相关功率获得其路径差,进而计算出空间飞行器的位置。介绍了利用延迟映射接收机(DMR)探测空间飞行器的系统组成,并给出了远程定位的数学模型、关键技术分析、DMR的设计原理及验证实验,实验结果表明所设计的DMR可以有效地接收到导航卫星反射信号。为空间飞行器的探测提供了一种新型的手段。     

Frequency Diversity in Low-Angle Radar Tracking

Elevation angle errors due to sea-reflected multipath are evaluated theoretically for a radar operating in an off-boresight monopulse tracking mode. The computer simulation accounts only for specular reflection. Angle estimates at three frequencies are assumed to be available. It is shown that one can improve upon a simple average of the three indicated angles by unequally weighting them according to rank (lowest, middle, or highest). Some sample computations show that there is an optimum difference between the three frequencies.     

航空声学风洞3/4开口试验平台地板对远场噪声测量的影响

利用Kirchhoff-Helmholtz声学理论建立了地板声波反射问题的时域理论模型,推导了地板面上反射波的幅值函数和延迟时间的表达式,并引入了恰当的数值方法对反射声场进行求解,理论方法的正确性得到了实验结果的验证。以典型的高斯调制正弦波脉冲和连续正弦波为例,用建立的理论方法对噪声信号的传播过程和平台地板对远场噪声测量的影响进行了数值模拟研究,结果表明：地板的存在将使得远场接收到的噪声信号中出现直达波和反射波,反射波包含了中心波和边缘波。在低频段,地板宽度增加,远场噪声声压级增大;在高频段,地板宽度增加对远场总噪声级影响不明显。对于高速列车（HST）模型,车底位置的声源比车顶位置的声源在高频段受到地板反射的影响更大。     

The Effect of Multipath Reflections on Spaceborne Interferometer Accuracy

This paper presents a discussion of multipath disturbance of a satellite interferometer. A mathematical expression is developed, in which an upper bound of the error of the measured angle difference and position locations owing to multipath is expressed in terms of geometrical system parameters and the amplitude and phase of the multipath signal. Because diffuse reflection from a rough surface is predominant, the spectrum of multipath reflections is calculated. Multipath rejection by filtering is discussed, and curves for the position errors are presented.     

雷达低空目标俯仰角跟踪的C2算法研究

在单脉冲测角体制下,由于多径回波信号的干扰,极大地降低了雷达低空目标仰俯角跟踪精度,甚至丢失目标。通过对多路径反射环境模型分析,得出了岸、海基单脉冲雷达低空目标跟踪时仰俯角测量误差的产生原因,提出将传统的多目标分辨算法(C2算法)应用于低角多径环境下目标俯仰角的跟踪测量,并在不同多径反射环境下对不同高度、不同飞行速度和飞行方向的目标进行了仿真,得到良好的仿真结果,表明该算法可较大地提高俯仰角跟踪测量精度。通过对仿真结果的分析,验证了该算法在低空目标跟踪中的有效性和可行性。     

海上舰船目标的宽带雷达散射特征信号仿真

提出一种将目标高频电磁（EM）散射计算多径回波模型与不同海况条件下粗糙海面前向复反射系数模型相结合的方法,对海上舰船目标宽带雷达回波进行仿真和分析。简要分析了目标-海面相互作用的多径回波模型,给出了目标体的电磁散射计算方法,研究了不同海况条件下海面前向复反射系数随频率和擦地角的变化特性,并通过仿真分析了海面对舰船目标雷达回波的影响。对比分析舰船目标体直接散射及在不同海况条件下考虑多径效应时的雷达散射截面（RCS）变化特性,并在不同海况条件下对目标进行二维逆合成孔径雷达（ISAR）成像仿真,验证了本文方法的有效性。     

Receiver-channel based adaptive blind equalization approach for GPS dynamic multipath mitigation

Aiming at mitigating multipath effect in dynamic global positioning system (GPS) satellite navigation applications, an approach based on channel blind equalization and real-time recursive least square (RLS) algorithm is proposed, which is an application of the wireless communication channel equalization theory to GPS receiver tracking loops. The blind equalization mechanism builds upon the detection of the correlation distortion due to multipath channels; therefore an increase in the number of correlator channels is required compared with conventional GPS receivers. An adaptive estimator based on the real-time RLS algorithm is designed for dynamic estimation of multipath channel response. Then, the code and carrier phase receiver tracking errors are compensated by removing the estimated multipath components from the correlators’ outputs. To demonstrate the capabilities of the proposed approach, this technique is integrated into a GPS software receiver connected to a navigation satellite signal simulator, thus simulations under controlled dynamic multipath scenarios can be carried out. Simulation results show that in a dynamic and fairly severe multipath environment, the proposed approach achieves simultaneously instantaneous accurate multipath channel estimation and significant multipath tracking errors reduction in both code delay and carrier phase.