Study Of Approaches To High Resolution Two-and Three-Dimensional ISAR Imaging

Inverse Synthetic Aperture Radar (ISAR), is an effective remote sensing tool in all-time and all-weather conditions, and has ability to provide an imagery of a target at very long range. ISAR has been playing more and more important roles in defense applications, such as surveillance of satellites and deep space targets. As the rapid development of industry and semiconductor, the advanced radar imaging technology has being studied. This dissertation focuses on study of some novel approaches to high resolution two-dimensional (2-D) and three-dimensional (3-D) ISAR imaging. The summary of this dissertation is given as follows:Chapter 1 is the introduction, which reviews the history of ISAR development and introduces the dissertation’s research background and related works.Chapter 2 presents the basic theory of high resolution ISAR. In this chapter, we review recent ISAR imaging algorithms, and some remaining problems are discussed breifly as well.Chapter 3 presents a novel approach to ultra-wideband ISAR imaging. First, a high-order approximation for instant range between target and radar is analyzed theoretically at the beginning of the chapter. Then DCFT (discrete chirp-fourier transform) is used to estimate the second-order phase error, which generates the migration through resolution cell. As the chirp rate is dependent on the range, the linear part can be obtained by searching the peak of the DCFT distribution. Subsequently, the second-order phase error can be compensated range by range. Finally, well-focused ISAR image can be generated by performming FFT in azimuth. In the last part of chapter, a backscatter model that has long coherent processing interval is presented. The new scattering characteristic is modeled as a function with respect to both the frequency and aspect angle, known as a parametric model. A method for target classification based on the scattering model is proposed. The validity is confirmed by real data and numerical simulations.Chapter 4 discusses the sparse aperture ISAR imaging technology. In the application of inverse synthetic aperture radar, the sparse data problem arises when continuous measurements are not possible or the measurements during certain periods are not valid. In this chapter, the interrupted or missing data model is described, and measurement matrix and dictionary are established. The thesis discusses a precise formulation of image formation as a sparse signal representation problem under a specific incomplete basis. Moreover a specific solution to the optimization function with regularization constraint is also introduced. Considering the phase error in the radar returns, a Quasi-Newton optimization is applied to minimize the cost function joint with retriving the phase error. Results with simulation data and real sparse ISAR data validate the feasibility and superiority of the approach.Chapter 5 studies the imaging processing for high speed spinning targets. It often occurs that the targets or some parts of a target are rotating along with the bulk translational motion, such as space debris, flying missiles, airscrews of airplanes, etc. The high-speed spinning turntable model with some assumptions is presented. Based on the time-frequency characteristic of the echoed signal, a novel coherent spectrogram redistribution method, coherent single range Doppler interferometry (CSRDI), is proposed, which is capable of generating high-resolution imagery by applying a phase matched filtering processing. In this chapter the cases of low SNR and Doppler Ambiguity are considered in detail. As the precision of spinning rate estimation is very important for both the existing approaches and CSRDI, the spinning rate error is taken into account and the estimation method based on image entropy is proposed in the last of chapter. The validity is confirmed by real data and numerical simulations.Chapter 6 studies 3-D ISAR imaging of spinning targets based on narrow-band stepped-frequency. A 3-D inverse synthetic aperture radar imaging method based on stepped-frequency waveforms for high speed spinning target is proposed in this chapter, by using narrow-bandwidth signal. Assuming the motion compensation is completed in advance, single-range matching filtering is performed in certain sub-pulse firstly to obtain ISAR image. Then parameters of scatters are estimated using CLEAN technique. ISAR images are obtained in different sub-pulse and different range bin. At last range synthetic profile processing is performed to get high resolution in range and the 3-D image is obtained. This method can void the Doppler influence of spinning targets on range synthetic and improve the resolution effectively. Simulations validate the feasibility and superiority of the approach.Chapter 7 discusses the 3-D ISAR imaging technology based on sum and difference beam. A wideband signal is transmitted in conditional angle-measurement monopulse radar. Firstly, the co-compensation for translational motion and phase error are performed in three receiver channels. Second, traditional processing is used to obtain the ISAR image and isolated scatters are chosen in image domain. At last the amplitude-comparison or phase interferometry with multi-channels is performed to scale the cross range bins. The results of real-data processing indicate that the algorithm proposed here performs better than traditional method, which validate the feasibility and superiority of the approach.