Research On Autofocus Algorithms For Synthetic Aperture Ladar

Synthetic Aperture Ladar(SAL)is a new imaging radar system with high resolution.The imaging resolution of SAL system can reach to centimeter even millimeter.The ultra-high resolution feature of SAL system promotes it a wide application prospect.The wavelength of SAL system is micron level,which is much shorter than traditional RF imaging radar system,thus the SAL system is very sensitive to error during the imaging process.Therefore,the autofocus algorithm plays an important role in SAL system.In this paper,autofocus algorithms in SAL system are for atmosphere disturb and platform vibration is investigated the main work is as follows:1.The main reasons of phase error during the SAL imaging process are analyzed.In addition,mathematical models of phase error induced by atmosphere disturb and platform vibration are set up.Typical autofocus algorithm,including signal model based PGA algorithm and image criterion based algorithm,is introduced.The weaknesses of these algorithms are also presented.2.The characteristic of SAL system makes SAL images do not fit traditional autofocus algorithms,which base on single scatter model.In this paper,a new autofocus algorithm based on covariance matrix from the estimates in different range gates is proposed.This algorithm fully consider the correlation between estimate errors in different range gates,and merge these estimates by a new fusion strategy.3.Targets in SAL images often extend in azimuth direction due to the ultra-high resolution.For this reason,an improved maximum likelihood phase error estimation algorithm is proposed.This algorithm first estimate the scatters distribution in every range gate,rather than considering there is only one scatter in every range gate,which is rare in SAL images.Then a maximum likelihood objective function is provided,and the phase error estimate is obtained by gradient descent method.Simulations and outdoor experiments have demonstrated the proposed algorithms effectiveness for atmosphere disturb and platform vibration.The research in this paper provides an important method for the development of the future SAL system.