The Application Of Fractional Fourier Transform In Inverse Synthetic Aperture Radar Imaging Processing

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.