SAR辐射定标精度设计与分析

合成孔径雷达（SAR）辐射定标是一项关联图像像素值和地物后向散射系数的重要工作。文中在分析SAR系统的误差源和辐射定标原理的基础上,研究了无源辐射定标和有源辐射定标的精度设计原理。以机载SAR、Pi-SAR定标结果为例,结合雷达散射计的测试结果,分析了相同条件相似地物的后向散射系数的差异及相应的原因。理论分析和数据比较表明了辐射定标精度设计和比较的合理性,为定标精度和结果分析提供了理论支持和技术途径。     

对SAR电子干扰的初步分析

在分析SAR对点目标回波、分布式地物杂波和噪声处理特点的基础上,总结了对SAR有源电子干扰的特点,给出了对设计SAR对抗系统有指导意义的结论。应用信号叠加的方法进行了仿真,验证了基于干扰等效后向散射系数统计特性分析的干扰效果评价方法。     

亚马逊热带雨林星载SAR辐射定标算法

天线外定标是实现星载SAR天线方向图在轨测试的主要手段。天线方向图的准确测量，对星载SAR图像的应用具有重要作用。分布目标辐射定标在星载SAR天线方向图测试中受到广泛重视。亚马逊热带雨林的后向散射系数呈现大面积的均匀性和稳定性，故成为分布目标定标最主要的定标场。该文在分布目标定标原理的基础上，对基于亚马逊热带雨林的星载SAR天线方向图测量算法进行了研究，提出了采用K．Pearson统计量非参数假设检验的一致性处理算法，并采用Levenberg-Marquard算法（LM算法）估计俯仰向天线方向图模型参数。仿真数据验证了算法的可靠性和准确性。     

一种复杂地物目标近垂直后向散射系数反演方法

复杂地物目标的近垂直后向散射特征是影响雷达高度表回波波形的重要因素之一。对近垂直后向散射系数的获取主要采用基于参数模型的实测数据拟合方法,但一般只适用于地势平坦且介质相对单一的地海面目标。本文基于机载雷达高度表,针对复杂地物目标的距离-多普勒图像仿真实验数据和实测数据,利用距离-多普勒域和地球空间域的映射关系,提出一种后向散射系数的反演算法,并对得到的后向散射系数进行定量分析。实验验证了提出的反演算法具有一定的适用性和参考性。     

基于GPU加速和RCS演算的SAR回波生成系统

提出了一种基于GPU加速的合成孔径雷达(SAR)回波生成系统。为了使生成的回波更加精确,该系统考虑了SAR照射场景中散射中心的雷达散射截面积(RCS)在合成孔径时间内的变化。这种现象在雷达入射角度变化较大时是不可忽略的。通过RCS的高频近似,给出了后向散射系数关于入射角和瞬时斜距的表达式。由此,在给定数字高程信息(DEM)和雷达入射角度范围后,照射场景中每个散射中心在不同方位时刻的后向散射系数就可以精确的计算得到。然后,详细介绍了基于GPU和CPU混合的SAR回波生成系统的实现过程。该系统根据SAR回波矩阵的对称特性降低了回波生成的运算复杂度,并运用了GPU矩阵运算的能力,大大提升了SAR回波矩阵的运算效率。文中的实验结果证明,该系统有很高的计算效率和输出精度。     

太赫兹雷达RCS测量中的分时定标技术

太赫兹频段的目标散射特性测量技术是当前太赫兹雷达的重要研究方向,其中系统定标技术决定了雷达散射截面积(RCS)测量结果的准确性。使用基于微波倍频源的太赫兹宽带雷达目标散射特性测量系统,该系统由微波源经倍频后,中心频率达到440 GHz,带宽达25.6 GHz。利用光滑表面金属球为标准体,采用分时定标技术对太赫兹雷达系统进行定标,再对金属材质的战斗机模型和吉普车模型进行近场RCS测量实验,获得以上2种典型人造目标的近场RCS测量结果。测试结果与理论趋势符合良好,证明了太赫兹雷达系统RCS测量中分时定标技术的有效性。     

低空小型无人机贝叶斯学习超分辨ISAR成像

本文针对低空小型无人机在雷达探测中散射截面积小、相干积累时间短等问题,提出一种基于贝叶斯统计机器学习的逆合成孔径雷达超分辨成像方法。利用无人机相对空域背景的稀疏性先验知识引入重尾的拉普拉斯先验概率分布,并基于观测系统噪声高斯分布假设建立贝叶斯后验推理模型。针对先验分布的非共轭性,引入分层贝叶斯模型。最后应用变分贝叶斯期望最大算法,解析求解目标后向散射系数后验概率密度函数,并校正目标非系统性平动误差及其造成的成像散焦。与传统方法相比,该方法能够有效解决无人机目标雷达散射截面积较小带来的成像信噪比低以及相干积累时间较短带来的成像分辨率低等问题。仿真实验结果证明了本文所提方法的有效性和优越性。     

复杂自然场景双站SAR成像模拟

以单站SAR复杂场景成像模拟的映射投影(MPA)算法为基础,实现三维投影和映射的双站SAR(BISAR)成像模拟.利用条带式BISAR成像的点目标响应解析表示式,在计算得到散射系数图后,模拟BISAR原始信号的产生与压缩,以模拟BISAR图像.经过对虚拟场景的BISAR成像模拟,讨论BISAR对植被、建筑物等复杂地物观测及其与单站SAR的对比.     

射频仿真系统中地杂波简单模型设计实现

主要介绍了地杂波各种概率分布统计模型、后向散射截面积以及特定地面条件下的后向散射系数。文章详细讨论了地杂波的工程实现,就工程中各种调制方式进行了论述,以期对射频仿真系统的建设起到积极作用。     

红外相机实时绝对辐射定标技术研究

辐射定标技术是实现定量遥感的关键环节。近年来,随着红外遥测技术愈发成熟,星上红外辐射定标已成为空间定量遥感技术的重要发展方向。文中从红外相机实时绝对辐射定标的背景出发,提出了半光路星上绝对辐射定标和基于多温度场的场地绝对辐射定标方法,结合实验数据,分别采用星上定标、场地定标、交叉定标三种方案进行在轨绝对辐射定标实验验证,并对其适用场景进行了分析。结果表明,通过结合半、全光路定标数据处理和转换技术,利用水面场和陆面场的场地绝对辐射定标方法,优选合适的定标场地,同时在陆面场中增加典型地物场景实现多温度场定标的方法,所提出的辐射定标方法实现了实时高精度绝对辐射定标,定标精度优于1.5 K。     

Calibration of a polarimetric imaging SAR

Calibration using point targets is discussed. The fourport network calibration technique is used to describe the radar error model. The processor ambiguity function and the radar distortion matrices are combined to form a generalized polarimetric ambiguity function. The polarimetric ambiguity function of the SAR is found using a single point target, namely a trihedral corner reflector. Based on the resultant polarimetric ambiguity function, an estimate for the backscattering coefficient of the terrain is found using a modified version of the single target calibration technique (STCT). A radar image recorded by the JPL aircraft SAR, which includes a variety of point targets, is used for verification of the new calibration method. The calibrated responses of the point targets are compared both with theory and responses based on the POLCAL technique     

多极化SAR 机载测量中的定标方法

针对极化合成孔径雷达由于发射和接收端受通道幅相不平衡、天线串扰、电磁波传播特性和系统噪声等因素影 响,从而造成遥感数据不能反映真实目标特性的特点,提出了应用方位对称的随机分布目标和多个三面角反射器回波数 据定标系统的方法,给出了应用该方法的具体步骤,并通过此方法对X 波段机载SAR 系统的实测数据进行了分析,试验 结果验证了该方法的可行性和有效性。     

Polarimetric calibration of SIR-C using point and distributedtargets

In preparation for the Shuttle Imaging Radar-C/XSAR (SIR-C/XSAR) flights, the University of Michigan has been involved in the development of calibration procedures and precision calibration devices to quantify the complex radar images with an accuracy of 0.5 dB in magnitude and 5 degrees in phase. In this paper, the preliminary results of the SIR-C calibration and a summary of the University of Michigan's activity in the Raco calibration super-site is presented. In this calibration campaign an array of point calibration targets including trihedral corner reflectors and polarimetric active radar calibrators (PARCs) in addition to a uniform distributed target were used for characterizing the radiometric calibration constant and the distortion parameters of the C-band SAR. Two different calibration methods, one based on the application of point targets and the other based on the application of the distributed target, are used to calibrate the SIR-C data and the results are compared with calibrated images provided by JPL. The distributed target used in this experiment was a field of grass, sometimes covered with snow, whose differential Mueller matrix was measured immediately after the SIR-C overpass using The University of Michigan polarimetric scatterometer systems. The scatterometers were calibrated against a precision metallic sphere and measured 100 independent spatial samples for characterizing the differential Mueller matrix of the distributed target to achieve the desired calibration accuracy. The L-band SAR has not yet been adequately calibrated for inclusion here     

Development of a ground-based radar for scattering measurements

A ground-based C-band scatterometer system has been constructed at the Multimedia University, Malaysia. This is an inexpensive FM-CW radar that was efficiently constructed from a combination of commercially available components and in-house fabricated circuitry. The system has full polarimetric capability for determining the complete backscattering matrix of a natural target. It will be used to conduct in-situ backscatter measurements on Earth terrain, such as vegetation fields, forests, and soil surfaces. This paper presents the system design and the evaluation results of the scatterometer system. The system was tested at a football field. A metal sphere was used as the calibration target. The backscattering matrices of an 8" trihedral corner reflector and of a 4" /spl times/ 8" dihedral corner reflector were measured. The dihedral was rotated at different angles to provide different sets of polarimetric data. The results were compared with the theoretical values to verify the effectiveness of the calibration technique. The external calibration and internal calibration procedures, as well as the approach used in measuring the relative phase response, are discussed.     

Cross-calibration experiment of JPL AIRSAR and truck-mountedpolarimetric scatterometer

When point calibration targets are used to calibrate a SAR image, the calibration accuracy is governed by two major factors. The first factor stems from the stringent requirement on the radar cross section (RCS) of the point calibration target. To reduce the effect of radar return from the background, the RCS of a point calibration target must be much larger than that of the background. Calibration targets with large RCS require large physical dimensions for passive targets or high amplifier gain for active targets, which in practice leads to uncertainty in the nominal RCS of the targets. The second factor is related to the fact that point calibration targets are used to develop a calibration algorithm which is applied to distributed targets. To this end, accurate knowledge of the impulse response (ambiguity function) of the SAR system is required. To evaluate the accuracy of such a calibration process, a cross-calibration experiment was conducted at a test site near Pellston, MI, using the JPL aircraft SAR and the University of Michigan truck-mounted polarimetric scatterometer. Five different types of distributed surfaces, all in the same area, were chosen: three of these were bare surfaces with varying roughnesses, and the other two were covered with vegetation. Trihedral corner reflectors were used for calibrating the aircraft SAR, and the UM scatterometer was calibrated using a metallic sphere. The scatterometer data were collected at L and C bands immediately after the aircraft flew over the test site. This paper presents results of the cross calibration between the polarimetric SAR and ground-based polarimetric scatterometer measurements at L and C bands. Comparison of the data measured by the two radar systems shows that SAR calibration with trihedrals may lead to unreliable results. A distributed-target calibration technique is introduced and applied to the data with good results     

The gridded trihedral: a new polarimetric SAR calibration reflector

The characteristics of the gridded trihedral used to calibrate polarimetric synthetic-aperture radar (SAR) systems are considered. The gridded trihedral is a normal trihedral with one of the conducting faces replaced with a grid of closely spaced (relative to a wavelength) parallel conductors over a layer of microwave absorber. This grid changes the incident wave's polarization and results in a calibration target with a significant cross-polarized reflection. The advantages of the gridded trihedral are that it has a broad backscatter beamwidth (unlike the dihedral), and that it is passive and simple to construct (unlike the active radar calibrator, or ARC). The performance characteristics of the gridded trihedral reflector are reviewed, using basic theoretical models and calibrated P-3 SAR imagery     

Radar polarimeter measures orientation of calibration corner reflectors

We have analyzed radar polarimeter signals from a set of trihedral corner reflectors located in the Goldstone Dry Lake in Cafifornia, and observed three types of scattering behavior: i) Bragg-like slightly rough surface scattering that represents the background signal from the dry lake, ii) trihedral corner reflector scattering that returns the incident polarization, and iii) two-bounce corner reflector scattering resulting from a particular alignment of a trihedral reflector. In the latter case, we can measure within about 3° the orientation angle of the apparent dihedral trough, even though the 2-m reflector is much smaller than the 10-m resolution element of our radar. Thus a radar calibration approach using trihedral corner reflectors should be designed such that precise alignment of the reflectors is ensured, as three-bounce and two-bounce geometries lead to very different cross sections and hence very different inferred calibration factors.     

Design criterion for inclined planar radomes on radar corner reflectors used for spaceborne SAR calibration

Larger corner reflector ground targets currently planned for spaceborne SAR calibration will require planar A-sandwich radome covers for weather protection and mechanical stability. The radomes must be inclined with respect to reflector apertures, however, to avoid scattering cross-section uncertainties in the target beam peak direction. A criterion for inclination angle is derived here.     

Polarimetric calibration of the SIR-C C-band channel using activeradar calibrators and polarization selective dihedrals

A polarimetric calibration experiment of Shuttle Imaging Radar C (SIR-C) is carried out using several different calibration targets. These are C-band polarimetric active radar calibrators (PARCs), polarization selective dihedrals (PSDs), 22.5° rotated dihedrals, and a trihedral. A novel polarimetric calibration algorithm is proposed that combines existing algorithms and uses one PARC and two PSDs. An error evaluation example is shown to estimate the typical hardware error value of the calibration targets allowable for a given calibration error. The novel algorithm gives polarimetric calibration results comparable to those obtained using the existing algorithm for three PARCs. Since PSDs work at frequencies lower than design frequency, and hence can be commonly used at multiple frequency bands, the simple addition of just one more frequency band PARC allows polarimetric calibration of a dual-frequency polarimetric synthetic aperture radar (SAR) by means of the proposed algorithm