Study Of ISAR High Resolution Imaging And Target Parameter Estimation Algorithms

Inverse synthetic aperture radar?ISAR?is able to generate high resolution images of non-cooperative targets and it has been applied in both military and civilian field.In this dissertation,the ISAR imaging algorithms,the extraction of anisotropic characteristics of scatterers and the motion compensation algorithm for synthetic aperture radar?SAR?imaging relative to ISAR imaging are studied.The main contents in this dissertation include the ultrahigh range resolution ISAR imaging algorithm,the ISAR imaging and cross-range scaling algorithm for maneuvering targets,the algorithm to extract anisotropic characteristics of scattering centers in wide-angle imaging and the algorithm to eliminate paired echoes in helicopter-borne SAR/ISAR.The main contents in this dissertation are summarized as follows:The imaging principle of ISAR is introduced and the resolution of ISAR images is analyzed in the Chapter 2 firstly.The algorithms for ISAR imaging processing are also introduced.In the second part of this chapter,we focus on the selection method of regularization parameter in the sparse signal recovery for ISAR imaging.Suitable regularization parameter plays an important role in sparse ISAR imaging algorithms.With proper regularization parameter,the quality of an ISAR image improves.In Chapter 2,the homotopy re-weighted L₁-norm minimization is applied to ISAR imaging.This method is able to choose the accurate regularization parameter for each point in an ISAR image with high efficiency.As a result,the imaging results processed by this method contain more details of the targets and less artificial points.Both simulated and real data experiments validate the effectiveness of the proposed method.The slant-range resolution of ISAR can be improved to 1.5cm by the application of microwave photonics technology.With the great improvement in range resolution,the ISAR images may contain more information on targets.However,ultrahigh resolution also induces severer space-variant range cell migration?RCM?.In Chapter 3,we propose an algorithm based on Keystone transform?KT?and time-domain chirp scaling?TCS?to deal with the space-variant RCM in ISAR imaging with ultrahigh range resolution.The algorithm consists of two steps.In the first step,the azimuth-variant linear components in the space-variant RCM are compensated for by Keystone transform.In the second step,the TCS is performed to remove the range-variant quadratic RCM.The processing results of both the simulation and the real data validate the effectiveness of the proposed algorithm.A long integration interval may lead to the instability of rotation speed and the variation of the image plane.As a result,imaging interval selection should be taken into consideration in order to obtain high quality ISAR image.In Chapter 4,the relationship between the effective rotating speed and the image plane is analyzed in detail firstly,by which the conclusion that the image plane is stable and the resolution of an ISAR image is best when the valid rotation velocity of a ship target is highest is drawn.Then a new method to select the favorable imaging interval based on the rotation speed of targets is proposed.In comparison with existing methods,the proposed one can select a better period for ISAR imaging which leads to an imaging result with a higher quality.The effectiveness of the proposed principle of imaging interval selection is verified by the simulation and real data processing.The azimuth resolution of ISAR images is determined by the look-angle diversity of the target.In order to improve the azimuth resolution,it is necessary to enlarge the observing angle,which is achieved by increasing the integration time.However,as the integration interval lengthens,the effective rotational speed of a target can no longer be regarded as a constant and rotational acceleration should be taken into consideration.In the second part of Chapter 4,we propose a novel ISAR imaging and scaling algorithm for maneuvering targets with both effective rotational velocity and acceleration.The existence of rotational velocity and acceleration can give rise to range-variant and azimuth-variant phase errors which are not considered in the traditional ISAR imaging and scaling algorithms.In the proposed scheme,the nonlinear chirp scaling algorithm?NCSA?is introduced to remove the azimuth-variant phase error.On top of that,the estimation of rotational velocity and acceleration is transformed into a nonlinear least squares?NLS?problem.This problem can be solved by Gauss-Newton method and FFTs with high efficiency.The results acquired from the processing of both the simulation and real data validate the effectiveness of the proposed algorithm.Based on the fact that anisotropic characteristic represents certain intrinsic properties of scattering centers,an algorithm for extracting anisotropic characteristic of scattering centers in wide-angle imaging is proposed in Chapter 5.Firstly,the components of the signals from a single scattering center are analyzed based on attributed scattering center model.Secondly,the estimation of anisotropic characteristic is transformed into an inverse issue of solving variation of a single scattering center's amplitude by using an identity matrix as the orthonormal basis.Finally,the inverse issue is solved under the constraints that the amplitude of scattering centers should be real and continuous.As a result,anisotropic characteristic is extracted from the solution of the inverse issue.Estimation results of both MATLAB simulated and electromagnetic computation data validate that the proposed algorithm is not only precise but also robust.Moreover,the proposed method is more efficient compared with traditional methods.A helicopter may vibrate at high frequencies during its flight,which can give rise to paired echoes in the helicopter-borne SAR/ISAR imagery.This phenomenon substantially degrades the quality of SAR/ISAR images since it may generate unwanted ghost targets in the images.In Chapter 6,we propose a method consisting of three steps to suppress paired echoes in helicopter-borne SAR/ISAR imagery based on the characteristics of the helicopter vibration.The proposed method can suppress paired echoes with high efficiency and accuracy in the case of vibrations with slowly varying amplitude.Furthermore,this method does not require the presence of isolated or strong points in a range cell.Both simulated and acquired data are processed to demonstrate the effectiveness of the proposed algorithm.