多子阵SAS方位空变运动补偿子孔径算法

为解决方位空变的侧摆和偏航误差存在情形下合成孔径声纳的快速运动补偿与成像问题,提出一种多子阵合成孔径声纳方位空变运动补偿子孔径算法。首先,建立运动误差存在情形下的双程距离历程模型,并利用泰勒级数展开对双根号形式距离历程进行近似;然后,利用子孔径运动补偿和单基等效处理,将含有方位空变的侧摆和偏航误差的多子阵回波数据转换为理想的单阵回波数据;最后,利用经典的单阵频域逐线成像算法,实现快速运动补偿和高分辨成像。仿真实验与实测数据成像结果均验证了所提算法的有效性。

多方位角融合SAR图像中“扇形”旁瓣分析

随着相控阵天线电子扫描能力的提升,多方位角观测成为星载合成孔径雷达(Synthetic Aperture Radar,SAR)发展方向之一,从多维度提高目标信息解译能力。然而,在公开的多方位角融合SAR图像中存在较强的二维“扇形”旁瓣现象,且二维分布特点不同,容易造成目标误判。因此本文针对此旁瓣方向和多方位角观测目标分辨率开展研究。首先,概述了多方位角观测模式和特性,提出了基于CUDA并行加速的快速后向投影(Back Projection,BP)算法,高效率生成多方位角序贯SAR图像,避免耗时的图像配准和几何校正;其次,基于多方位角观测中斜距平面和地距平面的几何投影关系,重点分析了地距平面目标方位向、距离向分辨率和旁瓣方向,揭示了融合SAR图像中“扇形”旁瓣的形成机理,并解释了二维旁瓣特性不同的原因,基于此可反演获取该SAR图像时卫星和观测场景之间的几何关系,以及融合SAR图像数;最后,通过点目标和扩展目标仿真开展实验,结果表明方位斜视观测中,地距平面方位向、距离向分辨率理论分析精度优于0.02 m,旁瓣方向理论分析精度优于1o,验证了所提分析方法的正确性。

多子阵合成孔径声纳距离-多普勒成像算法

基于相位中心近似(PCA)的传统多子阵合成孔径声纳(SAS)成像方法忽略了近似误差的方位空变性,使得分布式目标的聚焦结果发生畸变。为解决这个问题,该文从收发阵元空间分置采样与相位中心近似采样的几何模型出发推导了一种考虑近似误差方位空变性的双程斜距历程,并将多子阵合成孔径声纳2维频域系统函数分解为收发分置畸变项和类收发合置项。在此基础上,采用复数相乘、插值实现收发分置畸变项的补偿,并利用距离-多普勒算法进行成像处理。相对传统方法,该文方法在整个测绘带内的近似误差更小,不会带来方位向上的位置偏移,能得到与真实目标位置一致的成像结果。

SAR实时快速处理抗主瓣欺骗式干扰方法

主瓣欺骗干扰与真实目标场景具有相同的角度,二者在多个表征域具有高度相似性,严重降低了合成孔径雷达系统对真实目标的检测和判断能力。针对该问题,为保证有效抑制主瓣欺骗干扰,文中提出了一种实时快速处理抗主瓣欺骗干扰算法。首先,提出了一种基于方位相位编码的回波恢复框架,并且该框架能够对原始回波数据进行分组处理;其次,提出了一种模块化处理的快速算法来加快所提方法的处理效率;最后,通过点目标和分布式目标仿真实验验证了所提方法的有效性和可行性。通过与传统主瓣欺骗干扰方法进行对比,文中所提方法不仅能够降低计算复杂度,还能减少对系统资源的占用。

平面相控阵的色散误差非延时补偿

本文基于波束指向精度是相控阵天线重要的性能指标,频率色散误差是一种主要的天线指向误差,提出了一种非延时的色散误差补偿方法,给出了非均匀平面阵的色散误差机理和全空域的色散误差仿真结果。同时,通过将其对应的最大瞬时带宽进行理论推导,给出了基于指挥控制系统的补偿办法。结果表明:理论计算结果和仿真结果一致。

微小型无人机载FMCW SAR宽波束运动补偿算法

调频连续波(frequency modulated continuous wave,FMCW)体制的合成孔径雷达(synthetic aperture radar,SAR)具有较宽的方位向波束,运动误差在成像处理时会产生较为严重的方位空变性。对此,建立了具有三轴运动误差的信号模型;对宽波束情况下的距离和方位的耦合性以及由运动误差导致的方位空变性进行了分析;在视线向(line of sight,LOS)引入传统的频率分割算法来补偿方位空变误差,在航向将运动误差建模为包络误差与相位误差;提出了“逐方位块补偿法”来消除相位误差中的空变性。最后,通过仿真实验验证了所提算法的有效性。

PERFORMANCE ANALYSIS OF AZIMUTH ELECTRONIC BEAM STEERING MODE SPACEBORNE SAR

Pointing angle and pattern of the antenna can be changed swiftly to actualize the azimuth beam scanning by using electronic beam steering, which makes the Synthetic Aperture Radar (SAR) system more flexible and achieve a high resolution or cover a long strip within short time span. When the pointing angle is steered away from boresight, some aberrations may appear on the antenna pattern, e.g., the grating lobe appears, the main lobe gain decrease, and antenna pattern broadens, e.g., the aberrations result in the worsening of system performance, and complicate the corresponding performance analysis method. Conventional computation methods of performance parameters do not account for the rapid change of the antenna pattern. It introduces remarkable errors when the scanning angle is large. In this paper, a method of calculating performance parameters is proposed for the beam steering mode, which achieves the parameters by the energy accumulation in time domain. Actually, the proposed method simulates the working process of SAR and obtains accurate performance parameters. Furthermore, we analyze the effects of the grating lobe on the Azimuth Ambiguity to Signal Ratio (AASR), and present the generic Pulse Repetition Frequency (PRF) choosing principle which can also prevent the ambiguous area from weighting by the grating lobe. Finally, the effect of the antenna configuration on the performance parameters is analyzed by a system example.

Numerical simulation of high-resolution azimuthal resistivity laterolog response in fractured reservoirs

The high-resolution azimuthal resistivity laterolog response in a fractured formation was numerically simulated using a three-dimensional finite element method. Simulation results show that the azimuthal resistivity is determined by fracture dipping as well as dipping direction, while the amplitude differences between deep and shallow laterolog resistivities are mainly controlled by the former. A linear relationship exists between the corrected apparent conductivities and fracture aperture. With the same fracture aperture, the deep and shallow laterolog resistivities present small values with negative separations for low-angle fractures, while azimuthal resistivities have large variations with positive separations for high-angle fractures that intersect the borehole. For dipping fractures, the variation of the azimuthal resistivity becomes larger when the fracture aperture increases. In addition, for high-angle fractures far from the borehole, a negative separation between the deep and shallow resistivities exists when fracture aperture is large as well as high resistivity contrast exists between bedrock and fracture fluid. The decreasing amplitude of dual laterolog resistivity can indicate the aperture of low-angle fractures, and the variation of the deep azimuthal resistivity can give information of the aperture of high-angle fractures and their position relative to the borehole.

Detection, Estimation and Compensation of Ionospheric Effect on SAR Interferogram Using Azimuth Shift

An increasing interest in the use of low frequency Synthetic Aperture Radar(SAR)systems,e.g.,L-and P-bands,makes the research of the ionospheric effects on SAR interferograms become urgent and significant.As the most pronounced signature in interferograms,the ionosphere-induced azimuth streak was thoroughly investigated in this study through processing of the 19 L-band Advanced Land-Observing Satellite(ALOS)Phased Array type L-band Synthetic Aperture Radar(PALSAR)images over the Chongqing City,China.The investigations show that the visible ionosphere-induced stripe-shape azimuth shifts with the invariable direction of 26°E,113°N are observed in some interferometric pairs.Relating these anomalous azimuth shifts to the International GNSS Service(IGS)final ionospheric products shows that the detected ionosphere-contaminated SAR images display the relatively large ionospheric variation with time during SAR satellite travelled through the study area,indicating a somewhat correlation between them.After detecting the ionosphere-contaminated interferograms,we estimated the Ionospheric Phase Streak(IPS)based on an approximate linear relationship between IPS and azimuth shift,and then removed them from the original interferograms.The corrected results show that ionospheric phase patterns are largely removed from the ionosphere-contaminated interferograms.The investigation indicates that the direction of the IPS keeps approximately constant in space and time,which provides the potential chance to develop methods to correct the ionospheric effect.Furthermore,this study once more proves that the ionospheric effect on SAR interferogram can be detected,estimated and corrected from azimuth shifts.

A FAST APPROXIMATE IMAGING ALGORITHM FOR SPACEBORNE SYNTHETIC APERTURE RADAR

A fast algorithm of azimuth processing for spaceborne aperture radar (SAR), which is called SPECAN algorithm, is proposed. Compared with general frequency algorithms, SPECAN has advantages in computation, memory and structure. SPECAN algorithm is very important for surveying real time processing of spaceborne SAR. The structure and special problems for SPECAN algorithm used in spaceborne SAR are mainly engaged in this paper. Finally, raw data of SEASAT-A satellite is used to verify the feasibility of SPECAN algorithm.

Azimuth resolution improvement for spaceborne SAR images with quasi-non-overlapped Doppler bandwidth

The azimuth resolution improvement problem is solved via a coherent combination of synthetic aperture radar （SAR） ima-ges with the quasi-non-overlapped Doppler bandwidth. Prior to the spectra combination, SAR images should be coregistered, while phase biases induced by topography, atmospheric propagation delays and baseline measurement errors should be calibrated. However, the coregistration accuracy suffers from large Doppler decorrelation caused by the quasi-non-overlapped Doppler band-width. Furthermore, the method used to estimate phase biases from interferogram of azimuth prefiltered SAR image pairs wil fail when there is no overlapped spectrum. The fringe simulation and maximum sharpness optimization are adopted to deal with the problems. Accordingly, a novel algorithm to coherently synthesize SAR images is presented. The experiment with the Terra SAR X-band （TerraSAR-X） satel ite data validates the performance of the presented method.

Bistatic Synthetic Aperture Radar Point Spread Function Characteristic Analysis

Based on the point spread function （PSF） theory, the side-lobe extension direction of the impulse response in bistatic synthetic aperture radar （BSAR） is analyzed in detail; in addition, the corresponding autofocus in BSAR should be considered along iso-range direction, not the traditional azimuth resolution （AR） direction. The conclusion is verified by the computer simulation.

New slant range model and azimuth perturbation resampling based high-squint maneuvering platform SAR imaging

Strong spatial variance of the imaging parameters and serious geometric distortion of the image are induced by the acceleration and vertical velocity in a high-squint synthetic aperture radar(SAR)mounted on maneuvering platforms.In this paper,a frequency-domain imaging algorithm is proposed based on a novel slant range model and azimuth perturbation resampling.First,a novel slant range model is presented for mitigating the geometric distortion according to the equal squint angle curve on the ground surface.Second,the correction of azimuth-dependent range cell migration(RCM)is achieved by introducing a high-order time-domain perturbation function.Third,an azimuth perturbation resampling method is proposed for azimuth compression.The azimuth resampling and the time-domain perturbation are used for correcting first-order and high-order azimuthal spatial-variant components,respectively.Experimental results illustrate that the proposed algorithm can improve the focusing quality and the geometric distortion correction accuracy of the imaging scene effectively.

H_∞ controller design for a 4-meter direct-drive azimuth axis

To pursue a higher imaging resolution for exploring more details in the information conveyed by the Universe, the next generation of optical telescopes based on a direct drive widely employ the extremely large aperture structure, which also introduces more disturbances and uncertain factors to the control system. Facing this new challenge, the PID control method in main-axis control systems of traditional astronomical telescopes cannot suffice for the requirement of the tracking precision and disturbance sensitivity in angular velocity. To overcome this shortcoming, we establish a dynamic model and propose an H∞ controller for a 4-meter azimuth direct drive control system that consists of a revolving platform（azimuth axis）, a three-phase torque motor, a motor drive, an encoder, a data acquisition card and a small computers. Simulations are carried out to analyze the model and guide the real experiments.Experimental results show that the proposed H∞ controller reduces the tracking error by a maximum of 80.69%（average 57.8%） and the disturbance sensitivity by a maximum of 82.3%（average 50.96%） compared with the traditional tuned PI controller;furthermore, the order of the model describing the proposed controller can be reduced to three, thus its feasibility in real systems is guaranteed.

基于反正弦圆环天线阵列的二维成像

雷达成像技术可获得目标的丰富特征信息,从而为目标识别提供重要依据。涡旋电磁波雷达成像技术可获得相对静止目标的高分辨率二维像,是成像领域的研究热点之一。为获得更高的成像质量,利用反正弦圆环天线阵列获得雷达波束内的方位角分辨能力,提出了一种基于反正弦圆环天线阵列的二维成像算法。利用半个圆环天线阵列,获得与涡旋电磁波雷达成像技术相近的方位角分辨率,并降低了旁瓣高度。对回波进行Dechirp处理,获得一维距离像;在此基础上,构建参考信号,通过信号间的共轭相乘将相位中的余弦函数变为线性函数;将回波累加,获得目标二维像。仿真分析了阵元个数和波束宽度对方位角分辨率的影响,并与涡旋电磁波雷达成像算法对比,验证了所提算法的有效性和鲁棒性。

直反信号协同的GNSS-R BSAR距离多普勒成像算法

针对目前基于全球导航卫星系统反射信号的双基地合成孔径雷达(GNSS-R BSAR)在一站固定模式下的大斜视,斜距历程复杂,回波信号方位空变导致回波信号难以处理的问题,提出改进的距离多普勒成像新算法。所提算法采用GNSS信号作为辐射源,根据一站固定模式下GNSS-R BSAR合成孔径时间长的特点,引入高阶等效斜视距离模型,得到导航卫星与目标斜距相对时间变化的精确描述。先通过直射信号与回波信号时域对消进行距离徙动校正,实现全场景目标距离徙动的精确校正;再通过方位向分块混合相关处理来克服回波信号方位向的移变性质,实现全场景高效精确成像。所提算法的成像效率优于传统后向投影时域(BP)算法,成像精度与BP算法相当,且可根据需要通过调整方位分块的宽度来提升聚焦效果。最后,用GPS-L5信号进行仿真和实验,仿真和实验结果验证了所提算法的可行性和高效性。

舰船目标实时ISAR分辨率评估及成像方法

实时逆合成孔径雷达(Inverse Synthetic Aperture Radar,ISAR)分辨率评估是判断舰船目标成像质量的关键,其评估结果直接决定了是否需要重复成像和应用于目标分类与识别的价值。介绍舰船目标ISAR成像原理,提出基于实时成像数据的分辨率计算和误差评估方法,进而给出基于误差评估结果的自适应ISAR成像方法。所提的计算方法和分析结果对于工程应用具有一定的参考价值。

星载SAR图像方位角计算及其对风速反演影响评估

星载合成孔径雷达(Synthetic Aperture Radar, SAR)是海洋常规观测的重要技术手段,然而在其数据处理和关键物理参数反演中,SAR图像方位角SAR海面风速的反演精度。对此,根据SAR图像方位角的定义,结合空间卫星坐标系转换关系推导了?IMG,并进一步提出了基于SAR图像地面控制点的?IMG计算方法。通过对比分析2016年全球近70万景哨兵一号卫星波模式SAR图像,量化了?IMG与?SAT的系统偏差,发现该偏差与雷达入射角和轨道方向紧密相关。选用五次多项式拟合该偏差随纬度的变化规律,同时发现将卫星飞行方向角近似等于SAR图像方位角会引起海面风场反演误差,该误差空间分布不均,且升轨降轨有所差异,风速误差最大可达±0.5 m·s–1。研究结果表明,?IMG的正确计算和使用对于SAR地球科学研究具有重要意义,本文提出的?IMG计算方法对其他SAR卫星系列也具有实际参考价值。

调频连续波滑动聚束SAL信号特征分析

调频连续波-合成孔径激光雷达(Frequency Modulated Continuous Wave-Synthetic Aperture Ladar,FMCW-SAL)是一种重要的遥感观测手段,滑动聚束FMCW-SAL能够同时实现高方位向分辨率和较宽的观测范围。FMCW-SAL波束扫描导致方位向上出现混叠,距离向引入一次和二次位置偏移。通过对FMCW-SAL信号进行建模和仿真分析,通过实验验证了信号模型和分析的有效性。

SAR图像目标峰值特征提取与方位角估计方法研究

目标峰值特征是 SAR图像目标识别的重要特征之一 ,峰值特征提取是 SAR图像目标识别的一个重要步骤 ,为了由 SAR图像快速、精确地提取目标峰值特征 ,本文首先研究了 SAR图像目标峰值特征提取方法 ,提出了一种 "子像素 "级精度的 SAR图像目标峰值特征提取方法 ,并通过仿真实验分析了峰值位置、峰值幅度的估计精度。由于目标 SAR图像或 SAR图像特征矢量对目标方位角变化的敏感性 ,因此 ,为了提高 SAR图像目标识别系统的分类效率 ,本文还研究了 SAR图像目标方位角估计方法 ,提出了一种利用峰值特征基于线性回归的 SAR目标方位角估计方法 ,和现有方法相比 ,该方法除了计算速度快 ,估计精度较高之外 ,还能在估计方位角的同时 ,给出该估计的置信区间 ,从而更好的满足 SAR ATR的实际需要。文中通过对大量实测 MSTAR SAR图像目标方位角的估计实验 ,验证了本文目标峰值特征提取及方位角估计方法的有效性。