【作者】 尹治平；

【导师】 王东进；

【作者基本信息】 中国科学技术大学 ， 电磁场与微波技术， 2008， 博士

【摘要】 逆合成孔径雷达(ISAR)能够实现运动目标的高分辨二维成像,并具有全天候全时段的工作能力,因此受到各国研究者的广泛重视并得到了迅速的发展,成像对象已从最初的平稳运动目标扩展到复杂运动目标,例如强机动飞机目标、复杂海情中的舰船目标、高速飞行的空间航天器和空间碎片、旋翼飞机等。这些复杂运动目标的回波信号通常是非平稳的,具有多分量LFM信号的特征,此时传统的RD算法已不再适用,必须寻找新的处理方法。分数阶Fouire变换(FRFT)是一种新兴的非平稳信号处理方法,它是一种全时域的线性变换,具有很高的时频分辨率,不存在交叉项干扰问题,因而对分析和处理多分量LFM信号具有十分优良的特性。本文将FRFT应用到ISAR成像处理中,对高速目标、机动目标和含游动部件目标的成像进行了较深入的研究。本文首先研究了基于FRFT的LFM信号检测和参数估计问题,提出一种基于分数阶频谱原点矩的检测方法,并经数学推导和物理分析论证了该方法检测LFM信号的有效性。研究表明该方法与RAT检测方法等价,但它的计算量更小,能够更加有效地完成LFM信号检测和调频斜率估计。高速目标成像中的距离色散现象将严重影响目标的距离成像和ISAR图像质量,本文研究了基于FRFT距离压缩的解决方法。通过建立高速目标宽带回波模型和运动目标距离向点散布函数,详细分析了距离色散对目标距离像和ISAR图像的影响。利用高速目标回波是调频斜率相同的LFM信号的特点,采用FRFT替代传统的DFT完成距离压缩,从而消除了目标高速运动的距离色散效应,校正了距离像的模糊和畸变,避免ISAR图像的径向散焦。机动目标的多普勒回波多数情况下可以近似为LFM信号,因此FRFT能够解决机动目标的横向散焦问题。通过分数阶Fourier域滤波对距离单元回波完成子回波分离预处理,可以抑制距离-瞬时多普勒(RID)成像中的交叉项,提高RID图像质量。机动目标成像还可通过FRFT方位压缩实现,当散射点子回波在某分数阶Fourier域的能量聚集性最好时,它在该Fourier域的频谱就是其聚焦横向像,将距离单元内所有散射点在分数阶Fourier域的聚焦横向像相加则可实现该距离单元的横向成像,仿真表明基于FRFT方位压缩的成像结果好于RID方法。并且FRFT方位压缩过程完整地保留回波的相位信息,因此该方法能有效地实现机动目标的三维成像。此外,对于同时具有高速和机动两种运动形式的目标,可通过二维FRFT压缩实现其成像。本文研究的最后一个问题是含游动部件的目标成像,转动部件和振动部件的多普勒回波是正弦频率调制信号,它们与目标主体回波在信号参数上存在较大差异,因此可通过基于自适应高斯短时分数阶Fourier变换(AGSFRFT)的信号分解方法对两种回波进行分离,从而在ISAR图像中剔除游动部件的干扰和污染。本文对FRFT在ISAR成像处理中的应用做了若干尝试,较好地解决了高速目标距离色散、机动目标横向散焦和游动部件干扰等问题,但还有存在较多尚未完善的工作,有待于进一步的深入研究。更多还原

【Abstract】 As a high resolution radar with all-weather, all-time capability, Inverse Synthetic Aperture Radar (ISAR) can obtain the image of the moving targets from a long distance. In the recent ten years, ISAR theory and technique have got great development and, furthermore, been gradually applied to image the complex moving targets, such as the maneuvering-flight airplanes, the swaying ships in extreme sea condition, the airplanes with strong rotor blade, the high-speed moving spacecrafts and space debrises. The conventional Range-Doppler (RD) algorithm is no more applicable in the imaging of those targets, because the non-stationary signal, especially the LFM signal, is involved frequently. Therefore it is necessary to introduce a new method to process the signal.With good cross-terms reduction and high time-frequency resolution, Fractional Fourier Transform (FRFT) is a linear and full time domain analysis tool for non-stationary signal processing. It has been widely used in the multi-component LFM signal processing for its perfect property. In this thesis, the application of FRFT in the ISAR imaging processing is studied. The main contributions and conclusions of this thesis are summarized as follows.Firstly, a new method for the detection and chirp rate parameter estimation of LFM signal is proposed, which is based on the fractional spectrum origin moment. The physical significance and the mathematical derivation are given to prove its efficiency. The equivalence between the new method and the RAT method is demonstrated. However, the new method has a lower computational complexity.Secondly, the imaging of the fast-moving targets, such as spacecrafts and space debrises, is studied. The echo signal and the normalized range point spread function of fast-moving targets are established. For the fast-moving targets, range dispersion will occur in the DFT range compression, deriving from the high-speed radial motion. It will distort the range profile and defocus the ISAR image. A new reconstruction algorithm is proposed, which uses FRFT instead of DFT to implement the range compression. A simulation, which demonstrates the imaging of space objects using space-borne ISAR, indicates that this new algorithm can avoid the range dispersion and achieve refocused ISAR image effectively. Thirdly, four issues, with FRFT applied, on the imaging of maneuvering target are studied. 1) A method for cross-terms reduction in Range-Instantaneous-Doppler algorithm (RID) based on filtering in fractional Fourier domain and ’clean’ technique. 2) Anew azimuth compression method based on FRFT is proposed. 3) The imaging of the target, which has a fast radial motion and a maneuvering rotation at the meantime, is implemented successfully, by using two dimensional FRFT compression. 4) A new Interferometric ISAR reconstruction method, using FRFT azimuth compressoin for maneuvering target 3-D imaging, is proposed.Finally, the imaging of target with moving parts is studied. A signal representation method based on adaptive gauss short-time fractional Fourier transform (AGSFRFT) is proposed to distinguish the main rigid body echo from that of the rotating or vibrating parts. Therefore the pollution caused by moving parts in the ISAR image is eliminated.更多还原