Research On ISAR Imaging Technology For Targets With Complex Motion

Inverse synthetic aperture radar is used to produce high resolution two-dimensional(2D) and three-dimensional (3D) images of non-cooperative targets, which can providethe valuable target information of shape, size, structure and attitude. At present, ISARimaging has been an important technique for target recognition. In order to resolve theproblems of ISAR imaging for targets with complex motion, several special researchesare made in this dissertation, including the motion estimation for one-dimensional rangeprofile, ISAR imaging time selection, cross-range scaling and algorithms of2D and3DISAR imaging.In chapter1, the research background and significance are introduced, and thedevelopment of theory and system for ISAR imaging is reviewed. Then the currenetsignal processing technology in ISAR imaging is summerized and analyzed in detail,followed by the introduction of main content in this dissertation.In chapter2, the basic principle of ISAR imaging is analyzed; In order to avoid theeffect of range-Doppler couple on synthetic range profile of stepped-frequency chirpsignal, a method to estimate motion parameters of high moving targets based on thepolynomial phase transform and image contrast is proposed, and the well anti-noise,efficient and real-time performance of which can be obtained; After summarizing thebasic procedure of conventional ISAR imaging techniques, the effect of complexmotion on ISAR imaging is analyzed theoretically.In chapter3, the imaging time selection and cross-range scaling for targets withcomplex motion are inverstigated. Firstly, such an algorithm of ISAR imaging timeselection based on energy accumulation of time-frequency spectrum is presented. Thisalgorithm avoids the effect of low signal noise ratio (SNR) or signal clutter ratio (SCR)and have low computation cost. By choosing the optimal imaging time (i.e. the initialand coherent processing time), the complex motion can be approximated to the uniformrotation, and ISAR image of targets will be obtained by range-Doppler (RD) orrange-instanteous-Doppler (RID) algorithms; Secondly, for cross-range scaling oftargets with complex motion, the model of non-uniformly rotating platform targets withconstant acceleration is established, the exentsional and translational factors associatedwith the azimuth size of target are deduced, and then the cross-range scaling for ISARimage of non-uniformly rotation targets with constant acceleration based on prominentscatters is proposed, which uses echoes of most of prominent scatters with highback-scattering coefficients to obtain well anti-noise performance and develops thecross-range scaling for uniform rotation targets to obtain the rescaled RID images.In chapter4, the research of discrete chirp Fourier transform (DCFT) forhigh-order chirps and its application to ISAR imaging for targets with3D rotating motion is carried out. Firstly, DCFT for high-order chirps is generalized from that forquadratic chirps and its properties are verified theoretically, then DCFT for cubic andquartic chirps can be used for high-order chirp rate estimation, which is free from thecross-term effects between different components of multi-component signals and can beimplemented via the fast Fourier transform (FFT). After analyzing the relation betweenthe original and modified DCFT for cubic chirps, ISAR imaging algorithm of targetswith3D rotating motion based on modified DCFT for cubic chirps is proposed, which isable to avoid the effect of the azimuth echo cubic chirps caused by3D rotation on ISARcross-range imaging, and does not associate with the number of scatters so as to haslower comutation cost than ISAR imaging methods of time-chirp distribution andDechirpClean (TC-DechirpClean) and product high-order matched-phase transform(PHMT).In chapter5,2D and3D ISAR imaging for targets with rapadily spinning motionand precession are inverstigated. By using the spinning characteristics of targets,3DISAR imaging algorithm for rapidly spinning targets based on range–slow-timematched-filter and complex-valued back projention is proposed firstily, which is stillavalid in the case of shadowing, varying backscattering coefficients and low SNR;According to the precession characteristics of mid-course targets, the time-varyingscattering center model (SCM) is established, and the static data measured in darkroomis combined with the dynamic attitude angle to synthesize the dynamic ISARrange-slow-time echoes, and then three constraints on the the radar and targetparameters and three corresponding conclusions for ISAR imaging are deduced, basedon the derivaration of the analytical expression of the attitude angle and effectiverotating angle associated with the trajectory motion and precession. In the end, fromtime-varying SCM, the analytical expression of ISAR echo signals is derived and3DISAR imaging algorithm for mid-course precession targets based on therange–slow-time matched-filter and Clean technique is presented, which reduces thenumber of parameter searching dimensions and avoid the interfere between twodifferent signal components via2D matched-filter and Clean technique, repectively.In chapter6, to resolve the problem of2D ISAR imaging of moving targets withrotating parts, the sine-frequency demodulation Radon Wigner-Vill transfom (RWT)based imaging algorithm for rotating parts is proposed, which can obtain high accuracyof ISAR imaging for rotating scatters by synthesizing the amplitude and phaseinformation of echoes; meanwhile, the echoes of rotating part scatters are removed bythe filtering in frequency domain, then based on RWT, the echo signals of rigid parts areconstructed from the remained echoes and used to produce2D ISAR image by RDalgorithm.Charpter7summerizes this dissertation and discusses the future work.