Analysis And Research On Ionospheric Effects For P-band Spaceborne SAR Imaging

In recent years, the synthetic aperture radar (SAR) system has been developed rapidly, and the P-band spacebome SAR has been paid more attention by domestic and foreign scholars because of its strong penetration in the surface and vegetation. However, when the radio wave passes through the ionosphere, it’s transmission path will be changed by the ionospheric effects, especially the P band, which is the main factor to restrict the application of spacebome P-band SAR. So the ionospheric effects on imaging performance of the SAR system and the error compensation technologies have become hot research areas. As the basis of the research work, the imaging quality and error compensation methods of P-band spacebome SAR in ionospheric effects have been systematically studied on the base of radio wave propagation theory. The specific research works have been carried out from the following aspects.(1) Based on the Maxwell equation, the ionosphere influences of time delay are deduced, then the ionospheric effects on the ambiguity function are analyzed by Klein-Gordon equation and SAR imaging theory, which can be used to explain the reasons of SAR image distortion after pulse compression. Finally, the new pulse width and chirp rate are deduced in the influences of background ionosphere and irregular body disturbance.(2) The SAR signal and image quality of ionospheric effects are analyzed by modeling and simulation, then the correctness of the theoretical analysis and simulation are verified by the point target imaging. The ionospheric effects are studied from two aspects:①The dispersion effects on the SAR imaging performance such as range reslution, image position, peak sidelobe ratio, integral side lobe ratio, peak power and so on. ②The scintillation effects on the performance of SAR imaging such as azimuth resolution, peak value, peak sidelobe ratio, integral side lobe ratio and so on.(3) Two dispersion phase error compensation methods caused by background total electron content(TEC) have been studied. Through the theoretical derivation and simulation, the maximum contrast auto focusing algorithm and stochastic parallel gradient algorithm are used to compensate the dispersion phase error, and the validity of the two methods are verified by simulation results. ①The auto focusing algorithm based on maximum contrast is studied, which is use the quadratic phase error compensation to realize the image auto focusing. ②Parallel gradient descent stochastic (SPGD) algorithm is introduced to the correction of the matched filter function. The chirp rate is used as a control parameter, and the peak power is used as a performance evaluation function, then the convergence speed of the algorithm is accelerated by the way of bilateral perturbation. The simulation results of point target verify the validity and rapidity of the algorithm. ③The ionospheric TEC is measured by a narrow band dual frequency correlation method in the system, then the correction of matching filtering function is realized. The SAR image is obtained by the corrected matching filtering function.(4) Several compensation methods for scintillation phase errors caused by small scale TEC changes have been studied on the base of SAR imaging scene characteristics and scintillation phase error model. The simulation results show that these methods can effectively suppress the ionospheric scintillation effects, and improve the azimuth resolution. ①The minimum entropy auto focusing algorithm for motion object phase error compensation in ISAR imaging is applied to the ionospheric scintillation phase error correction of SAR in uniform scene. The method solves the problem that traditional algorithm can’t focus the image in uniform scene, and can be applied to the correction of defocus SAR image with low signal noise ratio (SNR). The simulation results verify the effectiveness of the method. ②In order to improve the calculating speed, the PGA algorithm is studied. The method uses Fourier transform instead of iteration in the process of phase error gradient estimation, so it can compensate phase errors fast to the images with multiple isolated strong points and have a good focusing effect. ③For the SAR image with two dimensional space variant characteristics, the phase errors are not present or have a poor redundancy, so the traditional PGA algorithm can’t carry out the auto focus, then the range direction sub-aperture auto focusing algorithm is proposed. The method can improve the image quality, and the image texture is more clear.