Modified Omega-K algorithm for processing helicopter-borne frequency modulated continuous waveform rotating synthetic aperture radar data

With appropriate geometry configuration, helicopter- borne rotating synthetic aperture radar （ROSAR） can break through the limitations of monostatic synthetic aperture radar （SAR） on forward-looking imaging. With this capability, ROSAR has extensive potential applications, such as self-navigation and self-landing. Moreover, it has many advantages if combined with the frequency modulated continuous wave （FMCW） technology. A novel geometric configuration and an imaging algorithm for helicopter-borne FMCW-ROSAR are proposed. Firstly, by per- forming the equivalent phase center principle, the separated trans- mitting and receiving antenna system is equalized to the case of system configuration with antenna for both transmitting and receiving signals. Based on this, the accurate two-dimensional spectrum is obtained and the Doppler frequency shift effect in- duced by the continuous motion of the platform during the long pulse duration is compensated. Next, the impacts of the velocity approximation error on the imaging algorithm are analyzed in de- tail, and the system parameters selection and resolution analysis are presented. The well-focused SAR image is then obtained by using the improved Omega-K algorithm incorporating the accurate compensation method for the velocity approximation error. FJnally, correctness of the analysis and effectiveness of the proposed al- gorithm are demonstrated through simulation results.

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.

Broadband printed antennas and dual-polarized microstrip and waveguide slot antenna arrays

In order to meet the urgent needs in wireless communications, microwave image synthetic aperture radars （SAR）, and electronic warfare systems, this dissertation studies several types of broadband dual-polarized planar antenna elements and arrays, and proposes a few of novel designs with experimental verification. The main accomplishments reported in the dissertation are as follows.

A New Dual Polarized Aperture-Coupled Printed Array for SAR Applications

This paper presents a new design of dual polarized aperture coupled printed antenna array. The finite difference time domain (FDTD) analysis of an aperture coupled microstrip element is performed, and the effects of antenna parameters on its characteristics are obtained to guide the design of the printed array. Then an 8×2 dual polarized array design in X band is introduced with configuration plots. In order to improve its isolation and cross polarization, an outphase displacement feeding technique is adopted in the feed network. Also, the round bends are used instead of conventional right angle bends so as to achieve better VSWR performance. Experimental results are presented, indicating the validity of the design. This dual polarized array can be applied as a sub array of spaceborne SAR systems.

基于频域分析的ROSAR成像算法

在现有的ROSAR(Synthetic Aperture Radar with Rotating antennas)成像算法中,基于斜距泰勒级数展开的算法存在方位向失配问题,时域相关卷积算法不能快速校正距离弯曲.频域ROSAR成像算法采用驻相点原理,直接对ROSAR回波信号进行二维频域变换,保留回波信号的频谱特征,进而构造相应的频域去耦函数和频域匹配函数,实现频域距离弯曲校正和目标场景重构.仿真结果表明,该算法能够快速校正距离弯曲及有效克服方位向失配问题.

直升机载ROSAR运动补偿及成像算法

针对传统旋转式合成孔径雷达(ROSAR)成像算法在直升机平台运动下难以实现有效的聚焦成像问题,提出一种融合直升机平台运动补偿的改进Chirp Scaling算法。首先建立了存在平台运动时直升机载ROSAR成像的几何模型,在此基础上,分析了平台运动引起的目标斜距误差随目标位置的变化关系,得到了平台运动引入的斜距误差的空变特性,并给出误差补偿方法。然后分析了雷达相位中心等效速度随目标斜距变化对成像质量的影响,并把其影响补偿融入到Chirp Scaling算法的距离徙动空变性校正和方位压缩中。仿真结果验证了所提方法的有效性。

基于RadarSat2数据的法拉第旋转校正算法实验

鉴于星载合成孔径雷达（SAR）系统受法拉第旋转效应引入的极化矢量偏移影响,研究基于极化SAR传输模型给出常用星载SAR法拉第旋转效应校正算法,还原目标真实散射特性以用于地面精确信息反演.实验给出了四种常用校正算法关于C波段全极化Radar Sat2数据的法拉第旋转角估计性能分析,对比得到Bickel＆Bates算法及Freeman算法具有较稳健的估计性能表现;并基于其对整景数据进行法拉第旋转效应估计,得到该雨林区域数据具有0°~2°的法拉第旋转角变化,对于一般遥感数据应用不会产生较大影响.这为进一步精确监测星载极化SAR系统失真变化打下了技术基础.

基于ROSAR的高分辨波数域成像算法

旋转式合成孔径雷达(rotor synthetic aperture radar,ROSAR)是一种新型成像雷达,在高分辨条件下,距离徙动对ROSAR成像结果影响严重。波数域成像算法利用插值操作实现距离徙动的精确校正,但由于ROSAR复杂的斜距历程,导致传统波数处理获得的二维波数谱形式复杂,难以进行插值操作。针对这一问题,提出了一种改进ROSAR波数域成像算法。重新构建角度维波数,通过求解隐函数获得易于插值操作的信号二维波数域表达式,设计参考函数消除高频调制影响,最终通过Stolt插值实现宽波束大场景下距离徙动无近似校正与精确聚焦。仿真实验验证了算法的可行性与有效性。

基于卡尔丹方程的高分辨ROSAR成像算法及性能分析

旋转式合成孔径雷达(rotor synthetic aperture radar,ROSAR)是一种新型的雷达成像模式,其圆形运动轨迹斜距历程造成传统二阶斜距近似法在宽方位波束场景下方位向散焦严重。针对这一问题,提出了一种基于卡尔丹方程解析表达式的ROSAR成像算法。考虑通过对回波信号的斜距表达式进行四阶近似,推导了回波信号的二维频谱,分析距离徙动对成像结果的影响,提出基于卡尔丹方程解析表达式实现距离徙动补偿,进而实现整个场景精确聚焦的成像算法并给出了算法的运算量。仿真实验验证了算法的有效性及在噪声环境下的稳健性。

Rotational parameters estimation of maneuvering target in ISAR imaging

The rotational parameters estimation of maneuvering target is the key of cross-range scaling of ISAR （inverse synthetic aperture radar）, which can be used in the target feature extraction. The cross-range signal model of rotating target with fixed acceleration is presented and the weighted linear least squares estimation of rotational parameters with fixed velocity or acceleration is proposed via the relationship of cross-range FM （frequency modulation） parameter, scatterers coordinates and rotational parameters. The FM parameter is calculated via RWT （Radon-Wigner transform）. The ISAR imaging and cross-range scaling based on scaled RWT imaging method are implemented after obtaining rotational parameters. The rotational parameters estimation and cross-range scaling are validated by the ISAR processing of experimental radar data, and the method presents good application foreground to the ISAR imaging and scaling of maneuvering target.

Progress in dual-band dual-polarization shared-aperture SAR antennas

The progress in dual-band dual-polarization(DBDP)shared-aperture antennas for the synthetic aperture radar(SAR)application in the last decade is reviewed.Several designs of DBDP SAR antenna arrays are introduced with their main performances,then their comparison is summarized.In addition,some techniques enhancing DBDP antenna performances are presented.

BEAM BROADENING SYNTHESIS FOR SAR ANTENNA ARRAY BASED ON PROJECTION MATRIX ALGORITHM

A new beam broadening synthesis technique for Synthetic Aperture Radar(SAR) antenna array, namely Projection Matrix Algorithm(PMA) is presented. The theory of PMA is introduced firstly, and then the iterative renewed manner is improved to resolve the unbalance problem under amplitude and phase control. In order to validate the algorithm correct and effective, an actual engineering application example is investigated. The beam synthesis results of 1.0~4.5 times broadening under the phase only control and the amplitude and phase control using improved PMA are given. The results show that the beam directivity, the beam broadening, and the side-lobe level requirements were met. It is demonstrated that the improved PMA was effective and feasible for SAR application.

一种联合TDCM和PFA的空间目标大转角ISAR成像方法

空间目标态势感知是当前航天技术领域的研究热点,逆合成孔径雷达(Inverse Synthetic Aperture Radar, ISAR)是空间目标感知的一种有效手段。为了得到利于目标鉴别的高分辨二维ISAR图像,需要通过增大积累转角来提高方位向分辨率。但是,随着转角的增大,平动与转动之间的耦合增大,使得传统的先平动补偿、后利用转动成像的两步式成像方法不再适用。本文提出了一种结合时域信号共轭相乘(Time Domain Conjugate Multiplication, TDCM)处理与极坐标格式算法(Polar Format Algorithm, PFA)的成像算法,首先利用雷达回波的二维相干性,通过TDCM处理进行平动粗补偿,然后借助PFA的插值操作校正大转角引起的散射点越分辨单元走动(Motion Through Resolution Cell, MTRC)并估计目标相对转动参数。通过迭代处理,持续对平动和转动参数进行优化,提升运动补偿效果。仿真验证了所提成像算法的有效性,与传统算法相比,成像聚焦性更好。

绕月SAR天线热控设计

合成孔径雷达(SAR)天线是遥感卫星的重要载荷之一,面对月球轨道舱外天线复杂的外热流环境,结合轨道特点,对天线开展了详细的外热流分析,解析了月球环境的热设计难点,进行了针对性的热控设计,解决了极地轨道下天线在一个很大的外热流和极小的外热流间不断循环的问题。采用TMG软件对天线进行了热仿真分析,并利用热模拟件进行了热平衡试验。分析和试验结果表明,天线各单机温度满足指标要求。本文适用于平板SAR天线,对有源SAR天线的热控设计具有一定的借鉴意义。

基于波束优化赋形方法的斜视星载SAR模糊抑制研究

距离模糊和方位模糊会严重影响星载合成孔径雷达(Synthetic Aperture Radar,SAR)的成像质量。现有的利用天线波束赋形来抑制模糊的方法在雷达正侧视成像时取得了优异的效果,但并不适用于雷达斜视的情况。针对这一问题,提出了一种基于平面阵列天线波束赋形的星载SAR二维模糊(距离和方位模糊)抑制方法。使用距离-方位模糊综合的模糊比(Ambiguity to Signal Ratio,ASR)指标来替代距离模糊比及方位模糊比,结合对不同斜视情形下天线波束变化的分析,充分考虑包含镜像模糊区在内的所有模糊区,建立了模糊比-天线权重优化模型。以模糊能量为目标函数、天线方向图掩模作为约束确立二次锥(Quadratic Cone Programming,QCP)优化问题,求解得到阵元幅度相位分布。仿真结果表明,所提方法可以通过调节模糊区对应的旁瓣幅值,灵活地抑制SAR斜视成像的距离和方位模糊,进而提高星载SAR的成像质量。

星载平面SAR卫星RCS特性研究

对星载合成孔径雷达(SAR)卫星结构精细估计是空间态势感知的主要内容,阵元间距的估计更是关注的重点。在微带星载SAR间距研究基础上,开展了波导裂缝SAR卫星电磁仿真和间距估计,利用SAR卫星不同倾斜角下的雷达散射截面(RCS)空间不变性,采用球体空间距离实现了倾斜角未知条件下的阵列间距估计。对多颗SAR卫星实测数据分析表明方法可靠可行,研究获得的RCS身份密码有助于提高SAR卫星识别和精细结构估计能力。

相控阵馈源反射面天线在星载SAR中的应用

5米S频段合成孔径雷达(S-SAR)卫星是一个采用相控阵馈源反射面天线体制的SAR卫星,从5米S-SAR天线技术要求出发,阐述了采用相控阵馈源反射面天线体制的理由,介绍了该种天线的工作原理、设计方法和性能特点。给出了天线系统组成和部分典型的实测结果。相控阵馈源反射面天线的创新点在于,该种天线是相控阵天线和反射面天线两种技术体制的融合,兼具相控阵天线可靠性高、波束控制灵活和反射面天线易实现大口径、轻量化、高收纳比的特点,在一些应用场合可以实现资源和效能的最优化平衡。对相控阵馈源反射面天线的特点做了说明,以期为该技术路线后续发展完善提供参考。

高隔离度双极化微带天线直线阵的设计

本文介绍一种新型的 1× 16双极化微带天线直线阵的设计 .该天线采用双极化角馈方形贴片单元 ,阵列的垂直极化端口采用共面馈电 ,水平极化端口采用口径耦合 ,并在馈电网络中应用反相馈电技术 .实验结果表明 ,天线二端口驻波比小于 1.5的相对带宽分别达到 15 %和 13.5 % ,两端口之间的隔离度在频带内高于 33dB ,最大达到4 3dB .本直线阵可用作星载综合口径雷达系统的天线子阵 ,便于与有源收发器件结合 .

一种新的无源压制性SAR干扰方法

该文针对SAR成像的特性,提出了一种基于旋转角反射器的无源SAR压制性干扰新方法,该方法利用旋转角反射器所产生的微多普勒调制,在方位上形成干扰条带,利用旋转角反射器的排列形成距离向压制干扰,这样可以有效地遮盖所保护的目标。同时,文中详细地论述旋转角反射器关键参数的选择方法,并在此基础上提出一种最佳旋转角反射器布阵方案。仿真和实测数据的处理结果证明该方法的有效性。

基于旋转角反射器的ISAR干扰新方法

根据旋转角反射器的雷达回波特点,提出一种新的ISAR干扰方法。该方法利用角反射器的旋转控制产生的微多普勒效应对ISAR回波进行有效的调制,在方位向上形成干扰条带,同时利用旋转角反射器的距离向布阵实现距离维的压制干扰,从而达到遮盖ISAR目标的目的。对旋转角反射器的干扰建模、旋转角反射器的关键参数选择和布阵策略进行了详细的分析,对该干扰方法的工程实现具有一定的参考价值。最后,仿真数据和实测数据的处理结果验证了该方法的有效性。