星载多发多收SAR多维编码波形设计与分析

多发多收合成孔径雷达(MIMO SAR)解决了传统SAR存在的高分宽幅矛盾。为提高MIMO SAR对通道之间干扰能量的抑制能力,采用多维波形编码技术进行系统设计,并研究了MIMO SAR的波形设计问题。针对两种新型信号形式:正负线调频信号及正交频分复用调制线调频信号,与短时移正交信号(STSO)进行对比分析及成像实验验证。根据MIMO SAR多个孔径同时发射信号、多个孔径同时接收回波的特点,结合多维波形编码技术进行回波处理。在俯仰维,利用数字波束合成技术进行多个子测绘带的划分,实现大测绘带成像;在方位维,利用多通道重构算法将周期非均匀采样的回波恢复为均匀采样,实现高分辨率成像,在此基础上开展新体制SAR发射信号的设计与成像实验验证。实验结果表明,较之其他两种信号,STSO信号的峰值旁瓣比、积分旁瓣比及分辨率等性能最优,对模糊能量抑制更为彻底,在MIMO SAR中性能良好,与传统SAR的性能近似。STSO在MIMO SAR体制中具有良好聚焦性能,且各项成像指标良好,在一定程度可降低系统设计的复杂度。     

基于傅立叶分析的非均匀采样信号内插重构方法

针对实际采样过程中出现的采样非均匀性,从随机过程的角度研究非均匀采样信号的谱特性,提出频域均匀抽样傅立叶逆变换的非均匀时域采样信号重构方法。将非均匀采样信号描述为不均匀采样时刻冲激函数代数和的形式,利用傅立叶变换得到非均匀采样信号的频谱曲线,由采样时刻随机均匀分布的特点,得到反应原信号频谱特性的非均匀采样信号频谱数学期望,再由频域抽样理论重构原信号。MATLAB仿真实验验证了这种非均匀采样信号分析与重构方法的正确性,将这一研究成果应用到机械抖动激光陀螺输出信号处理中,得到了可靠的符合实际的机械正弦抖动幅频曲线并重构出激光陀螺正弦抖动机构的输出信号。     

基于零阶保持器的非均匀采样信号恢复方法

在模/数转换过程中由于环境等因素会造成信号采样的非均匀性。非均匀采样信号使得数字控制系统数/模转换过程出现误差,提出了将非均匀采样信号通过零阶保持器进行恢复的信号重构算法,分析并证明了重构信号的频谱保持了原连续信号的谱特征。利用PSPICE仿真软件对非均匀采样信号及通过零阶保持器的连续输出信号进行了仿真验证,实验结果验证了本文提出方法的正确性。     

并行ADC采集系统的时间误差测量与校正

并行时间交替采样是提高采样率的一种有效方法,但并行通道间的失配将使拼接后的信号成为非均匀采样,严重降低了整个系统的性能。该文在分析并行时间交替采样信号频谱的基础上,给出了时间误差的测量方法,并采用FARROW结构的全通滤波器实现时间误差校正。仿真结果表明该方法能有效提高信号频谱质量,实现了对非均匀采样信号的时间误差校正。     

宽带信号采样的关键技术研究

提出了一种实现宽带信号实时采样的技术原理和应用设计方法;论述了混合滤波器组技术对传统并行交替采样结构进行非均匀采样的校准处理,多通道采样的时钟与触发同步机制,以及分相存储实现大容量采样数据存储等研究内容;给出了系统的可行性设计。     

地球同步轨道SAR凝视成像变脉冲重复频率技术

实现对特定区域凝视观测成像是地球同步轨道SAR（GEO SAR）非常重要的应用。针对地球同步轨道SAR凝视观测回波大距离徙动造成信号接收窗口难以选择的问题,提出了一种地球同步轨道SAR凝视成像变脉冲重复频率（PRF）设计方法。给出了变PRF设计的准则,详细推导了周期性变PRF过程中脉冲丢失的位置,分析对成像质量的影响,采用后向投影算法对方位非均匀采样信号进行成像聚焦。最后仿真验证变PRF设计的有效性。     

用于谐波测量的非均匀同步采样时钟产生方法

为了消除谐波采样中的频谱泄露并降低电路实现代价,提出非均匀同步过采样时钟产生方法.该方法使用延时锁定环路产生非均匀时钟,控制谐波采样的过采样间隔.通过合理设计过采样率、非均匀时钟频率的概率分布以及变化周期,使非均匀过采样噪声位于模数转换器输出带宽之外,减小了采样噪声对谐波频谱的调制影响,保证了非均匀时钟是统计意义上跟踪基波频率的同步时钟.过采样和时钟的非均匀特性大幅简化了延时锁定环路的结构,所需延时单元个数从3×104减少到125.采样数据可以作为同步采样序列直接进行快速傅里叶变换运算,无需消除非均匀采样噪声和频谱泄露的操作.在使用1.6384 MHz参考时钟、基波频率为46～54 Hz的情况下,63次谐波范围内的谐波幅度和相位测量误差分别小于0.02%和0.031°.     

带通信号非均匀采样理论的研究

为拓宽带通信号非均匀采样的采样频率范围,提出了一种周期性非均匀带通采样方法,该非均匀采样的平均采样频率在数值上等于带通信号无失真均匀采样的采样频率. 分析了采样后信号频谱的混叠问题,在此基础上,给出了无失真重建时内插函数的傅里叶像函数应该满足的条件. 计算机仿真实例验证了非均匀采样时内插函数的计算和带通信号的重建过程.     

非匀速平台SAR成像算法研究

提出一种波数域非匀速平台SAR成像方法．从波数和瞬时斜距的概念出发,通过对回波信号沿非匀速平台方位采样位置进行曲线积分得到理想的回波波数域表达式,再利用常规成像算法进行聚焦成像．该算法适用于任何已知平台方位采样位置的情况,不用进行插值操作,成像精度高．易于和已有成像算法结合．仿真结果验证了算法的有效性．     

一种机载MIMO雷达地面运动目标成像方法

针对传统机载合成孔径雷达（SAR）对地面非合作运动目标成像中存在的运动补偿难题,通过空间并行采样获取目标散射信息,提出了一种机载MIMO雷达的高分辨成像方法.与机载SAR不同在于该成像方法采用单次快拍的方式,从而可避免对非合作运动目标进行运动补偿.同时采用单次快拍成像方法可避免传统SAR方位向上长时间多脉冲采样积累,提高了信息获取的实时性.仿真结果证实了所提出方法的可行性.     

Filterbank implementation for multi-channel sampling in fractional Fourier domain

Reconstruction of a continuous time signal from its periodic nonuniform samples and multi-channel samples is fundamental for multi-channel parallel A/D and MIMO systems. In this paper, with a filterbank interpretation of sampling schemes, the efficient interpolation and reconstruction methods for periodic nonuniform sampling and multi-channel sampling in the fractional Fourier domain are presented. Firstly, the interpolation and sampling identities in the fractional Fourier domain are derived by the properties of the fractional Fourier transform. Then, the particularly efficient filterbank implementations for the periodic nonuniform sampling and the multi-channel sampling in the fractional Fourier domain are introduced. At last, the relationship between the multi-channel sampling and the filterbank in the fractional Fourier domain is investigated, which shows that any perfect reconstruction filterbank can lead to new sampling and reconstruction strategies. Supported by the National Natural Science Foundation of China for Distinguished Young Scholars (Grant No. 60625104), the National Natural Science Foundation of China (Grant Nos. 60890072, 60572094) and the National Key Basic Research Program Founded by MOST (Grant No. 2009CB724003)     

Improved channel error calibration method for the azimuth multichannel SAR

In multichannel synthetic aperture radar (SAR) imaging, the orthogonality of the noise subspace and the signal subspace is usually adopted to estimate the channel error. The division of the subspace is based on the number of ambiguity components. In order to avoid the effect of the inaccurate ambiguity components on the subspace division and the channel error estimation, an improved channel error calibration method is proposed, which can lead to the accurate information on the ambiguity components by the spatial spectrum estimation and calibrate the channel error precisely. The real data results validate the method.     

Multi-channel sampling theorems for band-limited signals with fractional Fourier transform

Multi-channel sampling for band-limited signals is fundamental in the theory of multi-channel parallel A/D environment and multiplexing wireless communication environment. As the fractional Fourier transform has been found wide applications in signal processing fields, it is necessary to consider the multi-channel sampling theorem based on the fractional Fourier transform. In this paper, the multi-channel sampling theorem for the fractional band-limited signal is firstly proposed, which is the generalization of the well-known sampling theorem for the fractional Fourier transform. Since the periodic nonuniformly sampled signal in the fractional Fourier domain has valuable applications, the reconstruction expression for the periodic nonuniformly sampled signal has been then obtained by using the derived multi-channel sampling theorem and the specific space-shifting and phase-shifting properties of the fractional Fourier transform. Moreover, by designing different fractional Fourier filters, we can obtain reconstruction methods for other sampling strategies. Supported partially by the National Natural Science Foundation of China (Grant Nos. 60232010 and 60572094) and the National Natural Science Foundation of China for Distinguished Young Scholars (Grant No. 60625104)     

星载多通道SAR的模糊特性研究

为解决星载SAR的高分辨率和宽测绘带之间存在的矛盾,引入了多通道模式．基于多通道SAR的工作原理,深入分析了多通道SAR模糊信号的来源;从子带滤波和多通道重构的角度,提出多通道SAR的方位模糊计算方法,最后给出了仿真实例．仿真结果表明,该方法能够有效应用于星载多通道SAR设计中,指导选择合适的脉冲重复频率,以降低方位模糊和距离模糊．     

基于相位匹配原理的宽带信号接收方法

针对传统数字信道化侦察接收结构难以采用简单方法对宽带跨信道信号进行无失真重构,以及存在兔耳效应的问题,提出了一种基于相位匹配原理的宽带信号接收方法。该方法采用改进的相位匹配算法对多通道延迟欠采样信号解频域模糊进而获取子信道信号。由于其任意相邻子频带可无缝拼接,所以该宽带信号接收方法能无失真重构宽带跨信道信号,同时避免了兔耳效应问题。仿真结果证明了该方法的有效性。     

Research Progress of the Sampling Theorem Associated with the Fractional Fourier Transform

Sampling is a bridge between continuous-time and discrete-time signals, which is important to digital signal processing. The fractional Fourier transform(FrFT) that serves as a generalization of the FT can characterize signals in multiple fractional Fourier domains, and therefore can provide new perspectives for signal sampling and reconstruction. In this paper, we review recent developments of the sampling theorem associated with the FrFT, including signal reconstruction and fractional spectral analysis of uniform sampling, nonuniform samplings due to various factors, and sub-Nyquist sampling, where bandlimited signals in the fractional Fourier domain are mainly taken into consideration. Moreover, we provide several future research topics of the sampling theorem associated with the FrFT.     

Simulation and Analysis of the Impacts of Anisotropic Ionospheric Scintallition on Geosynchronous Synthetic Aperture Radar

The impacts of ionospheric scintillation on geosynchronous synthetic aperture radar (GEO SAR) focusing is studied based on the multiple phase screen (MPS) theory. The power spectrum density of electron irregularities is first modified according to the ionospheric anisotropy. Then propagation wave equations in random medium are deduced in the case of oblique incidence in GEO SAR. The amplitude and phase errors induced by the random electron fluctuations are generated by the iterated MPS simulations and are superimposed into the GEO SAR signals. Through the following imaging and evaluation, the effects of the anisotropic ionospheric scintallition on GEO SAR are assessed. At last, the optimized integration time under different ionospheric scintillation conditions are recommended through Monte Carlo experiments. It is concluded that, greater ionospheric fluctuations and longer integration time will result in more severe deterioration, even no focus at all in the worst case.     

Ambiguity function of the ground-satellite bistatic SAR system

Base on the earth, moon and satellite trajectories, the geometric model of ground-satellite SAR is presented. The ambiguity function of ground- satellites bistatic SAR is defined and the ambiguity function formula is derived. Moreover, we analyzed the ambiguity function of ground-satellite SAR through numerical simulation. The simulation results demonstrate the validity of proposed function. Furthermore, it is the theoretical basis for the subsequent research on the imaging algorithm.     

机载三通道SAR/GMTI快速目标运动参数估计

针对常规SAR/GMTI系统无法获取快速目标信息的问题,提出一种基于解PRF模糊的快速目标检测和运动参数估计方法．首先将三通道SAR/GMTI数据在方位压缩后的距离频域两两对消抑制杂波,然后进行PRF模糊数估计．在方位压缩后的距离频域利用快速目标运动引起的模糊数与目标直线轨迹斜率的对应关系,通过Radon变换来搜索模糊数,同时得到目标轨迹直线斜率．已知直线斜率后进行方位徙动校正,并以最小熵准则搜索实现方位向聚焦,进而通过距离匹配滤波实现快速目标成像．仿真和实测数据结果均验证了该方法的有效性．