Study On Spaceborne Inverse Synthetic Aperture Ladar Imaging Algorithm

Inverse synthetic aperture ladar(ISAL)is a long-range high-resolution imaging method relying on the relative motion between the target and the radar,which can break through the diffraction limit of traditional optical imaging.In this dissertation,the ISAL imaging method and motion compensation algorithm are studied for the development of spaceborne ISAL.The content of this dissertation mainly includes:In chapter2,the basic principles of ISAL imaging which include turntable model,dechirp reception and translation motion compensation are briefly introduced at first.Second,after establishing the satellite orbit model,the ISAL imaging geometry is given,and the formulas of effection distance,scene width,synthetic aperture time and doppler bandwidth are derived.Finally,the common ISAL imaging algorithms and their application conditions are given,and the composition of ISAL imaging system are discribed.In chapter3,the unstable carrier frequency of the transmitted signal will cause the pulse compression position shift in the range direction,and the false targets and noise arise in the azimuth direction.Aiming at this problem,a carrier frequency error compensation algorithm based on the reference channel is proposed.The algorithm adds a reference channel in the system and makes a dechirp reception to two adjacent transmit pulses.Then we estimate the carrier frequency difference of the two adjacent transmit pulses according to the pulse compression position of the reference channel.By using the carrier frequency difference,the range and azimuth compensation functions are constructed and the influence of carrier frequency instability on ISAL imaging can be eliminated by compensating the echo signal.Simulation results verify the effectiveness of the proposed algorithm.In chapter 4,aiming at the problem that the traditional "stop and go" assumption is not tenable due to the high-speed movement of targets,a intra pulse doppler compensation and RVP(residual video phase)compensation algorithms are proposed.First,the echo signal model of high-speed moving target is established.According to the analysis,the high-speed movement of the target will introduce linear and secondary phases of fast time,which are called intra pulse doppler,and which will result in the position deviation and mainlobe widening of the pulse compression result of the scattering point.Then,the intra pulse doppler is compensated according to the speed,and the expression of the residual phase after RVP compensation and translation compensation is given.It is considered that when the residual phase is less than π/2,the compensated signal can be equivalent to the range pulse compression signal under the "stop and go" model,and the compensated signal can be directly imaged by RD,keystone or instantaneous imaging algorithm.Simulation results verified the effectiveness of the proposed algorithm.In chapter 5,aiming at the problem of platform vibration,a vibration phase compensation algorithm based on time-delay conjugate multiplication is proposed.Firstly,the vibration signal model is established.According to the analysis,the main influence of platform vibration on ISAL imaging is to introduce sinusoidal modulation phase in the azimuth direction,so after taking the Fourier transform of the received signal along the azimuth direction,the paired echoes will be generated,which makes the "ghost targets" appear in the imaging results.Then,the amplitude attenuation function of the first pair of paired echoes relative to the real target is defined,and it is considered that when the attenuation function is less than-30dB,the vibration phase can be ignored.Finally,a delay conjugate multiplication algorithm is proposed to estimate and compensate the vibration phase through iteration until the amplitude attenuation is less than-30dB.Simulation and real data processing results verified the effectiveness of the proposed algorithm.In chapter 6,aiming at the problems of low echo signal-to-noise ratio and target rotation speed change of spaceborne ISAL imaging,an ISAL multi-look imaging algorithm based on rotation speed estimation is proposed.Firstly,the imaging data is divided into multiple subapertures,in which the target speed is assumed to be constant.Secondly,the rotation speed estimation algorithm based on the image contrast is used to estimate the target rotation speed in each subaperture.Thirdly,using the estimated target rotation speed,all sub-images drived from subapertures are processed into the images with the same pulse compression position of scattering points through interpolation and interception operations.Then,by doing multi-look processing on the processed sub-images,the ISAL image with high signalto-noise ratio is obtained.Finally,the ISAL image an be scaled according to the estimated rotation speed.