Study On The High Resolution Imaging Techniques For UAV Airborne Mini-SAR

The combination of mini-UAV platform and frequency modulated continuous wave(FMCW)synthetic aperture radar(SAR)system paves the way for high-resolution radar imaging with flexibility,low-cost and low-power.The high-resolution imaging of FMCW SAR is one of the trends in the community of SAR.But the real-time imaging processing,high-efficiency and high-resolution imaging in the presence of severe trajectory deviation and squint angle are the main challenges in the practical applications of FMCW SAR.On the other hand,circular SAR is another hotspot in the field of SAR,and the flexibility of mini-UAV is very helpful to carry out circular observation trajectory.However,the fast imaging processing methods as well as the related motion compensation methods for CSAR are underdeveloped.In this dissertation,the detailed research work of high resolution imaging processing of linear-trajectory FMCW SAR and circular SAR is carried out,during which the properties of miniature FMCW SAR system have been taken into account.The main contributions of this dissertation can be summarized as follows:1.The signal model of FMCW SAR is established first,based on which the two-dimensional spectrum of FMCW SAR is presented and the influence of intra-pulse movement on the SAR imaging is analyzed in detail.To have a better insight,the generalized derivation of frequency-domain imaging algorithm of FMCW SAR is carried out.Moreover,a modified frequency scaling algorithm which can remove the second range compression term exactly is proposed.2.The signal model of FMCW SAR in the presence of trajectory deviation is established,and the influence of motion errors on the imaging processing of FMCW SAR is proposed.According to the motion compensation,the difference between FMCW SAR and pulsed SAR are analyzed in detail.Besides,to reduce the computation burden of imaging processing,a real-time imaging flow by simplifying the Range-Doppler algorithm is presented.In addition,an autofocus processing by Doppler rate estimation is proposed to produce high resolution SAR images in the presence of coarse navigation information.3.The signal model of circular SAR is established and the exact and completed spectrum expression of circular SAR is proposed first.Based on the signal model,the difference between circular SAR and strip–map SAR is comparied.In addition,the feasibility of multi-pass circular SAR and FMCW CSAR is analyzed.Specifically,the influence of intra-pulse movement on the imaging processing of FMCW CSAR is analyzed and the corresponding correction methods are presented too.4.In practice,almost all the targets in the observed area of CSAR only maintain their electromagnetic scattering within a limited angle range.Accordingly,a novel imaging strategy of CSAR is proposed: coherent processing within sub-aperture and non-coherent superposition of sub-aperture images leads to the final CSAR image.Moreover,a matched filtering based projection method is presented to map the spectrum of CSAR from the slant plane into the ground plane.Eventually,a Fourier-based imaging algorithm of CSAR is proposed,it is efficient and suitable for large observed area.5.To apply the proposed Fourier-based imaging method of CSAR in the presence of trajectory deviation,a three-step MOCO strategy contains space-invariant MOCO,space-variant MOCO,and autofocus after the polar formatting resampling is given.Simulation and experimental CSAR test proved that the MOCO strategy is able to remove the motion errors and can figure out well-focused CSAR images.In summary,the main contribution of this thesis is to provide verified theoretical analysis and practical algorithms for the high-resolution imaging of mini-UAV SAR.All the proposed signal model as well as the presented methods or algorithms are verified by the experimental test data(provided by mini FMCW SAR system),the Gotcha data provided by the American Airforce research laboratory(AFRL),and the simulated data.