SAR方位向非均匀采样频谱重构算法及误差分析

SAR DPCMAB（方位向多相位中心多波束模式）能够提高方位向采样率,同时却带来非均匀采样的问题,通过频谱重构算法能够有效恢复原信号频谱。但是在SAR传感器运行过程中存在通道特性不一致、采样时刻偏差、噪声等干扰因素,导致重构后的信号出现频谱噪声,重构精度受到影响。文中通过对混叠干扰因素后的非均匀采样LFM信号进行频谱重构,分析各种干扰因素变化与重构误差的关系。通过仿真,得到了干扰条件下的重构频谱,并通过改变干扰条件进一步得到了频谱误差与干扰强度的关系曲线。仿真结果表明．超过一定阈值后,随着干扰强度的增加,重构频谱误差呈线性增长趋势,最终逼近重构前非均匀采样时的频谱误差。     

PRF可变的星载方位向多相位中心多波束SAR

方位向多相位中心多波束(DPCA)SAR可以解决测绘带宽和分辨率之间的矛盾,但严格限制了对脉冲重复频率(PRF)的选择。该文分析了方位向信号的频谱,给出了当PRF偏离理想值时均匀采样方位谱的重构方法,提出了PRF可变的DPCA模式。计算机仿真验证了新模式的正确性。     

基于多通道重构的DPCA SAR方位向信号处理

研究了一种基于方位向相位中心偏移天线技术的机载合成孔径雷达(SAR)实现方法。通过在方位向上安置相位中心偏移的多个波束来降低系统对脉冲重复频率的要求,从而在保证方位向分辨率不变的条件下能够扩展机载SAR的测绘带宽。针对由脉冲重复频率的变化造成的方位信号非均匀采样,采用了多通道重构算法来恢复均匀采样信号,以两通道机载SAR为例,通过仿真实例验证了该算法的有效性。     

星载SAR DPCMAB技术的方位向非均匀采样研究

在星载SAR分离相位中心方位多波束（DPCMAB）系统中，发射脉冲盲区平和星下点回波盲区的存在会引起方位向非均匀采样，从而在方位向冲激响应的主瓣两侧产生不期望的尖峰。该文采用Spectral—Fit法和时域重构法在不影响图像性能的基础上，消除了非均匀采样带来的影响。最后通过对两者运算量的分析，指出时域重构法优于Spectral-Fit法。     

基于Spectral—Fit法的星载SAR方位多波束方位向非均匀采样的处理

介绍了方位多波束技术的基本原理，指出了其在星载情况下必然会产生方位向非均匀采样，而非均匀采样会在雷达图像中产生虚假目标。Spectral—Fit法的目的是从原始信号的非均匀样本中得到该信号的真实频谱，为了消除方位向非均匀采样的影响，文中将Spectral—Fit法与方位多波束SAR的成像算法结合，对方位向非均匀采样的目标回波信号进行处理。最后通过计算机仿真验证了该处理的有效性。     

利用相位中心偏移方位多波束来实现宽测绘带SAR成像

该文提出了一种基于相位中心偏移方位多波束的宽测绘带星载SAR实现方案.通过在方位向上配置相位中心偏移的多个波束使雷达在一个脉冲周期内完成多个多普勒历程的信号采样,显著降低系统对脉冲重复频率的要求,实现宽测带SAR成像.针对卫星速度的非恒定性和不同波位所需PRF的多样性造成的方位信号非均匀采样,该方案采用了多通道信号重建技术来恢复均匀采样信号的算法.与采用高轨技术和分布式技术的宽测绘带SAR方案相比,这种方案的技术难度较小、易于实现.     

基于方位向多波束-多相位中心的星载SA R多维波形编码技术研究

通过方位向多波束与多相位中心结合,采用方位向-快时间二维波形编码发射技术获得了星载SAR系统高分辨-宽测绘带成像的优良性能。所提方案在发射端的方位向采用脉内扫描形成多个方位窄波束,以减小子多普勒带宽,距离向通过波束展宽获得宽测绘带;在接收端沿距离-方位向形成多个等效相位中心,通过俯仰DBF技术分离各子波束回波信号来抑制距离模糊,方位向通过谱重构方法解方位多普勒模糊,并通过多个子多普勒谱拼接获得方位高分辨。文中研究了本系统的主要系统参数典型设计及性能优势。仿真结果表明,该系统能有效的完成高分辨-宽测绘带任务,相对已有的其它星载SAR系统,系统性能优势明显,系统结构更加轻便灵活。     

非均匀采样对偏置相位中心多波束SAR性能影响的分析

该文从理论上推导了方位向的非均匀采样对偏置相位中心多波束SAR性能的影响。分析表明方位向的非均匀采样会造成脉冲压缩的主峰两侧存在虚假峰值,同时它会造成脉冲压缩主峰增益降低、主峰展宽。文中给出了峰值-假目标比及主峰增益损失的计算公式,表明了它们与不均匀性及方位向过采样率等因素之间存在定量关系。这些公式的正确性通过仿真得到了进一步验证。     

双基星载HRWS鄄SAR系统方位向信号重构的最小二乘算法

双基星载高分辨率宽测绘带合成孔径雷达系统(HRWS-SAR)的方位向信号普遍为非均匀采样,重构其均匀采样信号或多普勒频谱是成像处理的关键步骤。文中将方位照射时间内时变的发射接收距离比近似为常数,利用双基系统与单基系统方位向通道间传递函数的等效关系,建立了一般双基构型星载HRWS-SAR 系统的方位向信号模型,构建了方位向信号重构的最小二乘算法,利用传递函数矩阵的Vandermonde矩阵表示有效降低算法复杂度,并给出了重构性能指标信噪比缩放因子及方位模糊比的计算公式。文中对几种典型双基构型的星载HRWS鄄SAR 系统进行方位向信号重构仿真,结果表明最小二乘算法能较高性能的重构出方位向信号的多普勒频谱。     

基于自适应滤波的DPC-MAB SAR方位向信号重建

偏置相位中心方位多波束(DPC-MAB)SAR系统利用方位向多个子孔径同时接收回波,在PRF较低时根据多波束回波的相关性对其进行联合处理,实现方位向信号的重构,从而在保证高方位分辨率的同时增加测绘带宽。该文针对DPC-MAB SAR系统研究了基于波束形成理论的多普勒解模糊方法。传统非自适应的波束形成算法敏感于通道幅相误差以及各类噪声,而基于Capon算法的自适应最小方差无畸变(MVDR)最优波束形成器在多个模糊分量相关性较强时性能大大衰减,针对以往方法的局限性与不足之处,该文提出了一种改进的最小输出能量(MOE)波束形成算法,该算法首先利用多通道回波数据自适应的估计统计自相关矩阵,根据子空间投影理论对理想导向矢量进行修正,然后利用多重约束条件有效抑制相关模糊分量,解模糊性能明显提升。仿真实验和实测数据处理均验证了其有效性。     

分数阶Fourier域上非均匀采样信号的频谱重构研究

 本文研究了分数阶Fourier变换域上非均匀采样信号的重构问题.首先得到周期非均匀采样信号经非均匀分数阶Fourier变换后的频谱表达式,研究了该分数阶频谱和信号连续分数阶频谱之间的关系,并基于该关系式提出了一种分数阶Fourier域周期非均匀采样信号的频谱重构算法;其次,讨论了分数阶Fourier变换域上更加一般情况下非均匀采样信号重构问题;最后,给出了周期非均匀采样信号频谱重构的仿真结果.     

方位多通道合成孔径雷达数据重建方法

多通道合成孔径雷达可以突破传统SAR不能同时实现高分辨宽测绘带的限制,是SAR发展的必然趋势,在成像处理时,它必须重建方位均匀采样数据。该文提出了一种基于降升调频斜率技术的处理方法,利用线性调频信号特性对方位非均匀数据进行解线性调频处理,然后采用升采样参考函数进行逆解线性调频处理,即完成方位数据的均匀重建。文中给出了信号处理流程,并与重建滤波器组法进行了比较。仿真结果证明了理论分析的正确性。     

幅度非均匀取样信号的数字频谱研究

在并行交替式超高速数字化系统中,由于各个A/D客观上存在的偏置(offset)及增益(gain)的不一致性,造成取样信号幅度的不均匀性.本文首先提出了幅度非均匀取样信号的概念,然后研究了由于偏置产生的幅度非均匀取样信号的数字频谱,推导出了它的数字频谱表达式,详细分析了存在偏置时正弦信号的数字频谱,得到了系统的信噪比公式,最后给出了计算机仿真结果.     

Least-Squares and Minimax Design of Polynomial Impulse Response FIR Filters for Reconstruction of Two-Periodic Nonuniformly Sampled Signals

This paper proposes polynomial impulse response finite-impulse response filters for reconstruction of two-periodic nonuniformly sampled signals. The foremost advantages of using these reconstruction filters are that on-line filter design thereby is avoided and subfilters with fixed dedicated multipliers can be employed in an implementation. The overall implementation cost can in this way be reduced substantially in applications where the sampling pattern changes from time to time. The paper presents two different design techniques that yield optimum filters in the least-squares and minimax senses, respectively. Design examples are included that illustrate the benefits of the proposed filters     

不规则加权小波框架共扼梯度算法

信息传输中的数据丢失、存储数据的硬件损坏等因素常导致采样数据是非均匀的,文章根据不规则小波框架的稳定性理论和不规则加权小波框架算子提出了针对非均匀采样信号重建的不规则加权小波框架共扼梯度算法,进行了数值仿真实验,并和同类重建算法进行比较,新算法在逼近误差与重建速度等方面有较大优势.     

On the optimality of the gridding reconstruction algorithm

Gridding reconstruction is a method to reconstruct data onto a Cartesian grid from a set of nonuniformly sampled measurements. This method is appreciated for being robust and computationally fast. However, it lacks solid analysis and design tools to quantify or minimize the reconstruction error. Least squares reconstruction (LSR), on the other hand, is another method which is optimal in the sense that it minimizes the reconstruction error. This method is computationally intensive and, in many cases, sensitive to measurement noise. Hence, it is rarely used in practice. Despite their seemingly different approaches, the gridding and LSR methods are shown to be closely related. The similarity between these two methods is accentuated when they are properly expressed in a common matrix form. It is shown that the gridding algorithm can be considered an approximation to the least squares method. The optimal gridding parameters are defined as the ones which yield the minimum approximation error. These parameters are calculated by minimizing the norm of an approximation error matrix. This problem is studied and solved in the general form of approximation using linearly structured matrices. This method not only supports more general forms of the gridding algorithm, it can also be used to accelerate the reconstruction techniques from incomplete data. The application of this method to a case of two-dimensional (2-D) spiral magnetic resonance imaging shows a reduction of more than 4 dB in the average reconstruction error.     

Nonuniform sampling techniques for antenna applications

Recent investigations have demonstrated that uniform sampling techniques can be effectively applied for construction of far-field patterns of antennas. There are, however, many circumstances for which it may not be practical to directly utilize uniform sampling techniques. A two-dimensional sampling technique which can employ irregularly (nonuniformly) spaced samples (amplitude and phase) in order to generate the complete far-field patterns is presented. The technique implements a matrix inversion algorithm which depends only on the nonuniform sampled data point locations and with no dependence on the actual field values at these points. A powerful simulation algorithm is presented to allow a real-life simulation of many reflector/feed configurations and to determine the usefulness of the nonuniform sampling technique for the co-polar and cross-polar patterns. Additionally, an overlapped window concept and a generalized error simulation model are discussed to identify the stability of the technique for recovering the field data among the nonuniform sampled data. Numerical results are tailored for the pattern reconstruction of a 20-m offset reflector antenna operating atL-band. This reflector is planned to be used in a proposed measurement concept of large antennas aboard the space shuttle, whereby it would be almost impractical to accurately control the movement of the shuttle with respect to the radio frequency (RF) source in prescribed directions in order to generate uniform (u, v) sampled points. Also, application of the nonuniform sampling technique to patterns obtained using near-field measured data is demonstrated. Finally, results of an actual far-field measurement are presented for the construction of patterns of a reflector antenna from a set of nonuniformly distributed measured amplitude and phase data.     

Efficient signal reconstruction scheme for M-channel time-interleaved ADCs

In time-interleaved analog-to-digital converters (TI-ADCs), the timing mismatches between the channels result in a periodically nonuniformly sampled sequence at the output. Such nonuniformly sampled output limits the achievable resolution of the TI-ADC. In order to correct the errors due to timing mismatches, the output of the TI-ADC is passed through a digital time-varying finite-length impulse response reconstructor. Such reconstructors convert the nonuniformly sampled output sequence to a uniformly spaced output. Since the reconstructor runs at the output rate of the TI-ADC, it is beneficial to reduce the number of coefficient multipliers in the reconstructor. Also, it is advantageous to have as few coefficient updates as possible when the timing errors change. Reconstructors that reduce the number of multipliers to be updated online do so at a cost of increased number of multiplications per corrected output sample. This paper proposes a technique which can be used to reduce the number of reconstructor coefficients that need to be updated online without increasing the number of multiplications per corrected output sample.     

Vector sampling expansion

The vector sampling expansion (VSE) is an extension of Papoulis' (1977) generalized sampling expansion (GSE) to the vector case. In VSE, N bandlimited signals, all with the same bandwidth B, are passed through a multi-input-multi-output (MIMO) linear time invariant system that generates M (M⩾N) output signals. The goal is to reconstruct the input signals from the samples of the output signals at a total sampling rate of N times the Nyquist rate, where the Nyquist rate is B/π samples per second. We find necessary and sufficient conditions for this reconstruction. A surprising necessary condition for the case where all output signals are uniformly sampled at the same rate (N/M times the Nyquist rate) is that the expansion factor M/N must be an integer. This condition is no longer necessary when each output signal is sampled at a different rate or sampled nonuniformly. This work also includes a noise sensitivity analysis of VSE systems. We define the noise amplification factor, which allows a quantitative comparison between VSE systems, and determine the optimal VSE systems     

非均匀取样周期信号取样时间偏差估计算法数学证明

本文证明了在非均匀取样周期信号中只要满足寄生频线不为周期信号基波频线的整数倍这一工程条件,周期信号的各次谐波将不会与非均匀取样产生的寄生谱线发生混叠。