Research On Circular SAR And Its Signal Processing Methods

Circular Synthetic Aperture Radar(Circular SAR)is a new imaging radar that can work all day and all weather.In the model the radar moves in a circle with the carrier at a certain height,and the antenna beam always points to the center of the imaging area.Such movement pattern can make the radar obtain the reflection information of the target in all directions so that the spectrum information in the azimuth direction can be extended to the most extent.Therefore,circular SAR not only obtains more target information,but also greatly improves the azimuth resolution,which has advantages that traditional linear SAR cannot match.Among the developed imaging algorithms of circular SAR,the Back Projection(BP)algorithm of time domain is accurate,but its calculation is too large to real-time imaging.The Polar Format Algorithm(PFA)in the wave-number domain based on the assumption of the far-field plane wave can bring about the image defocusing and geometric distortion,which is one of the problems to be solved in high-resolution imaging of circular SAR.In addition,motion error is also a key factor to restrict the improvement of the resolution of circular SAR,so accurate motion compensation is also necessary.Firstly,in order to improve the accuracy of the imaging algorithm of circular SAR,this thesis has analyzed the difference distance of the PFA algorithm under the condition of plane wave approximation.Starting from the essence of mathematics,we make the Taylor expansion of the difference distance on the base of the target distance? and retain the quadratic term of the Taylor expansion which is exactly the phase increment that causes the decline of imaging accuracy.The thesis suggests a phase all-pass system for correcting quadratic terms and a sub-regional compensation algorithm to correct quadratic phase increment.The feasibility of the presented method is deduced in theory,then the results of the experimental simulation proves that the above method can effectively eliminate the imaging defocusing and geometric distortion caused by the quadratic term and improve the resolution of circular SAR imaging.Secondly,the problem of motion compensation for circular SAR is studied.The concept of motion error and their classifies according to the mode of motion are given.The translational motion error which has greatly affected the phase of radar echo is analyzed,then the influence and compensation requirement of the error in slant distance are put forward under three conditions of uniform angular velocity error,constant angular acceleration and sinusoidal fluctuation.Finally,the motion error model of circular SAR is established,then the two-dimensional spatial-variant properties of the motion error is discussed in detail,where the motion error of slant distance is divided into space-variant motion error and space-invariant motion error which are respectively used for two-step motion compensation in Polar Format Algorithm.The effectiveness of the proposed algorithm is verified by experimental simulation.