基于先验误差模型的机载高分宽幅DBF-SAR自聚焦算法

高分宽幅（HRWS）数字波束形成（DBF）合成孔径雷达（SAR）利用多通道空间采样代替部分时域采样,可以有效缓解SAR成像时高分辨率与宽测绘带间的矛盾,具有重要的军用和民用价值。现有常规DBF-SAR成像算法都假设雷达传感器相对位置精确已知,实际应用中受传感器位置测量误差影响,由位置不精确导致的相位误差会严重影响DBF-SAR高精度成像能力。在极坐标格式算法（PFA）框架下,推导了DBF-SAR成像处理后,残留相位误差的解析模型,分析了该误差对成像质量的影响。依据推导的先验相位误差解析结构模型,提出了一种基于图像对比度最优化准则的自聚焦算法。新算法通过引入先验相位结构信息,极大降低了待估参数的空间维数,可以同时改善自聚焦算法的参数估计精度和计算效率。数据处理结果验证了理论分析的正确性和所提算法的有效性。     

机载斜视SAR改进ETF快速成像算法

 根据机载斜视SAR(Synthetic Aperture Radar)成像模式,详细推导了任意场景位置的准确传递函数,在此基础上,提出了一种改进的ETF（Exact Transfer Function）成像算法。并推导得到了算法使用中分块处理应该满足的边界条件。由于推导过程基于准确运动模型和参考距离模型,方位相位的计算不做近似截取而是做准确计算,使得该算法具有更高的相位精度。同时该算法可运行于现有的成像系统,兼容性好;对数据分块并行运算,解决了斜视情况下大距离徙动带来的问题并且提高了运算速度。最后,点目标仿真成像结果证明了算法的有效性。     

基于数值计算的机载SAR空变运动补偿算法

针对机载合成孔径雷达(SAR)高分辨率宽测绘带(HRWS)成像问题,在分析结合两步运动误差补偿的距离徙动算法基础上,提出一种基于数值计算的空变运动误差补偿算法。通过对粗聚焦图像进行分块,在子块的两维波数域进行空变运动补偿,补偿的相位包括方位相位误差、距离相位误差以及方位和距离的耦合相位,因此该算法在复杂航迹、高分辨和宽测绘带情况下仍具有较好的鲁棒性。最后对SAR仿真数据和实测数据进行处理,并与结合两步运动误差补偿的距离徙动算法进行比较,处理结果表明该算法能够更好地补偿空变运动误差。     

一种横向机动弹道下SAR成像算法

针对横向机动弹道下SAR成像回波方位向和距离向严重耦合、弹载SAR平台实时性要求高的特点,提出了一种基于频谱分析的扩展SAR成像算法。首先,以初始距离相同的目标作为成像处理对象,建立了横向机动下弹载SAR成像模型,分析了回波相位和瞬时距离的泰勒展开;然后,采用包含水平面速度和偏航加速度参数的相位因子依次进行距离徙动校正、二次相位补偿和多普勒中心频率补偿,实现了SAR图像的精确聚焦。该算法处理流程简单、实时性高,适合横向机动弹道下的中等分辨率的大斜视成像,给出了算法流程,仿真验证了算法有效性。     

Conceptual design of near-space synthetic aperture radar for high-resolution and wide-swath imaging

Airborne synthetic aperture radar (SAR) has the capability of high-resolution, and spaceborne SAR has the capability of wide-swath. Inspired by recent advances in near-space defined as the region between 20 km and 100 km, this paper conceptually designed near-space vehicle-borne SAR. The near-space vehicle-borne SAR has the synthetical advantages of the satellite and airplane platforms. By placing SAR transmitter or receiver in near-space vehicles, many functions that are currently performed with satellites or airplanes could be performed in low cost way. These advantages make simultaneous high-resolution and wide-swath SAR imaging possible. As such, this paper focuses on the role of near-space vehicle for high-resolution and wide-swath SAR imaging, and deals with conceptual performance, as opposed to technological implementation. The concepts, models and processing algorithms are provided. To further suppress the azimuth ambiguities and extend swath width, multiple beams in azimuth is applied. Furthermore, an example near-space vehicle-borne SAR is designed. It is shown that the use of cost effective near-space vehicles can provide the solutions that were previously thought to be out of reach for remote sensing scientists and customers.     

An extended chirp scaling algorithm for spaceborne sliding spotlight synthetic aperture radar imaging

A system impulse response with low sidelobes is critical in synthetic aperture radar（SAR） images because sidelobes contribute to noise and interfere with nearby scatterers. However,the conventional tricks of sidelobe suppression are unable to be exactly applied to the case of spaceborne sliding spotlight SAR due to great azimuth shifts in both time and frequency domains. In this paper, an extended chirp scaling algorithm is presented for spaceborne sliding spotlight SAR data imaging. The proposed algorithm firstly uses the spectral analysis（SPECAN） technique to avoid the azimuth spectrum folding effect and then employs the chirp scaling（CS） algorithm to achieve data focusing, i.e., the so-called two-step approach. To suppress the sidelobe level, an efficient strategy for the azimuth spectral weighting which only involves matrix multiplications and short fast Fourier transformations（FFTs） is proposed, which is a post-process executed on the focused SAR image and particularly simple to be implemented. The SAR image processed by the proposed extended CS algorithm is very precise and perfectly phase-preserving. In the end, computer simulation results verify the analysis and confirm the validity of the proposed algorithm.     

A Generalized Approach to Multiresolution Complex SAR Signal Processing

Multiresolution synthetic aperture radar (SAR) image formation has been proven to be beneficial in a variety of applications such as improved imaging and target detection as well as speckle reduction. SAR signal processing traditionally carried out in the Fourier domain has inherent limitations in the context of image formation at hierarchical scales. We present a generalized approach to the formation of multiresolution SAR images using biorthogonal shift-invariant discrete wavelet transform (SIDWT) in both range and azimuth directions. Particularly in azimuth, the inherent subband decomposition property of wavelet packet transform is introduced to produce multiscale complex matched filtering without involving any approximations. This generalized approach also includes the formulation of multilook processing within the discrete wavelet transform (DWT) paradigm. The efficiency of the algorithm in parallel form of execution to generate hierarchical scale SAR images is shown. Analytical results and sample imagery of diffuse backscatter are presented to validate the method.     

Performance improvement for constellation SAR using signal processing techniques

A new concept of spaceborne synthetic aperture radar (SAR) implementation has recently been proposed - the constellation of small spaceborne SAR systems. In this implementation, several formation-flying small satellites cooperate to perform multiple space missions. We investigate the possibility to produce high-resolution wide-area SAR images and fine ground moving-target indicator (GMTI) performance with constellation of small spaceborne SAR systems. In particular, we focus on the problems introduced by this particular SAR system, such as Doppler ambiguities, high sparseness of the satellite array, and array element errors. A space-time adaptive processing (STAP) approach combined with conventional SAR imaging algorithms is proposed which can solve these problems to some extent. The main idea of the approach is to use a STAP-based method to properly overcome the aliasing effect caused by the lower pulse-repetition frequency (PRF) and thereby retrieve the unambiguous azimuth wide (full) spectrum signals from the received echoes. Following this operation, conventional SAR data processing tools can be applied to focus the SAR images fully. The proposed approach can simultaneously achieve both high-resolution SAR mapping of wide ground scenes and GMTI with high efficiency. To obtain array element errors, an array auto-calibration technique is proposed to estimate them based on the angular and Doppler ambiguity analysis of the clutter echo. The optimizing of satellite formations is also analyzed, and a platform velocity/PRF criterion for array configurations is presented. An approach is given to make it possible that almost any given sparse array configuration can satisfy the criterion by slightly adjusting the PRF. Simulated results are presented to verify the effectiveness of the proposed approaches.     

合成孔径雷达系统建模、仿真与信号处理初探

作为雷达技术发展历史中的里程碑,合成孔径雷达(SAR)通过使用空中合成天线阵列技术及先进的目标回波信号处理技术能够提供清晰的地球表面图像。由于它的这一突出特点,SAR已经成为许多飞行器的重要任务载荷并被广泛应用于军事及民用领域。从航空电子系统总体需求角度出发,为了深刻理解SAR系统的工作原理并得到更好的SAR图像产品,有必要对系统数学模型的建立、SAR回波信号的仿真以及信号处理算法进行深入的研究。本文试图从这三方面讨论SAR系统设计中的关键问题。     

一种孔径和频率二维稀疏的步进频SAR成像方法

 步进频率信号(SFWs)在不增加雷达系统瞬时带宽的情况下能够获得高的距离向分辨率的同时,也存在着抗干扰能力较差及其等效重复频率较低的问题,并且在方位向积累时间内由于雷达载机工作状态的变化,会导致方位向的数据录取不完整。针对上述问题,提出一种孔径和频率二维稀疏的步进频合成孔径雷达(SAR)成像方法。首先,分析了稀疏步进频率信号(SSFWs)的SAR成像模型,然后基于压缩感知理论完成距离向成像处理。其次,针对稀疏孔径的回波数据,通过构造成像算子和压缩感知重建模型的方法实现其距离徙动校正和方位压缩处理,进而获得二维成像结果。相比于传统的步进频率信号SAR成像,利用所提方法能够在少量的频率资源和雷达回波数据情况下实现准确的SAR成像。最后,通过对仿真和实测的步进频率雷达数据进行成像处理,验证了所提方法的有效性和可行性。     

Digital Beamforming on Receive: Techniques and Optimization Strategies for High-Resolution Wide-Swath SAR Imaging

Synthetic Aperture Radar (SAR) is a well-proven imaging technique for remote sensing of the Earth. However, conventional SAR systems are not capable of fulfilling the increasing demands for improved spatial resolution and wider swath coverage. To overcome these inherent limitations, several innovative techniques have been suggested which employ multiple receive-apertures to gather additional information along the synthetic aperture. These digital beamforming (DBF) on receive techniques are reviewed with particular emphasis on the multi-aperture signal processing in azimuth and a multi-aperture reconstruction algorithm is presented that allows for the unambiguous recovery of the Doppler spectrum. The impact of Doppler aliasing is investigated and an analytic expression for the residual azimuth ambiguities is derived. Further, the influence of the processing on the signal-to-noise ratio (SNR) is analyzed, resulting in a pulse repetition frequency (PRF) dependent factor describing the SNR scaling of the multi-aperture beamforming network. The focus is then turned to a complete high-resolution wide-swath SAR system design example which demonstrates the intricate connection between multi-aperture azimuth processing and the system architecture. In this regard, alternative processing approaches are compared with the multi-aperture reconstruction algorithm. In a next step, optimization strategies are discussed as pattern tapering, prebeamshaping-on-receive, and modified processing algorithms. In this context, the analytic expressions for both the residual ambiguities and the SNR scaling factor are generalized to cascaded beamforming networks. The suggested techniques can moreover be extended in many ways. Examples discussed are a combination with ScanSAR burst mode operation and the transfer to multistatic sparse array configurations.     

高信噪比、高分辨宽测绘带成像

针对传统高分辨和宽测绘带以及高信噪比和宽测绘带之间的矛盾,提出一种基于脉内扫描面阵合成孔径雷达(SAR)系统的二维空域联合处理算法实现高信噪比、高分辨宽测绘带成像。文中首先建立脉内扫描面阵SAR系统模型,该系统采用低脉冲重复频率(PRF)获得宽测绘带信息,同时利用脉内扫描方式获得高信噪比的回波信号。对于低PRF采样宽多普勒谱(对应方位高分辨)引起的多普勒模糊以及脉内扫描引起的距离模糊,提出一种二维空域联合处理算法解距离和多普勒模糊,并且详细地分析了地形高度变化对解模糊算法的影响。最后,通过仿真实验验证了本文算法的有效性。     

An improved imaging algorithm for spaceborne MAPs sliding spotlight SAR with high-resolution wide-swath capability

Conventional synthetic aperture radar(SAR) systems cannot achieve both highresolution and wide-swath imaging simultaneously.This problem can be mitigated by employing multiple-azimuth-phases(MAPs) technology for spaceborne sliding spotlight SAR systems.However, traditional imaging algorithms have met challenges to process the data accurately, due to range model error, MAPs data reconstruction problem, high-order cross-coupling phase error and variation of Doppler parameters along the azimuth direction.Therefore, an improved imaging algorithm is proposed for solving the above problems.Firstly, a modified hyperbolic range equation(MHRE) is proposed by introducing a cubic term into the traditional hyperbolic range equation(THRE).And two curved orbit correction methods are derived based on the proposed range model.Then, a MAPs sliding spotlight data reconstruction method is introduced, which solves the spectral aliasing problem by a de-rotation operation.Finally, high-order cross-coupling phases and variation of Doppler parameters are analyzed and the corresponding compensation methods are proposed.Simulation results for point-target scene are provided to verify the effectiveness of the proposed algorithm.     

基于CZT的机载并行双站斜视SAR成像算法

建立了机载并行双站斜视合成孔径雷达(SAR)的几何模型,给出了雷达回波的数学表达式,推导了它的二维频谱并对其特点做了分析。在二维频域内,先用聚焦函数对观测场景中心的点目标做精确成像,然后用Chirp-Z变换(CZT)校正中心点两侧目标回波的距离徙动,再通过方位向逆傅里叶变换得到了雷达图像。该算法利用了CZT能够处理非线性调频信号的特点,简化了处理过程,提高了计算效率和成像精度。仿真实验验证了这种基于CZT的新算法在处理并行双站斜视SAR数据时的有效性。     

一种新的非平行轨迹机载双站斜视SAR成像算法(英文)

本文提出了一种用于非平行轨迹机载双站斜视SAR条带模式成像的新的解析算法。该算法用收、发雷达的多普勒调频率贡献比为加权系数推导了点目标回波的二维频谱。通过解目标位置相对于收、发载机飞行轨迹的耦合,将这个二维频谱中目标的距离参数和方位参数进行了分离。在二维频域内,补偿掉双站扭曲项后利用二维Chirp-Z变换(2D-CZT)校正了距离向和方位向的徙动,获得了精确聚焦的目标图像。雷达回波的二维残余徙动用沿距离向和方位向的分块来限制,推导了数据分块的条件,由此可以实现宽场景成像。仿真试验验证了这种2D-CZT算法的有效性。     

Precision tracking algorithms for ISAR imaging

The simultaneous imaging and tracking of a moving target is one of the most difficult problems in inverse synthetic aperture radar (ISAR) signal processing. The problem is addressed here using a two-antenna imaging radar. The target range shift is sensed with an exponentially averaged envelope correlation algorithm while the angle change is measured with a differential phase shift algorithm. The three state Kalman filters are used in range and azimuth dimensions to provide both a filtered estimate and a one-sample-ahead prediction for the purpose of target tracking. The filtered range shift provides an accurate information for range bin alignment, and the target angle change provides the angular positions of the synthetic array elements. Therefore, the ISAR imaging simply becomes processing of a circular-arc aperture. The algorithms are verified both by computer simulations and also with experimental data processing     

调频连续波SAR改进的频率尺度变换算法(英文)

本文提出了一种调频连续波SAR改进的频率尺度变换算法。调频连续波SAR在运动过程中持续不断地发射和接收信号,这种运动导致了回波信号的伸缩,对波前重建产生了严重的影响。推导了回波信号模型,给出了信号的距离-多普勒域表达式,分析了由于天线不断运动而导致的相位变化与方位向频率之间的关系。改进的频率尺度变换算法补偿了这种相位变化,实现了目标的精确成像,仿真结果表明了分析的正确性和算法的有效性。     

一种新颖的超高分辨率SAR频率域反投影成像算法

超高分辨率条件下,机载合成孔径雷达（SAR）发射信号带宽大,合成孔径时间比较长,对成像处理算法的精度和效率要求较高。现有近似频率域处理和时间域滤波反投影（FBP）算法聚焦SAR数据时均存在诸多问题。基于微局部分析方法,提出了一种新颖的频率域滤波反投影（FD-FBP）成像处理方案。首先,利用Keystone变换简化了数据距离多普勒（RD）域徙动表达式。然后,在RD域进行反投影操作,对参考位置处反投影数据进行移位、相位补偿和FFT等操作即可以得到图像,从而在保证算法精确性的前提下有效降低了运算效率,实现了频率域方法的高效率和时间域方法的精确性特点的结合。最后,点目标仿真和实测数据处理以及与FBP等算法的对比验证了该方法的有效性。     

一种超高分辨率机载聚束SAR两维自聚焦算法

受运动参数测量误差和大气扰动等因素影响,合成孔径雷达(SAR)图像通常会发生散焦,利用自聚焦对散焦的SAR图像进行后处理是一种有效的重聚焦手段。传统的自聚焦算法都只是针对方位一维相位误差的估计和补偿。随着成像分辨率的提高,自聚焦时残留距离徙动的校正成为SAR成像面临的一个新挑战。本文推导得到了极坐标格式算法处理后残留距离徙动和方位相位误差的解析表达式,分析了两者之间的内在关系,并利用该关系,提出了一种能够同时进行残留距离徙动和方位相位误差补偿的两维自聚焦算法。实测数据处理结果表明,在残留距离徙动效应不可忽略的条件下,该方法能够极大地改善原有自聚焦算法的聚焦性能。