Research On NLCS Imaging Algorithm For Squint Synthetic Aperture Sonar

Synthetic Aperture Sonar(SAS)is a new type of underwater imaging device with high resolution,which is widely used in underwater exploration and underwater imaging.Synthetic aperture sonar uses a specific device to launch a sound wave signal to form a beam to scan the seabed area and process the received echo signal with the technologies of synthetic aperture and signal processing to obtain the imaging of the seabed area.Synthetic aperture sonar has two imaging modes: side-looking model and squint model.The beam of side-looking synthetic aperture sonar points to an invariant direction vertical to the moving direction of sonar platform,resulting in a narrow application field.The beam direction of squint synthetic aperture sonar can be changed within a certain range with greater flexibility,which can not only detect the front or rear area of the SAS,but also can improve the concealment and safety of the emitting source,so it has a wider range of application.At present,there are few researches on squint SAS imaging algorithms.The echo data of squint SAS have many problems,such as low Doppler frequency modulation efficiency,long synthetic aperture time and intense range azimuth coupling,which make the traditional SAS imaging algorithms not applicable anymore,like range Doppler algorithm and chirp scaling algorithm.This dissertation mainly studies the nonlinear chirp scaling(NLCS)imaging algorithm for squint SAR,and the specific content are as follows:To remove the range-azimuth coupling of the echo,this dissertation modifies the original NLCS algorithm,and proposes the method of Doppler center frequency removing in the range processing,which ensures the validity of Keystone transform.Then,a residual range migration correction method is proposed to improve the accuracy of the range processing,which lays a good foundation for the subsequent azimuth focusing.Digital simulation experiment results show that the proposed method to remove Doppler center frequency and residual range migration can make the NLCS imaging algorithm suitable for high-resolution SAS imaging.According to the properties of range history of the echo,the extended NLCS(ENLCS)algorithm is studied,and then a circular model is proposed to construct the relation between the center slant range of an arbitrary point target and the reference point in a same range cell.By using this relation,an azimuth-variant residual range cell migration correction(RCMC)is deduced to achieve more accurate range processing.Then,based on the circle model,the azimuth frequency modulation(FM)rate is expanded to the second-order term,and an improved extended NLCS algorithm is derived,which obtains high-quality azimuth focusing.Simulation results show that the proposed slant range relation constructed by the circular model to process the echo has higher accuracy than the traditional method.The quadratic phase error(QPE)caused by the traditional NLCS imaging algorithm and the ENLCS algorithm based on the circular model for azimuth focusing is analyzed,which shows that the ENLCS algorithm based on the circular model has lower error.Simulation results show that when the moving velocity of SAS increases to a certain extent,the QPE at azimuth edge caused by the traditional NLCS algorithm will exceed the threshold value,which will lead to the defocusing of the azimuth.However,the QPE caused by the ENLCS algorithm based on the circle model is smaller than the threshold value within the entire azimuth scene,and can get a better azimuth focusing.