Missile-borne SAR Imaging Algorithm Research

Synthetic Aperture Radar(SAR)is an active microwave remote sensing imaging system capable of providing all-time,all-weather,long-range imaging and multi-polarization,strong anti-jamming capability.It can provide high-resolution,high-precision 2-D images and has great application prospects in military area.In particular,the application of SAR imaging technology to missiles is an effective way to achieve precise guidance in information warfare.Because the missile's flight speed is very high,and the requirement of real-time imaging is strict,the missile-borne SAR imaging algorithm is often more complicated than the air-borne and space-borne algorithm.Especially in the terminal guidance stage,the missile should avoid enemy air defense fire when attacking the targets,so the flight process is no longer a straight line,but often changes in attitude,height and speed.Thus,it's necessary to study different missile-borne SAR imaging algorithms in view of different flight states.In terms of the characteristics of missile,this paper studies the missile-borne SAR imaging algorithms in highly-squinted and diving side-looking conditions respectively.The main contents include the following aspects:(1)This paper analyses the basic principle of SAR from the aspects of RCM and high resolution imaging,and gives an introduction of the R-D and CS algorithms.According to the flying characteristics of the missile,the terminal guidance stage is divided into three parts: the diving stage,the flat flight stage and the target-attacking stage.It is pointed out that the missile-borne SAR is operating in the side-looking,the highly-squinted and the forward-looking imaging modes respectively.(2)When the missile-borne SAR is operating,the echo signal will have serious coupling in the range-azimuth direction,and a large RCM error will be generated,due to the fast flight of the platform and the high squint angle.At the same time,in order to meet the strict requirement of real-time,the amount of calculation in the SAR imaging algorithm should be reduced as much as possible,so as to make it less complicated.This paper elaborates a missile-borne high-squint R-D imaging algorithm,which is based on range walk correction in time domain and range curvature correction in frequency domain,by using the cubic phase coefficient correction to eliminate the RCM error.(3)In the terminal guidance stage,the missile often need to do diving movement to the scheduled attack height,at the same time,search the target-area through side-looking imaging and modify the flight errors.In this process,the motion of the missile in the vertical direction will bring about a series of problems such as range walk,geometric distortion and Doppler parameter change.Therefore,the diving side-looking SAR imaging algorithm not only should realize high-resolution,high-precision focusing imaging,but also to solve the impact brought about by squint angle and other space movement parameter changes.In the diving stage,the signal of side-looking SAR also generate a greatly range-azimuth coupling,and another problem of azimuth focusing depth is usually brought about as we try to solve the coupling issue by correcting the range walk in time domain.In view of this,according to the characteristics of sub-aperture imaging,using frequency domain nonlinear chirp scaling formulation,this paper eliminates Doppler frequency-modulation rate and high-order azimuth phase coefficient cross-dependent by constructing high-order phase filtering function in the azimuth frequency domain,and achieves uniform processing of azimuth.Finally,the image is focused in the Range-Doppler domain.