IMPROVED SYNTHETIC APERTURE SONAR MOTION COMPENSATION COMBINED DPCA WITH SUB-APERTURE IMAGE CORRELATION

Estimation precision of Displaced Phase Center Algorithm(DPCA) is affected by the number of displaced phase center pairs,the bandwidth of transmitting signal and many other factors.Detailed analysis is made on DPCA's estimation precision.Analysis results show that the directional vector estimation precision of DPCA is low,which will produce accumulating errors when phase cen-ters' track is estimated.Because of this reason,DPCA suffers from accumulating errors seriously.To overcome this problem,a method combining DPCA with Sub Aperture Image Correlation(SAIC) is presented.Large synthetic aperture is divided into sub-apertures.Micro errors in sub-aperture are estimated by DPCA and compensated to raw echo data.Bulk errors between sub-apertures are esti-mated by SAIC and compensated directly to sub-aperture images.After that,sub-aperture images are directly used to generate ultimate SAS image.The method is applied to the lake-trial dataset of a 20 kHz SAS prototype system.Results show the method can successfully remove the accumulating error and produce a better SAS image.

Image Motion Compensation of Off-Axis TMA Three-Line ArrayAerospace Mapping Cameras

To enhance the image motion compensation accuracy of off-axis three-mirror anastigmatic( TMA)three-line array aerospace mapping cameras,a new method of image motion velocity field modeling is proposed in this paper. Firstly,based on the imaging principle of mapping cameras,an analytical expression of image motion velocity of off-axis TMA three-line array aerospace mapping cameras is deduced from different coordinate systems we established and the attitude dynamics principle. Then,the case of a three-line array mapping camera is studied,in which the simulation of the focal plane image motion velocity fields of the forward-view camera,the nadir-view camera and the backward-view camera are carried out,and the optimization schemes for image motion velocity matching and drift angle matching are formulated according the simulation results. Finally,this method is verified with a dynamic imaging experimental system. The results are indicative of that when image motion compensation for nadir-view camera is conducted using the proposed image motion velocity field model,the line pair of target images at Nyquist frequency is clear and distinguishable. Under the constraint that modulation transfer function( MTF) reduces by 5%,when the horizontal frequencies of the forward-view camera and the backward-view camera are adjusted uniformly according to the proposed image motion velocity matching scheme,the time delay integration( TDI) stages reach 6 at most. When the TDI stages are more than 6,the three groups of camera will independently undergo horizontal frequency adjustment. However, when the proposed drift angle matching scheme is adopted for uniform drift angle adjustment,the number of TDI stages will not exceed 81. The experimental results have demonstrated the validity and accuracy of the proposed image motion velocity field model and matching optimization scheme,providing reliable basis for on-orbit image motion compensation of aerospace mapping cameras.

MOVING OBJECT TRACKING IN DYNAMIC IMAGE SEQUENCE BASED ON ESTIMATION OF MOTION VECTORS OF FEATURE POINTS

An improved estimation of motion vectors of feature points is proposed for tracking moving objects of dynamic image sequence. Feature points are firstly extracted by the improved minimum intensity change （MIC） algorithm. The matching points of these feature points are then determined by adaptive rood pattern searching. Based on the random sample consensus （RANSAC） method, the background motion is finally compensated by the parameters of an affine transform of the background motion. With reasonable morphological filtering, the moving objects are completely extracted from the background, and then tracked accurately. Experimental results show that the improved method is successful on the motion background compensation and offers great promise in tracking moving objects of the dynamic image sequence.

ISAR MOTION COMPENSATION USING MODIFIED DOPPLER CENTROID TRACKING METHOD

With regard to the phase compensation in inverse synthetic aperture radar (ISAR),the modified Doppler centroid tracking (MDCT) method is developed which applies the phase gradient autofocus (PGA) algorithm developed by Wahl[1]to improve the Doppler centroid tracking (DCT) method[2].When the phase compensation is performed,the proposed approach smartly eliminates the effect of the rotational phase component (RPC) on the estimation of the translational phase component (TPC) by circular shifting,windowing and iteration steps. After several iterations,the maximum likelihood estimation and compensation of the TPC of the target can be realized more effectively.The processing results of live data show that the proposed method can improve the imaging quality of ISAR significantly.

Reference-driven method for MR image reconstruction based on wavelet sparsity and nonlocal total variation

A novel reference-driven method for MR image reconstruction based on wavelet sparsity and nonlocal total variation（NLTV）is proposed.Utilizing the sparsity of the difference image between the target image and the motion-compensated reference image in wavelet transform domain,the proposed method does not need to estimate contrast changes and therefore increases computational efficiency.Additionally,NLTV regularization is applied to preserve image details and features without blocky effects.An efficient alternating iterative algorithm is used to estimate motion effects and reconstruct the difference image.Experimental results demonstrate that the proposed method can significantly reduce sampling rate or improve the quality of the reconstructed image alternatively.

Wavelet-Based Mixed-Resolution Coding Approach Incorporating with SPT for the Stereo Image

With the advances of display technology, three-dimensional(3-D) imaging systems are becoming increasingly popular. One way of stimulating 3-D perception is to use stereo pairs, a pair of images of the same scene acquired from different perspectives. Since there is an inherent redundancy between the images of a stereo pairs, data compression algorithms should be employed to represent stereo pairs efficiently. The proposed techniques generally use block-based disparity compensation. In order to get the higher compression ratio, this paper employs the wavelet-based mixed-resolution coding technique to incorporate with SPT-based disparity-compensation to compress the stereo image data. The mixed-resolution coding is a perceptually justified technique that is achieved by presenting one eye with a low-resolution image and the other with a high-resolution image. Psychophysical experiments show that the stereo image pairs with one high-resolution image and one low-resolution image provide almost the same stereo depth to that of a stereo image with two high-resolution images. By combining the mixed-resolution coding and SPT-based disparity-compensation techniques, one reference (left) high-resolution image can be compressed by a hierarchical wavelet transform followed by vector quantization and Huffman encoder. After two level wavelet decompositions, for the low-resolution right image and low-resolution left image, subspace projection technique using the fixed block size disparity compensation estimation is used. At the decoder, the low-resolution right subimage is estimated using the disparity from the low-resolution left subimage. A full-size reconstruction is obtained by upsampling a factor of 4 and reconstructing with the synthesis low pass filter. Finally, experimental results are presented, which show that our scheme achieves a PSNR gain (about 0.92dB) as compared to the current block-based disparity compensation coding techniques.

A rate control approach to distributed source coding for interferential multispectral image compression

Distributed source coding （DSC） is applied to interferential multispectral image compression owing to strong correlation among the image frames. Many DSC systems in the literature use feedback channel （FC） to control rate at the decoder, which limits the application of DSC. Upon an analysis of the image data, a rate control approach is proposed to avoid FC. Low-complexity motion compensation is applied first to estimate side information at the encoder. Using a polynomial fitting method, a new mathematical model is then derived to estimate rate based on the correlation between the source and side information. The experimental results show that our estimated rate is a good approximation to the actual rate required by FC while incurring a little bit-rate overhead. Our compression scheme performs comparable with the FC based DSC system and outperforms JPEG2000 significantly.

用于航空相机像移补偿的三轴稳定平台设计与研究

航空相机在工作过程中,由于载机的抖动干扰,拍摄过程中会产生像移,而像移是影响图像质量的主要原因。为航空相机配备一套稳定平台系统是解决像移问题有效的解决办法,稳定平台系统的原理是通过将航空相机成像传感器与干扰隔离来稳定视轴。为了稳定视轴,实现推扫拍摄,这里设计了一种三轴稳定平台,对三轴稳定平台的姿态补偿方式和结构设计进行了研究。通过分析像移对像质的影响,确定了轴系的旋转精度,推导了基于角速度的横滚轴、俯仰轴与航向轴框架交叉耦合的平台补偿方程。然后,根据轴系精度和运动补偿方程,设计并分析了三轴稳定平台结构,通过实验来验证视轴稳定的性能。实验结果表明,平台在载波干扰下具有良好的稳定性能。

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。

一种改进的基于惯导运动补偿的实时成像方法

合成孔径雷达(SAR)作为一种主动式的对地微波遥感技术,具有全天候、全天时和远距离的特点。通过发射大带宽信号,获得距离向的高分辨。通过平台相对静止场景的运动形成合成孔径,获得方位向的高分辨,SAR平台的运动是成像的基本条件。对于机载SAR,由于载机在大气中飞行时受到大气湍流等因素的影响,不可能保持理想的运动状态,必然会产生运动误差,运动误差会导致相位误差和包络移动,从而导致图像散焦等问题。如何对SAR进行运动补偿是实现成像的关键。基于惯导数据的运动补偿是实现高分辨成像的重要手段之一。根据惯导数据的特点,结合卡尔曼滤波原理和SAR成像的几何模型,提出了一种基于卡尔曼滤波的惯导数据滤波和重采样方法。该方法可以有效剔除运动参数野值,精确地获得雷达平台的运动参数。文中所提出的运动补偿方法能和SAR成像算法很好的结合,实测数据的成像结果验证了该方法的准确性和有效性。

某型舰载机目标的ISAR成像算法研究

逆合成孔径雷达(ISAR)成像技术因其能够提供目标的高分辨率二维图像,已经成为军事侦察、地球观测和灾害监测等领域的重要工具,但是,由于目标的复杂运动,传统的ISAR成像方法往往会出现图像模糊和失真的问题。距离多普勒(RD)算法通过对雷达回波数据进行二维傅里叶变换,可以有效地抑制目标的复杂运动对ISAR图像的影响,从而获得更清晰、更精确的图像,然而,在处理目标加速度和微动方面存在局限性。文中首先说明了ISAR的成像原理,介绍了运动补偿中的包络对齐,在此基础上,提出一种改进的包络对齐方法,通过某卫星成像对该算法进行验证,证明了该方法的适用性和灵活性,运用改进的包络对齐方法对某型舰载机成像,发现该方法可以有效地提高ISAR图像的质量和分辨率。

基于次镜像移补偿的折反式航测相机光机系统设计

针对航测相机对地摆扫成像过程中的动态像移问题,采用两镜望远系统的矢量像差理论,将次镜作为像移补偿元件,通过次镜多维运动实现航测相机的综合像移补偿。次镜在对像移进行补偿的过程中会发生偏心和倾斜,导致次镜离轴,影响成像质量,因此,建立了失调折反光学系统成像模型,研究次镜像差场偏移矢量与次镜失调量之间的关系,并分析次镜多维运动对成像质量的影响。为验证次镜多维运动的像移补偿能力,搭建实验平台对该航测相机进行了实验室内以及外场成像试验。结果表明,采用该像移补偿技术的航测相机动态分辨率可达74 lp/mm,且像移补偿精度优于0.5个像素,达到设计预期。

基于谱压缩的大斜视TOPS BP图像自聚焦算法

在机动平台大斜视TOPS模式SAR成像时,通过使用地平面直角坐标系BP成像算法,能够在短时间内获取地距平面无畸变的宽幅SAR图像,但实际应用中如何对BP图像快速完成运动误差补偿与旁瓣抑制仍是一个难点。针对此问题,提出了一种改进的谱压缩方法,基于此能够快速实现机动平台大斜视TOPS模式地平面BP图像自聚焦等后续操作。首先,考虑到传统BP谱压缩方法仅适用于聚束成像模式,结合大斜视TOPS SAR虚拟旋转中心理论与波数谱分析,推导出了改进的精确谱压缩函数,能够通过全孔径压缩获得无模糊的地平面TOPS模式BP图像频谱。在此基础上,利用相位梯度自聚焦(PGA)能够快速完成全孔径运动误差估计与补偿。此外,基于提出的改进谱压缩方法得到的无模糊对齐BP图像频谱,可以在方位频域统一加窗实现图像旁瓣抑制。最后,通过仿真数据处理验证了所提算法的有效性。

基于PSO联合运动补偿的ISAR成像算法

在传统的逆合成孔径雷达(ISAR)成像中,目标的运动补偿通常需要先进行包络对齐实现一维距离像对准,再进行相位校正,这种补偿方法称为级联运动补偿。低信噪比下的级联运动补偿容易增加误差积累,最终导致图像模糊散焦,严重影响成像质量,针对该问题文中提出了一种基于粒子群优化(PSO)联合运动补偿的ISAR成像算法,该算法分解目标的运动模型,通过高阶次展开,设置参数向量,通过图像的锐化度作为代价函数,寻找向量参数的最优值进行目标的联合运动补偿,使得ISAR成像得到最佳效果。在最优参数求解过程中,本文采用PSO算法,运算时实现更快收敛,该算法提高了模型的求解速度,得到聚焦效果更好的ISAR成像。通过仿真数据和外场实测数据验证了算法的有效性。

Autofocus technique for ISAR imaging of uniformly rotating targets based on the ExCoV method

The inverse synthetic aperture radar (ISAR) imaging can be converted into a sparse reconstruction problem and solved by the l1norm minimization algorithm. The basis matrix in sparse ISAR imaging is usually characterized by the unknown rotation rate of a moving target, thus the rotation rate and the sparse signal should be jointly estimated. Especially due to the imperfect coarse motion compensation, we consider the phase error correction problem in the context of the sparse signal reconstruction. To address this issue, we propose an iterative reweighted method, which jointly estimates the rotation rate, corrects the phase error and reconstructs a high resolution ISAR image. The proposed method gives a gradual and interweaved iterative process to refine the unknown parameters to achieve the best sparse representation for the ISAR signals. Particularly, in ISAR image reconstruction, the l1norm minimization algorithm is sensitive to user parameters. Setting these user parameters are not trivial and the reconstruction performance depends significantly on their choices. Then, we consider an expansion-compression variance-component (ExCoV) based method, which is automatic and demands no prior knowledge about signal-sparsity or measurement-noise levels. Both numerical and electromagnetic data experiments are implemented to show the effectiveness of the proposed method. It is shown that the proposed method can estimate the rotation rate and correct the phase errors simultaneously, and its superior performance is proved in terms of high resolution ISAR image. © 2017 Beijing Institute of Aerospace Information.

天基光学成像系统像移研究及补偿技术发展现状及趋势(特邀)

天基各类光学系统在成像过程中,受卫星轨道变化、姿态角变化等因素以及跟踪速度误差、平台及各运动部件颤振的影响会造成像移。像移会导致图像分辨率降低,图像模糊,严重影响最终的图像质量,因此如何抑制动态像移的影响已成为获取高分辨率清晰图像的瓶颈。目前已有的研究基本都是从单独的应用领域去分析像移的产生因素,没有系统全面的将天基光学系统像移的产生和补偿技术进行综合呈现。本文在简述像移影响机理及像移补偿技术的基础上,对天基成像按照用途分类,详细介绍近年来国内外在对地遥感、空间天文探测、火星探测和空间目标探测成像领域像移影响及补偿的研究进展。通过对国内外研究现状的跟踪分析,预测天基光学成像系统像移研究的发展方向,进一步提升对我国空间光学技术发展方向的理解,为我国航天光学技术和装备的发展提供指导。

结合帧差的核相关滤波弱小红外目标检测

为了提高红外目标检测的性能,提出了一种结合帧差的核相关滤波弱小红外目标检测算法。算法首先通过核相关滤波训练当前帧获得最大回归值,相对间隔帧求取差值,以此进行循环移位,从而实现对帧间背景运动的补偿;再者借助帧间差分法提取当前帧相对运动特征,增强区分弱小目标和红外背景的能力;最后对相对运动特征进行阈值分割获得最终检测结果。仿真实验显示本算法能有效检测出复杂环境下红外弱小目标,与其他同类算法相比,本算法可以很好地对杂波和点状干扰源进行抑制,获得较高的目标检测率,同时将大量运算置于频域中,运算效率也优于其他算法。

微光条件下遥感器像移补偿机构设计及控制方法

夜间成像时,遥感成像地面目标的辐亮度可低至白天的10–6,在提升弱信号获取能力的各种措施中,通过长曝光时间提升信噪比是一种直接而有效的解决方式,但是在低轨遥感任务中其实现却受到极大限制。为完成探测目标凝视,文章提出通过光路中的像移补偿机构实现光轴稳定的解决方案,补偿了相机相面与地物的相对运动,在曝光时间内使探测器相面景物与地物景物保持相对静止,从而实现灵活微光成像模式;具体阐述了微光条件下遥感器像移补偿机构的设计和控制方法,对其原理及设计进行了详细分析,并进行仿真及试验验证,结果表明所设计的像移补偿系统可将图像抖动补偿至0.2像元之内,为低轨遥感微光成像系统提供了优异的解决途径。

海底小目标高分辨合成孔径声呐成像技术研究进展

海底小目标的成像探测问题是海底探测领域的经典难题,合成孔径声呐技术为小目标高分辨成像与探测提供了主要技术手段。本文分析了现阶段合成孔径声呐所面临的主要能力提升问题,介绍了合成孔径声呐高分辨成像的基本原理以及多子阵成像技术,并且针对合成孔径声呐面临的技术难点,对运动误差估计与补偿技术、高测绘效率成像技术、非稳定直航下的高分辨成像技术进行了讨论,介绍了相关技术路线与研究进展,并在此基础上展望了海底小目标高分辨成像技术的发展趋势与应用前景。

基于OMP-CADMM的非合作运动人体ViSAR成像方法

合成孔径雷达(Synthetic Aperture Radar,SAR)人体成像技术凭借其高分辨、强穿透、易部署和高安全性,已成为SAR拓展至日常民用领域的关键技术之一.然而现有SAR人体成像方法大多针对固定场景下的静态人体目标进行合作式成像,难以满足开放场景非合作成像应用的需求.为此,本文基于人体运动的块稀疏性与一致性约束,提出了一种面向非合作运动人体的视频SAR(Video SAR,ViSAR)成像方法 .该方法首先结合ViSAR子孔径快速成像技术,简化人体非刚性运动模型,建立非刚性运动相位误差参量模型;随后结合人体运动的块稀疏性与时间连续性,基于正交匹配追踪(Orthogonal Matching Pursuit,OMP)稀疏恢复算法估计人体运动相位误差;最后基于一致性交替方向乘子法(Consensus Alternating Direction Multiplier Method,CADMM)优化框架,保留人体目标局部结果的一致性共有特征,从多个局部补偿结果集中优化出人体的全局聚焦成像结果,实现人体的非刚性运动补偿与全局精细优化.实验结果表明该文所提方法可实现非合作走动人体的视频化高分辨精细成像.