The effect of topography on radar scattering from vegetated areas

The ways in which radar scattering from vegetated areas is affected by the topography of the surface underneath the vegetation are discussed. It is shown, using a discrete scatterer model, that the dominant scattering mechanism may change drastically when the ground surface is tilted relative to the horizontal. For a horizontal ground surface, for example, the total scattering may be dominated by scattering off the tree trunks, followed by a reflection off the ground surface. For a relatively small tilt in the ground surface, the ground-trunk interaction term may be replaced by scattering from the branches alone as the dominant scattering mechanism. The effect of the topography is more pronounced for scattering by longer wavelengths, and the implications on algorithms designed to infer forest woody biomass and soil and vegetation moisture using polarimetric SAR data are discussed. The effect of the topography on the scattering behavior from forested areas is illustrated with images acquired by the NASA/JPL three-frequency polarimetric SAR over the Black Forest in Germany     

Segmentation and classification of vegetated areas usingpolarimetric SAR image data

Classification of radar images based on the information provided by individual pixels cannot generally produce satisfactory results due to speckle. The classification based on area analysis is therefore expected to be more accurate, as a uniform area, which usually consists of multipixels, provides reliable measurement statistics and texture characteristics. However, the area analysis requires partitions of uniform areas to be performed first. In this paper, an approach to the classification of radar images is developed based on two steps. First an image is partitioned into uniform areas (segments), and then these segments are classified. Both segmentation and classification are achieved by using the Gaussian Markov random field model. Test images are classified to demonstrate the method     

PolInSAR analysis of X-band data over vegetated and urban areas

This paper investigates the polarimetric and polarimetric interferometric synthetic aperture radar (PolInSAR) information contained in the high-resolution X-band data acquired by the RAMSES airborne SAR system over an area around Avignon, France containing bare surfaces, vegetation, and urban areas. The interferometric coherences are computed over natural and urban areas for all possible baseline copolar polarizations. In the complex plane, the obtained regions of coherence corresponding to most vegetation areas display small angular extents, meaning that if penetration occurs in the foliage, it is shallower than the system height accuracy. To quantify the PolInSAR information, an analysis of the interferometric height accuracy is first performed, and the results are compared with those associated with a theoretical and an empirical model. Concerning vegetation, a 6-m height difference is measured between the different polarimetric phase centers over a sparse pine forest, probably due to the presence of holes in the canopy. Crop study reveals also that wheat-type fields present oriented media properties at X-band due to their vertical structure. Over urban areas, in most cases, building height can be accurately obtained by using Pauli polarimetric phase center information.     

Antenna-polarization and terrain-depolarization effects on pulse-radar return from extended areas at the near vertical

The methods now in the literature for calculating the terrain-backscattered radar-altimeter echo utilize the antenna power-gain patterns and the terrain radar cross section per unit area, usually designated σ0. In this paper, the polarization characteristics of the antennas and the echo depolarization by the terrain are taken into account for the first time. The formulas for the backscattered return are presented in terms of antenna and terrain parameters that can be measured directly or calculated from other measurements. These parameters are the four transmitting- and receiving-antenna gain patterns for the orthogonal polarization components, the two functions giving transmission- and reception-phase difference between the polarization components, the terrain-depolarization factor that accounts for the average backscattered power cross polarized with respect to the incident wave, and the radar cross section σ0per unit area. It is shown that the older formulas used for radar return from the near vertical are correct only when both antennas are linearly polarized and have the same direction of polarization. The errors that result from failure to consider polarization effects are illustrated by examples.     

Use of radar and optical remotely sensed data for soil moisture retrieval over vegetated areas

This work assesses the possibility of obtaining soil moisture maps of vegetated fields using information derived from radar and optical images. The sensor and field data were acquired during the SMEX'02 experiment. The retrieval was obtained by using a Bayesian approach, where the key point is the evaluation of probability density functions (pdfs) based on the knowledge of soil parameter measurements and of the corresponding remotely sensing data. The purpose is to determine a useful parameterization of vegetation backscattering effects through suitable pdfs to be later used in the inversion algorithm. The correlation coefficients between measured and extracted soil moisture values are R=0.68 for C-band and R=0.60 for L-band. The pdf parameters have been found to be correlated to the vegetation water content estimated from a Landsat image with correlation coefficients of R=0.65 and 0.91 for C- and L-bands, respectively. In consideration of these correlations, a second run of the Bayesian procedure has been performed where the pdf parameters are variable with vegetation water content. This second procedure allows the improvement of inversion results for the L-band. The results derived from the Bayesian approach have also been compared with a classical inversion method that is based on a linear relationship between soil moisture and the backscattering coefficients for horizontal and vertical polarizations.     

基于低秩杂波子空间的杂波对消器

提出了一种利用低秩杂波子空间（L RCC ）的杂波对消器,该方法利用相对较少的杂波子空间正交基构造出原杂波,然后再对消相邻脉冲间的杂波回波,所需要的杂波信息矩阵维数相对于原二维脉冲杂波对消器（TDPC）来说明显减小,而杂波滤波性能没有损失。     

Tracking Refractivity from Clutter Using Kalman and Particle Filters

We address the problem of tracking the spatial and temporal lower atmospheric variations in maritime environments. The evolution of the range and height-dependent index of refraction is tracked using the sea clutter return from sea-borne radars operating in the region. A split-step fast Fourier transform based parabolic equation approximation to the wave equation is used to compute the clutter return in complex environments with varying index of refraction. In addition, regional statistics are incorporated as prior densities, resulting in a highly nonlinear and non-Gaussian tracking problem. Tracking algorithms such as the extended Kalman, unscented Kalman and particle filters are used for tracking both evaporative and surface-based electromagnetic ducts frequently encountered in marine environments. The tracking performances and applicability of these techniques to different types of refractivity-from-clutter problems are studied using the posterior Cramer-Rao lower bound. Track divergence statistics are analyzed. The results show that while the tracking performance of the Kalman filters is comparable to the particle filters in evaporative duct tracking, it is limited by the high non-linearity of the parabolic equation for the surface-based duct case. Particle filters, on the other hand, prove to be very promising in tracking a wide range of environments including the abruptly changing ones.     

混沌学与海杂波

主要介绍混沌学形成的概要情况,同时还介绍了混沌学的主要特点和分析确认一个系统是否服从混沌的判断准则和方法,描述了分析海杂波服从混沌特征的方法,并提出了雷达处理海杂波的分析方法和实施途径.     

海面杂波模拟

首先推导了特定海情下海杂波的平均雷达截面积,然后讨论了海杂波的幅度起伏模型,并给出了一种产生服从对数正态分布随机序列的算法。海杂波的功率谱模拟通常采用的是高斯模型,这与实际情况有所出入,所以采用频域取样法对模拟序列进一步处理,产生即具有特定概率分布,又具有所要求的功率谱的海杂波模拟序列。     

Radar return from a continuous vegetation canopy

The radar backscatter coefficientsigmadegof alfalfa was investigated as a function of both radar parameters and the physical characteristics of the alfalfa canopy. Measurements were acquired with an 8-18 GHz FM-CW mobile radar over an angular range of0deg-70degas measured from nadir. The experimental data indicate that the excursions ofsigmadegat nadir cover a range of nearly 18 dB during one complete growing cycle. An empirical model forsigmadegwas developed, which accounts for its variability in terms of soil moisture, plant moisture, and plant height.     

瑞利与非瑞利杂波背景下单、双参数杂波图的性能分析

对瑞利和非瑞利杂波中的单、双参数杂波图的控制虚警性能和检测性能进行了比较分析，并得到了双参数杂波图门限调整值的取值范围，得出了几点有益的结论。     

一种多频道运动杂波抑制技术

研究雷达在杂波环境下的自适应杂波抑制技术,介绍了参差时变多凹口滤波器设计方法,给出了一种带通滤波器实现方法,从而说明全频道杂波轮廓图的工作原理,利用杂波轮廓图信息确定杂波所在的多普勒频率范围,以此选择相应频道的MTI滤波器,实现AMTI,事实已证明该技术的可行性。     

PD中重频下主瓣杂波特性及抑制滤波器设计

针对中重频PD雷达应用背景,设计了一种主瓣杂波抑制滤波器。对主瓣杂波在实际情况下的参数特点进行了详细分析,得出杂波强度、带宽和功率与俯仰角的对应关系,据此提出了采用查表法设计主瓣杂波抑制滤波器方法;该方法克服了对消器和AMTI滤波器的不足,能有效改善杂波抑制性能,其算法存储容量和实时性满足设计要求,具有良好的工程应用价值。     

动杂波速度估值的工程实现

在AMTI雷达信号处理中,为综合利用目标回波的各项信息,提高目标的分辨精度及检测概率,需要有效去除动杂波,其中,杂波速度估值对于动杂波抑制具有重要作用。着重讨论了杂波速度估值原理和试探滤波器时变加权设计,并给出了在某雷达信号处理系统中杂波速度估值的设计及其实现。     

Clutter suppression method for heterogeneous environment

Clutter suppression is a key process in ground moving target indication (GMTI). Adaptive filtering is an effective method for clutter suppression but the performance degrades severely in heterogeneous clutter environment. To solve this problem, a new clutter suppression method is proposed. The approach uses subspace tracking technique to update clutter subspace of different clutter patches which may mitigating the heterogeneous effects. Simulation results illustrate that the method performs well when lacking of the independent and identically distributed (i.i.d.) samples.     

提升小波去地杂波方法

风廓线雷达去地杂波一直是一个较难解决的问题,基于提升小波理论的滤除风廓线雷达地杂波信号具有很多优势,例如抑制杂波效果好、运算量小、易于工程实现等。介绍了提升小波理论,然后分析风廓线雷达信号特征,提出提升小波去地杂波方法,最后给出仿真结果和结论。这种方法将被应用于某型在研边界层风廓线雷达实时信号处理中。     

雷达杂波的仿真与实现

论述了一种雷达杂波信号的模拟方法及其硬件实现。该模拟器采用软硬件相结合的方法,由计算机实时解算每个杂波回波单元的方位、距离、散射系数等参数,然后将数据发送到硬件电路中,由硬件电路完成对雷达信号的采样、调制、恢复、幅度控制,最终产生杂波信号并输出。     

基于统计模型的杂波仿真

准确而有效地模拟雷达杂波信号,对于机载PD雷达的目标检测具有重要意义。提出一种基于统计模型的ZMNL法对杂波仿真,阐述了该方法的思路和实现步骤,给出实现模型和仿真结果。计算机仿真的结果证明,用该方法来描述雷达杂波的＂长拖尾＂是合理的,为机载PD雷达检测提供了依据。     

雨杂波的特性及仿真

主要研究并总结雨杂波的一些基本特性及降水回波的涨落现象,用ZMNL(零记忆非线性)方法对雨杂波的幅度特性和功率谱特性进行仿真,并与理论值进行比较,结果表明两者符合较好。