Study On Long-time ISAR Imaging Techniques Of Ship Target

Inverse Synthetic Aperture Radar(ISAR)is an imaging radar that the theoretical aperture depends on the motion of the observation target,thus the main observation targets of the ISAR system are the non-cooperate targets.The range resolution is determined by the bandwidth of the transmitted signal and the azimuth resolution relies on the size of the synthetic aperture.The longer the aperture is,the higher azimuth resolution is.Owing to its ability to detect and monitor the moving target,especially the non-cooperative targets,ISAR has play a very important role in reconnaissance and monitoring fields of space,air and ocean environment.To improve the reconnaissance ability of the ocean fields,there is an increasing demand for obtaining the high-resolution images of ocean targets.The ship targets influenced by the wave will produced six degree-of-freedom movements which denotes three-dimensional swing characteristics(yaw,pitch and roll)when sailing in the ocean.In the process of long-time observation,not only does the ship target echo present a high-order phase,but also its imaging plane changes during the observation.At present,for ship targets with complex motion,the current algorithms mainly use short-term processing to obtain a well-focused image,scarifying the azimuth-resolution of the radar image.To improve the monitoring and reconnaissance ability of the sea targets,the demand for obtaining a high-resolution image is getting increased in recent years,by studying the long-time imaging algorithm of sea target.In this paper,with the support of the national "973" project,the long-time motion parameter estimation and high-resolution imaging algorithms for ship targets are studied.The main contents of this dissertation include: the parameter estimation method for high order phase signal of ship target,high resolution imaging algorithm for ship target with short aperture echo data,and long-time Doppler tracking imaging algorithm.The contents of this dissertation are summarized as follows:(1)Long time and frequency parameter estimation algorithmDue to the complex motion of the ship target under the long observation process,the echo presents as a high-order phase signal along the azimuth direction.Thus,estimating the parameter of the echo signal plays an important role to obtaining the high-resolution image of the ship target.In chapter 3,a second frequency modulation signal is applied to represent the echo along the azimuth,and a parameter estimation algorithm based on improved integral kernel function is presented.The algorithm avoids the iterative operation with large amount of calculation,and can accurately estimate the parameters of each order of multi-component signal to improve the quality of the ISAR image.The kernel function used in the algorithm has the characteristics of low-order nonlinearity,and introduces fewer cross-terms which can effectively improves the accuracy of estimation for multi-component signals.Also,this algorithm has a high anti-noise performance.The algorithm first uses the symmetry of the signal to estimate the quadratic coefficient of the echo.Then,the signal is transformed from a one-dimensional time domain to a two-dimensional time-delay domain applying the kernel function transformation.After that,these two-dimensional signals are transformed by second-order matching transformation(SMT)and fast Fourier transformation(FFT)along the time axis and time-delay axis,respectively,so that the signal energy can accumulate effectively.Finally,by searching the peak position of energy accumulation,the parameters of multicomponent signals are estimated which denote the first and third-order parameters.(2)Long-time phase tracking imaging algorithmIt is difficult to obtain high-resolution images of ship targets directly because of their complex motion during the long-term observation.Mostly,after applying the appropriate imaging time choosing algorithm can obtain a well-focused radar image with low azimuth resolution.During a short observation aperture,the rotation of a ship target can be simply approximated as a uniform rotation,however the azimuth-resolution of the obtained image is low.To overcome the resolution limitation of the short-aperture data,a phase tracking high-resolution imaging algorithm based on the Kalman filter is presented in the chapter 4.For a linear system,the Kalman filter can estimates and updates the state vector with state transition matrix,observation matrix and the observation values.The transition matrix describes the variation of the state vector between the adjacent two observations.And the observation matrix describes the relationship of the state vector and the observation values.Under the assumption that the target rotates with a uniform speed,the echo can be thought as the result of the inverse Fourier transform of the radar image along the azimuth direction,which meets the linear requirements of the Kalman filter.The algorithm uses the Kalman filter’s tracking ability to estimate and update the high-resolution image at each observation time in order to obtain a stable high-resolution image results within fewer iterations.The KF tracking algorithm can effectively reduce the aperture length required for a high-resolution image.In addition,KF has good anti-noise performance,which makes the algorithm also suitable for the low signal-to-noise ratio situation.(3)Long-time Doppler tracking imaging algorithmWhen the ship target is greatly affected by the sea waves,the phase change is complex during the long radar observation process and the rotation of the targets always has acceleration.In Chapter 5,a Doppler tracking and estimating algorithm,which based on the all-pole model,is proposed.Based on the idea of the short-time-frequency analysis,the algorithm divides the long aperture into a series of sub-aperture using the sliding window.Then,for each sub-aperture,the echo can be established by applying the full-pole model.After tracking the phase changing of the poles between each sub-aperture,the motion parameters can be estimated and the high-resolution image can be obtained.The Doppler center of each scatter is extracted from the echo rely on the corresponding relationship between the scatter and the phase of the pole.The frequency modulation parameters of the echo signal are estimated from the variation of the phase of the poles at different scales to obtain a focused high-resolution ISAR image.