Research On High Resolution Radar Imaging Technology For Targets With Complex Motion

Wideband radar imaging is a primary technique for target recognition, due to its capability to provide the shape and structure information. Four main subjects have been researched in this dissertation, which are the imaging performance analysis for targets with complex motion, the high resolution range profile (HRRP) imaging, translation compensation, and two dimensional imaging for targets with complex motion.Firstly, the research background and significance is introduced, and the development of high resolution radar system for target imaging is reviewed. Then the work about signal processing technology of radar imaging is compared and analyzed in detail. The problems of high resolution radar imaging of targets with complex motion are pointed out.Secondly, the analytic relationships between the phase of echos and the movement parameters are derived, and the formulas for calculating the effection of HRRP are given. For the targets which are equably accelerative rotating, rapidly spinning or precessing, the Doppler signatures are analyzed respectively, including Doppler, Doppler rate and spread of Doppler.Thirdly, in order to obtain the HRRP of targets with complex motion, a method based on predesigned pulse repetition time (PRT) is presented for stepped frequency signals and frequency-stepped chirp signals, respectively. The target velocity will not cause higher order phase terms for the new signals, and the higher order phase terms caused by the acceleration can be eliminated by phase cancellation technology. For a nearly constant acceleration target in imaging time, the parameters estimation is not required for the new technology. The conclusion is verified by a series of experiments conducted both on the simulated and measured data.Fourthly, in the research of translation compensation and the rotational angle estimation, the dissertation proposed a novel range alignment algorithm, which uses both the phase information and the envelope information. Interpolation is not needed for the new algorithm, as a result, good performance can be achieved without a heavy computation burden. Then the inherent consistency between phase compensation and image auto- focusing method in ISAR imaging is analyzed and a new image auto-focusing method is presented, which has more consistency performance with the ideal result of phase compensation. In order to estimate the rotational angle, a method based on phase cancellation (PC) is proposed. In this method, PC is used to eliminate the linear terms of the phase, which is a disadvantage to the rotational angle estimation. By employing the weighted linear least squares algorithm, the rotation angle can be estimated efficiently and robustly.Fifthly, the dissertation has researched the imaging technology for three typical targets with complex motion, i.e., equably accelerative rotating targets, rapidly spinning targets and precessing targets. For an equably accelerative rotating target, the ratio of the second order terms of the phase to the linear terms is derived, and a novel imaging method based on the chirp-Fourier transform is presented. In the new method, the ratio is estimated by calculating the entropy of all the scattering points in cross-range. The steps of choosing range cell and the separation of the scattering points are not need in the proposed method, so the computation burden is greatly reduced. For a rapidly spinning target, a new spinning-rate estimating method is proposed, which is a precondition for two dimensional imaging. In this method, a primary estimation using many coherent pulses is put forward, and it is refined in specified range by calculating the entropy of the whole image, which uses a few coherent pulses to guarantee the convergence. Simulations results confirm its effectiveness and robustness. For a precession target, the cosine of the aspect angle and the RCS can be treated as the input and the response of a linear system, respectively. Both the frequency characteristic of the input signals and the frequency transmission characteristic of the system are analyzed. Then the frequency characteristic of RCS is achieved and a new method of estimating the precession cycle based on the frequency characteristic is presented. The simulation results show that the algorithm is effective. After that, the scattering model of a target with rotational symmetry is analyzed and a novel imaging method is proposed , which is based on the back prejection transform. The echos of the target in the range-time domain are transformed to the scatterer distribution plane by means of coherent cumulation. The algorithm is verified by the experiment results of both the simulated and anechoic chamber data.Chapter 6 is the summary of the dissertation. It also discusses the future work to be further researched.