| Linear array 3D SAR can overcome the defect of can't obtain the spatial information of observed space of 2D SAR imaging and resolve its problems of occlusion, spatial fuzzy and top-bottom upside down. And LASAR has the advantage of easy control flight path and down-looking Imaging, which gives it a good prospect and high research value. In this thesis, to overcome the limits of signal bandwidth and array aperture to get higher 3D spatial resolution, we study the 3D super-resolution imaging method, signal transmitting and receiving mode and flight programme of LASAR. The main research contents and contributions of this thesis are as follows:1. The basic principles and data processing methods of LASAR is introduced. First,the working mechanism as well as four operating modes and their features are introduced. Then the mathematic model of LASAR signal and 3D spatial resolution are illustrated. Finally, the Range Doppler(RD) and Back Projection(BP) algorithm of 3D SAR imaging are discussed.2. A range focusing algorithm of LASAR based on single snapshot MUSIC is proposed. To resolve the problem that the range resolution is limited by the signal bandwidth, we treat the range focusing as an estimation of the target's position,amplitude and phase in the range image. Then a single snapshot MUSIC is used to estimate the time delay of the target's echo to determine its position. After that, the Least Squares Estimate(LSE) method is adopted to get the amplitude and phase of the target. By combining the two estimation methods, we can reconstruct the range image with super-resolution. And the phase needed to azimuth and cross-azimuth imaging is reserved as well. Result of simulation experiment shows that this algorithm can still get a good super-resolution effects in low SNR conditions. And result of measured data processing also shows that the algorithm can achieve range super-resolution imaging and sidelobe suppression.3. Improved the azimuth and cross-azimuth super-resolution imaging methods based on MUSIC of LASAR. To resolve the problem exists in the original methods that the synthesis array's effective aperture is sacrificed when using sliding average method to construct the covariance matrix, we adopt a new covariance matrix construction method and obtain a better super-resolution effect than the original method. Result of the simulation experiment shows that the improved method has stronger ability of super-resolution.4. 3D SAR experimental system based on stepped-frequency signal is studied. As the wide-bandwidth LFM signal is difficult in hardware implementation and needs high sampling rate, we use Vector Network Analyzer as the transceiver devices of the 3D SAR system to confirm stepped-frequency signal instead of the LFM signal to realize range high resolution. In this way, we can obtain wider signal bandwidth with lower cost. The principle of range high resolution is illustrated and the mathematical expression of range resolution is deduced. After that, applicability of the"move-stop-move mode" and the stepped-frequency signal's influence to range focus is analysed and two solutions is given. Result of field experimental demonstrates the feasibility of this mode of signal transmitting and receiving.5. 3D SAR experimental system based on helical trajectory is studied. We designed a helical trajectory based 3D SAR experimental system, moving the antenna along a helical trajectory to synthesize a circular array to obtain the 2D resolution in the the antenna's motion plane. And combine the range resolution obtained by stepped-frequency signal to finish 3D imaging. The radial blur function is deduced and the influence of the array to the radial resolution and sidelobe is analysed. The array's resolution of different direction is the same and its radial blur has lower sidelobe compares to single circular array, which provides a solution to solve the problem that the cross-azimuth resolution is far lower than the azimuth when the LASAR system fly with a straight trajectory. |
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