Study Of High-Resolution Muti-Mode Spaceborne SAR Imaging Method

Synthetic Aperture Radar(SAR)is a high-resolution imaging radar that actively emits microwaves.It has the characteristics of all-weather,all-day,and strong penetration.It is widely used in disaster prediction,geological detection,and intelligence reconnaissance.Compared with airborne SAR,spaceborne SAR has the advantages of large mapping bandwidth and no limitation of airspace sovereignty,but its system configuration and imaging processing are also more complicated.High-resolution large mapping bandwidth has always been the goal pursued by the SAR systems.At present,dozens of SAR satellites have been launched in various countries around the world,but there are not many high-resolution,especially decimeter-level SAR satellites.The difficulty of high-resolution spaceborne SAR imaging is that when the imaging resolution is very high,the premises of "stop-and-go" approximation,orbital straight line approximation,and equivalent speed invariant for traditional SAR are no longer applicable.The deduced imaging algorithm is no longer effective,coupled with uncontrollable factors such as satellite attitude error,atmospheric propagation error,and earth rotation,all increase the difficulty of imaging high-resolution spaceborne SAR..Therefore,this paper mainly focuses on the three high-resolution spaceborne SAR systems of Sliding Spotlight Mode,Spotlight Mode and Mosaic Mode,and derives three improved NCS(Nonlinear Chirp Scaling)imaging algorithms suitable for these three imaging modes.The main work of the thesis includes:(1)The imaging characteristics of high-resolution spaceborne SAR are analyzed in detail,and the limitations of the minimum antenna area and azimuth spectrum aliasing of high-resolution spaceborne SAR are described.At the same time,different slant range models are studied,and their applicable imaging resolutions and approximation errors are analyzed and compared.(2)Aiming at the high-resolution and large-azimuth scene imaging of the sliding spotlight mode spaceborne SAR,the traditional two-step imaging method can easily cause the aliasing of the focused image in the time domain while eliminating the azimuth spectrum aliasing.In order to solve this problem,under the condition that the azimuth resolution and imaging scene are satisfied,it is deduced that the mixing degree factor has a critical value that makes the azimuth time domain alias,and then the two-step imaging method is improved.The simulation results of point target imaging show that this method can effectively eliminate the aliasing of azimuth spectrum while avoiding the aliasing problem in time domain.(3)Aiming at the spotlight mode high-resolution spaceborne SAR,an NCS imaging method based on an improved hyperbolic slant range model is proposed.First,an analytical method is used to derive the nonlinear factor expression of the distance migration trajectory with distance.Secondly,the accurate two-dimensional spectrum based on the improved hyperbolic slant range model is expanded to the fourth order term,and the influence of the fourth order term is eliminated while filtering the third term.Based on the above two points,the NCS algorithm is extended and improved,and the accuracy and effectiveness of the proposed imaging method are verified by point target imaging simulation experiments.(4)Aiming at the working characteristics of high resolution and Large Squint Angle in mosaic mode,a processing method of track segmentation based on Equivalent Squinted Range Model is proposed.First,a method different from the general Doppler parameter estimation is adopted to provide an Equivalent Squinted Range Model with different parameters for each mosaic unit.Secondly,in view of the problem of large oblique of edge mosaic units,the traditional NCS imaging algorithm is improved.On the basis of the linear change of the frequency modulation in the distance frequency domain with the distance,the nonlinear change with the distance is considered.Finally,the simulation experiment of point target imaging verifies the correctness and effectiveness of the proposed imaging method.