弹载SAR俯冲段斜视成像算法

弹载SAR在俯冲段具有较大的下降速度和加速度,这使得距离徙动校正(Range cell migration correction,RCMC)变得困难,并且由于导弹在攻击目标前需要有一定的转弯机动时间,弹载SAR还往往工作在斜视模式下,此时常规的SAR成像算法不再适用.本文提出了一种弹载SAR俯冲段斜视成像算法,该方法在建立弹载SAR回波信号模型的基础上,首先在距离频域-方位时间域校正距离走动的主要部分,然后结合级数反演思想,推导出SAR回波信号的二维频域表达式,在距离频域-方位多普勒域进行剩余的距离走动和距离弯曲的校正,最后完成二维脉冲压缩,得到聚焦的图像.运算量的定量分析和实验仿真的结果验证了该算法的可行性和有效性.     

前斜视SAR成像目标高精度模拟技术

前斜视合成孔径雷达(SAR)成像的距离-方位存在强耦合,目标径向运动信息的模拟精度不仅影响雷达的距离像分辨性能,同时影响方位像分辨性能。通过对距离调制分辨率与SAR成像结果的理论分析,提出了一种频域插值升采样调制的SAR成像回波模拟方法,通过提高采样率以减小距离徙动引起的幅度误差,同时采用GPU并行处理,提高回波模拟的实时性。结果表明:所提出的方法能够达到0.1 m的距离延时模拟精度,并对256×256点场景回波的运算周期小于10μs,可适用于前斜视SAR目标高精度模拟。     

一种改进的前斜视SAR距离多普勒成像算法

根据大斜视角SAR成像的空间几何模型和回波信号特点,提出了一种改进的适用于前斜视场景的距离-多普勒成像算法;对补偿二维频域耦合相位项的条件进行了分析,分析表明只需根据设计参数决定是否补偿到三阶耦合相位即可,且不必补偿更高阶的耦合相位;对距离徙动的校正方法进行了研究,通过在频域直接对距离徙动进行校正可以避免使用传统的插值方法,有效减小了计算量,提高了算法的运算效率,有助于算法的实时性实现;对改进的距离-多普勒算法的仿真表明,其成像性能能够满足前斜视条件下SAR实时成像处理的要求。     

基于切比雪夫多项式的双基地前视SAR成像算法

双基地前视合成孔径雷达(SAR)的特殊构型使其距离历程具有双根号和大斜视的特点,给回波信号的成像处理带来困难,也对成像算法的精度提出更高要求,为此提出一种基于切比雪夫多项式的双基地前视SAR RD成像算法.首先,建立双基地前视SAR的几何模型与回波信号模型,指出收发分离构型对二维频谱公式推导过程中驻定相位点求解的影响;然后,引入切比雪夫多项式展开代替传统成像算法中的泰勒级数展开,在此基础上进行双基地前视SAR斜距等效模型和二维频谱模型的推导;最后,通过分析确定双基地前视SAR RD成像算法的相位匹配函数,给出成像算法的完整流程.仿真结果表明,所提出算法可极大减小双基地前视SAR的斜距等效误差与频谱展开误差,有效提高成像算法的聚焦深度和成像质量.     

斜视SAR分数阶距离多普勒成像算法

针对传统距离多普勒(RD)算法在斜视合成孔径雷达(SAR)成像时运算量不足和边缘存在干扰的问题,将传统的RD算法中的匹配滤波用分数阶Fourier变换替代,提出基于分数阶Fourier变换SAR斜视距离多普勒成像(FrRD)算法。理论分析斜视SAR回波信号分数阶Fourier变换域模型,徙动校正在距离分数阶Fourier域方位频域完成,给出FrRD算法仿真流程。仿真结果表明,与传统RD算法相比,FrRD算法成像副瓣更低,成像边界更加清晰,成像时间缩短近一半。     

SAR大斜视RMA快速成像算法研究

在快速成像问题的研究中,针对常规距离徙动算法RMA中STOLT插值引起的高计算开销、效率低,不能有效进行快速成像的问题,提出了一种将非均匀傅立叶变换NUFFT和RMA相结合大斜视快速成像算法.通过详细分析大斜视SAR回波信号的时频特性,介绍了NUFFT算法原理以及实现步骤.再采用NUFFT代替STOLT插值和距离向IFFT,实现频域非均匀的格点向时域均匀格点变换.仿真结果表明改进后算法具有较高精度,同时兼顾了效率,满足大斜视SAR成像要求,并且为SAR成像算法中非均匀时频变换提供了一条新途径.     

机动目标双基地ISAR越距离单元徙动校正算法

逆合成孔径雷达（Inverse syn thetic aperture radar, ISAR）使用距离多普勒算法成像时,当目标尺寸较大或转角过大 ,会发生越距离单元徙动（Migration through resolution cells, MTRC）现象,严重影 响 目标成像效果。基于双基地ISAR转台模型,针对成像的MTRC问题,对解线频调后的目标回波 进行研究,分析了机动目标MTRC的产生机理,并将匹配傅里叶变换（Match Fourier trans f orm, MFT）应用到MTRC的校正中,消除了快时间频率与慢时间的耦合项,有效解决了MTRC引 起的ISAR图像散焦问题。仿真结果表明,基于MFT的双基地ISAR MTRC校正算法能够有效校正 机动目标的距离徙动,提高ISAR成像质量。     

基于Legendre多项式展开的双基SAR二维频域成像算法

针对任意构型双基SAR基于传统Taylor级数展开斜距方法的边缘点散焦问题,提出了一种基于Legendre正交多项式逼近的双基SAR二维频域成像算法。该算法基于对斜距函数的Legendre多项式展开推导了点目标二维频谱,解除了回波信号距离—方位的耦合,使成像处理更高效。在二维频域对场景区域的点目标采用二维频域成像算法进行成像。该算法改善了场景边缘点的聚焦效果,增加了场景边缘点的聚焦深度。理论推导和仿真结果验证了该算法的有效性和可行性。     

双站聚束SAR极坐标格式成像算法

双站聚束SAR(Synthetic aperture radar)相对于双站条带SAR可以达到更高的方位分辨率,其成像算法受到越来越多的关注.本文介绍了双站聚束SAR成像几何关系,建立了回波信号模型,从Dechirp后的回波数据域出发,提出了一种适合双站聚束SAR的极坐标格式成像算法.该PFA算法采用斜视坐标旋转及沿视线方向的极坐标插值LOSPI,更好地利用空间域数据,适用于正侧视和斜视模式成像.最后分析了距离弯曲对成像区域的限制,以及残余视频相位误差对成像性能的影响,并将此算法与不进行Dechirp的PFA算法相比较.实验结果表明该方法降低了数据率和运算量,算法有效、可靠.     

带距离徙动校正的压缩感知PFA成像方法

目的 极坐标格式算法（PFA）是合成孔径雷达（SAR）聚束模式下的一种高分辨率成像算法,方位向增加孔径长度带来了数据存储和传输的负担,利用压缩感知进行合成孔径雷达成像可以减小采样率,以前的研究往往认为图像是2维可分离的而忽略距离徙动的影响,造成了图像质量的下降。提出一种在方位向利用压缩感知处理的PFA成像算法,可以校正距离徙动,保证压缩感知成像的图像分辨率。方法 在方位向进行压缩感知处理的过程时,采用了随距离空间频率变化的傅里叶基。结果 该方法可以有效代替PFA处理过程中的方位向插值,消除距离徙动的影响,保证距离向和方位向的分辨率。结论 仿真和实测数据的处理结果证明了该方法的有效性。     

An exact wide field digital imaging algorithm

Abstract

A new imaging algorithm is presented for Synthetic Aperture Radar (SAR) that is exact in the sense that it is capable of producing a complex image with excellent geometrical, radiometrical and phase fidelity. No interpolations or significant approximations are required, yet the method accomplishes range curvature correction over the complete range swath. The key to the approach is a quadratic phase perturbation of the range linearly frequency modulated signals while in the range signal, azimuth frequency transform (Doppler) domain. Range curvature correction is completed by a phase multiply in the two-dimensional frequency domain. Other operations required are relatively conventional. The method is generalizable to imaging geometries encountered in squint and spotlight SAR, inverse SAR, seismics, sonar, and tomography.     

Focusing one-stationary bistatic forward-looking synthetic aperture radar based on squint minimization and modified nonlinear chirp scaling algorithm

ABSTRACT

This article deals with the imaging problem of bistatic forward-looking synthetic aperture radar with a stationary transmitter (one-stationary BFSAR). In one-stationary BFSAR, due to the large forward-looking (or squint) angle, it causes more serious range cell migration and two-dimensional spatial variation. To solve this problem, an imaging algorithm based on squint minimization and modified nonlinear chirp scaling (NLCS) method is proposed. The squint minimization method can decrease the coupling between range and azimuth and correct the linear range cell migration. The modified NLCS can eliminate the variation of azimuth reference function more accurately than traditional NLCS algorithm, which improves the focus quality of marginal targets. Finally, simulation results confirm the efficiency of our proposed algorithm.     

基于四阶传递函数大斜视角SAR成像算法

针对研究SAR大斜视工作时,为增加天线实时侦察效果,对低阶ETF(Exact transfor function)算法处理方位向相位的近似误差进行改进,在传统ETF成像算法基础上研究了四阶传递函数算法.采用四阶距离模型,分析了距离向聚焦深度为分块数据处理提供了依据,同时在距离多普勒域对非参考距离处目标进行了相位补偿.通过场景目标仿真对算法有效性进行了验证.仿真结果表明,算法可有效补偿大斜视角时方位向近似误差,提高了精度,满足大斜视时成像要求,同时避免插值处理,计算量小.便于实时成像.     

一种斜视滑动聚束SAR子孔径处理成像方法

子孔径法是解决滑动聚束SAR成像方位频谱混叠的有效方法,具有内存占用量低、灵活性高的特点。子孔径的划分不仅影响成像 质量,还会影响成像效率;但目前子孔径的划分大多采用经验值,理论上并没有给出明确分析。因此在斜视成像过程中,未合理划分的子孔径会导致拼接谱出现“空隙”现象。本文首先介绍了斜视滑动聚束SAR工作模型,分析了其方位多普勒历程,提出了斜视下基于方位频域Scaling的子孔径成像算法。根据相邻子孔径的二维频谱,研究了子孔径频谱拼接的准则,推导了确定子孔径长度与重叠率的解析式,给出了子孔径划分的流程,最后通过仿真 实验和实测数据处理验证了子孔径划分方法及斜视下成像算法的有效性。     

基于去调频技术的斜视聚束SAR成像方法

针对具有方位谱混叠现象的斜视聚束式合成孔径雷达(SAR)成像问题,本文研究了一种基于去调频技术的成像方法.文中首先分析了斜视聚束SAR中的方位谱混叠现象,从理论上找出了斜视角影响方位粗聚焦的根本原因.基于分析结果,引入了回波信号非线性相位校正预处理法,彻底消除了斜视角的影响,从而得到正确的粗聚焦结果.然后基于斜视成像模型,推导了预处理回波信号的方位粗聚焦和精聚焦处理过程.在精聚焦的方位滤波处理中,针对非线性相位校正预处理导致的场景聚焦深度限制问题,提出了一种改进的非线性调频变标(NLCS)算法,实现了回波信号的高精度方位压缩.仿真实验结果证明了文中理论分析的正确性和所提方法的有效性.     

利用四参数Chirplet分解方法估计SAR多普勒调频斜率

提出了基于一个距离门信号的四参数Chirplet分解方法，估计合成孔径雷达(SAR)回波的多普勒调频斜率。距离压缩后的回波数据，每个距离门的信号都是由一系列的Chirp分量组成的。利用四参数Chirplet分解方法，能够检测出这一信号中的一个或几个Chirp分量，估算出多普勒调频斜率，其结果被应用于合成孔径雷达成像的方位压缩处理。仿真及真实回波数据的计算表明，无论在精度还是运算时间上，其结果都优于目前已有的实时估计方法。     

Doppler properties of radars in circular orbits

Abstract

Expressions are presented for Doppler frequency shift, Doppler bandwidth, zero Doppler offset angle in spacecraft yaw and, in the SAR mode, the rate of Doppler frequency modulation, azimuth time-bandwidth product, resolution, available integration time and the location of principal azimuth ambiguities. The equations are simply expressed with virtually no approximations based on angles referenced to the satellite. Earth rotation is included, as is the geosynchronous case. (These results differ from expressions to be found elsewhere in the literature, most of which have been derived using flat-Earth approximations.).     

A simple algorithm for the direct extraction of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar

Abstract

A simple method is derived for 1the determination of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar (SAR) without explicitly producing an image. The algorithm is similar in structure to a two-dimensional Fourier transformation, but the transformed function also depends explicitly on the wavenumber components. The function consists of the quadratic product of the complex return signal amplitude and the time-delayed complex conjugate return signal, the time delays with respect to range and azimuth time being proportional to the corresponding wavenumber components in these directions. The algorithm appears to be sufficiently simple to be implemented in real time aboard a satellite. Because of the considerable data compression achieved in reducing the original image to its statistical variance spectrum, this opens the possibility of obtaining global surface-wave spectral data from satellites without the excessive costs of sophisticated telemetry and a ground station network required for real-time line-of-sight transmission of the unprocessed signal data. The contracted signal-image-Fourier-transform (SIFT) algorithm may be interpreted as the application of the SAR as a continuous, two-dimensional Δκ (dual frequency) scatterometer. The difference frequencies arise through the multiplication of the chirped return signal with the time-lagged (i.e. frequency shifted) complex return signal. Since the return signal is chirped with respect to both range time and azimuth time, the introduction of two time lags corresponds to a two-dimensional Δκ modulation. The standard dual frequency scatterometer yields a modulation wave only in the radar propagation direction, but the basic principle of producing a beat wave by multiplying two chirped signals which are displaced in time relative to each other can be applied also to the azimuthal Doppler chirp of a broad beam dual frequency scatterometer. The generalized two-dimensional Δκ-scatterometer obtained in this manner differs from a SAR-SIFT processor only in the manner in which the spectrum of difference frequencies is generated: in a Δκ-scatterometer, the duration of the difference frequency sweep is normally large compared with the signal travel time, whereas in a SAR, the entire spectrum of difference frequencies is generated within a single pulse in a time shorter than the average signal travel time. The relative operational advantages or disadvantages of the two methods of obtaining microwave surface image spectra will need to be clarified in a more detailed technical analysis.