Study On High Squint Imaging Algorithms For SAR Mounted On Maneuvering Platforms Of Xidian University

Synthetic aperture radar(SAR)has been widely applied to military and civil fields for it can provide 2-D high resolution image of the observed scene in day and night under all weather conditions.For future applications,more and more SAR systems are mounted on maneuvering platforms(MPs),such as missiles,unmanned aerial vehicles(UAVs)and helicopters.This kind of SAR has been an important research orientation in the radar imaging field and its characteristics,such as flexible trajectory,observation in advance,real-time imaging,make the conventional imaging algorithms questionable.Therefore,more flexible processing approaches must be sought after.For the development requirements,this dissertation studies some key points on the imaging algorithms of SAR mounted on MPs under the foundation of real-time processing frame with small-aperture imaging and multi-apertures synthesizing.The whole dissertation mainly contains five aspects as follows.Chapter 2: Based on the flexible characteristics of SAR mounted on MPs,the trajectory is divided into two parts: one is the straight track with constant velocity and another one is the curved trajectory with acceleration.Due to the fact that the conventional third-order range model approximation is inaccurate in the circumstance of high squint SAR mounted on MPs,a free-approximation range model is firstly introduced to protect the following imaging procedures.In the traditional range model of high squint SAR mounted on MPs with curved trajectory,the complicated algebraic equations limit the development of imaging algorithms while the fourth-order equivalent range model includes the complexity of numerical calculation and the variation of equivalent factors with range cell.In order to accommodate these issues,a modified equivalent range model(MERM)which decomposes the real range history into the standard range model and the disturbing components is proposed.The MERM simplifies the complexity of fourth-order range model and lays a foundation for wavenumber-domain high resolution imaging.Chapter 3: For the conventional high squint SAR full-aperture imaging algorithms cannot be directly applied in the case of small-aperture imaging,a frequency phase filtering algorithm based on free-approximation range model is proposed.With the difference between full-aperture and small-aperture in the azimuth focusing,a novel filtering factor isintroduced in the azimuth frequency domain to equalize the azimuth dependence of Doppler parameters and realize the azimuth unified focusing.Further,the influences of the resulting residual Doppler centroid are addressed in the high squint SAR imaging and a frequency nonlinear chirp scaling(FNCS)algorithm with highly varying residual Doppler centroid correction over azimuth is proposed.Compared with the previous work,this procedure greatly improves the focusing quality of the final image.Chapter 4: The imagery of SAR mounted on MPs with curved trajectory is a challenging task due to the existence of acceleration,real-time imaging requirement and image distortion.For these problems,a tandem two-step nonlinear chirp scaling(TNCS)coherent with geometric correction is proposed.Firstly,an acceleration-deramping(AD)operation is performed to recover the original spectrum and protect the following procedures.Furthermore,the first-step NCS(Fs NCS)in tandem with the second-step NCS(Ss NCS)are introduced to realize the unified RCM correction and equalize the cross-range dependent Doppler parameters.Finally,a novel geometric correction method which can be integrated with TNCS based on inverse projection is utilized to eliminate the negative effects caused by the imaging processing.The innovative idea is that we display an equally spaced grid on the ground and calculate range and Doppler information of the pixels in the grid,then project the ground image to the slant range plane inversely and the distortion will be considered during the calculation.The whole procedure is computationally efficient and facilitates hardware real-time implementation.Chapter 5: For the traditional frequency-domain imaging algorithms are not suitable for the future high resolution high squint(HRHS)SAR,this chapter explores the application of an improved wavenumber-domain imaging algorithm based on MERM for processing HRHS SAR data acquired at MPs with a curved trajectory.Due to the fact that the conventional wavenumber-domain imaging algorithm cannot make full use of data support region in the case of the skew spectrum in squint mode,a novel idea is proposed to protect the resolution by axis rotation.With this rotation,the skew spectrum can be corrected into line-of-sight(LOS)and the cross LOS,which resembles the spectrum of broadside SAR.Under the circumstance of small-aperture data processing,wavenumber-domain focusing without padding zeroes,which is much more efficient than distance-domain focusing,is performed.The proposed wavenumber-domain imaging algorithm paves the way for the future high resolution imaging of HRHS SAR mounted on MPs with curved trajectory.Chapter 6: For the application of MPs SAR ship detection in coastal region,a hierarchical scheme including ship prescreening with the statistical model of background and targets and ship discrimination by motion properties is proposed.Firstly,a preprocessing is performed which is introduced to eliminate the effect of coastal regions and extract the ship candidates by adaptive window before prescreening.After the preprocessing,the modified mixture distribution model K-lognormal combined with constant false alarm rate(CFAR)for ship candidate detection is analyzed in detail.Then,the discrimination via the difference between the true ship targets and the false ones in the aspects of motion properties is performed to reduce the false alarm rate.Finally,space-borne SAR and helicopter-borne SAR real data processing are presented to validate the proposed scheme.