Research On The Imgaing Algorithm Of Missle Borne Synthetic Aperture Radar

Synthetic Aperture Radar(SAR)imaging technology obtains the range resolution by transmitting broadband signal,obtains azimuth resolution by Doppler phase generated by the accumulation platform motion,it is a kind of imaging method with the advantages such as:all-day,all-weather,long distance and strong anti-jamming capability,etc.The missile borne SAR technology combines SAR imaging with the seeker technology,is the leading direction in the field of missile guidance and has gradually become a research hotspot in recent years.On the one hand,missile borne SAR can carry out high-resolution two-dimensional imaging to the observation area,to obtain abundant topographic feature information,and calculate the real time position coordinates of the missile by image matching and geometric relations,so as to achieve the purpose of correcting ins accumulation error and positioning navigation;On the other hand,the missile borne SAR makes two-dimensional imaging to the target area,to extract the target's size and shape feature,then select the attack point and improve the accuracy and efficiency of the attack.However,compared with traditional airborne and spaceborne SAR imaging system,the missile borne SAR needs to solve a series of technical problems due to its special imaging system:the purpose of the missile borne SAR imaging is to attack the target,in order to have a certain steering adjustment time,the missile borne SAR cannot use normal side looking or small squint mode,but must work in large squint imaging state;In order to improve the success rate of the search target area,the missile borne SAR should be as much as possible to get landform and other environment information,so as to improve the matching success rate of scene image,which requires missile borne SAR has the ability of large scene imaging;Terminally guided projectile is in the state of two or three dimensional acceleration,and the flight trajectory is different from the general SAR imaging mode,which will lead to serious coupling of the range and deterioration of imaging quality;In the attach flight stage,the missile beam antenna directly points to the target area,the missile borne SAR will work in forward-looking state,will lead to fuzzy left and right and deterioration of imaging quality.Aiming at the problems of missile borne SAR,this paper focuses on the specific application environment of missile borne platform and the imaging requirements in missile flight,to develop the following research work:(1)It studies the imaging method of missile borne SAR in large squint stripe mode under plane flight trajectory.It focuses on the strong coupling of range azimuth in large squint imaging mode to analyze the characteristics of range migration in squint mode,to analyze the causes of the failure of the traditional frequency domain algorithm,and put forward the method of two-dimensional decoupling;then,after analyze the influence of time domain linear walk correction on the range envelope,to describe that in imaging,the same distance gate has the phenomenon of frequency modulation error,and propose DFT filter bank's azimuth frequency compensation method by using time-frequency convolution theorem and block focusing idea.The simulation results and the measured data show that the new algorithm is superior to the traditional algorithm.(2)It studies TOPSAR imaging method of missile borne large squint.Large squint TOPSAR imaging has the ability of large scene imaging,but also has a series of processing difficulties:beam scanning in azimuth and large squint imaging geometry will cause the echo to have a serious range-azimuth coupling,and make the azimuth signal not be sampled;in the imaging stage,the flight path of the radar platform is much smaller than the scene length,which brings difficulties to the final azimuth focusing.The fourth chapter analyzes these imaging problems,and also puts forward a kind of improved azimuth scaling compensating imaging algorithm.First,the algorithm uses Doppler center spatial variation compensation effect brought by nonlinear walk correction,to solve range azimuth coupling,and at the same time,solve the aliasing in the azimuth frequency domain and time domain;then,by the improved nonlinear scaling algorithm to compensate azimuth frequency modulation spatial variation effect brought by nonlinear walk correction;Finally,by two-dimensional scaling to do geometric correction to focus image.The simulation experiment shows that compared with the traditional algorithm,this method is simple and clear,it can avoid the interpolation and massive data expansion,has the advantages of high efficiency and easy implementation.(3)It studies the characteristics of missile borne SAR echo of non-uniform trajectory of terminally guided projectile,puts forward sub-aperture imaging method of azimuth frequency domain projection focusing.We can know from theoretical analysis and simulation experiment that,due to the influence of acceleration,both the radial velocity and Doppler process of the scene target have spatial variation when imaging in non-uniform trajectory,which requires compensate for the spatial variation in linear compensation and azimuth focusing processing.In order to solve these problems,this paper uses the relationship between the instantaneous Doppler and the velocity to derive the error influence of traditional linear motion compensation,and based on the analysis of the error sources to put forward the method of compensating the linear item spatial variation by the introduction of auxiliary frequency modulation;Azimuth focusing starts from the view of time domain projection,uses azimuth signal bandwidth limit after coarse focusing to complete the projection focusing in the frequency domain.The simulation experiment shows the correctness of the algorithm.(4)It studies forward-looking defuzzification imaging method of three-antenna missile borne SAR.When the missile borne platform is in forward-looking state,the slant distance and the Doppler course of the two sides' target echo of projectile motion plane will be exactly the same,resulting in left and right ambiguous and increase the difficult of imaging.In order to solve this problem,the sixth chapter is based on beam weighting and null theory to put forward three-antenna forward-looking SAR imaging method:first,according to equivalent phase center principle of the antenna,three-antenna array is equivalent to five-antenna array by reasonable distribution of receiving and transmitting of antenna;then on the basis of the fifth chapter sub-aperture imaging,uses the phase difference of each array element to do beam weighting and null,to defuzzify each pixel in image domain.The simulation results show that compared with the traditional dual antenna imaging method,this algorithm can be more effective to solve left and right ambiguous problem in forward-looking imaging.