Study Of ISAR Ship Imaging

Nowadays much work has been done on ISAR imaging of airborn targets. Imaging of ship targets is also very important for national defence. However, the investigation of ship imaging is in leading strings in our country. Because of the influence of ship voyage as well as sea wave fluctuation, rotation of ship is more complex than airborn targets. Furthermore, the influence of multi-path is more severe. This paper will investigate the main problems in ship imaging such as signal model, imaging algorithm, muti-path and rotation estimation. This paper is arranged as follows:Firstly, we introduce the research status of radar imaging. Then we discuss the main theories of ISAR imaging. Many basic issues related to ISAR imaging such as range compression, movement compensation and imaging algorithm is analysed.According to movement complexity of ships at sea, we obtain the signal model through theorectical analysis. Then we discuss the characteristics of received signals while one or more dimentional rotation exists and corresponding simulation is gaven.On the condition of low sea level the rotation of ship targets is unconspicuous and target images can be obtained through Range-Doppler algorithm. However, when sea level is high three-dimentional rotation is obvious which results in the variation of Doppler frequency of scatters. On this condition instantaneous imaging is needed. We divide instaneous imaging algorithms into two categories the first one of which is based on time-frequency analysis. Several representative linear and bilinear time-frequency distributions are choosen and corresponding instantaneous imaging results are gaven. Towards the conflict between time-frequency centralization and cross-term depression two kinds of new instaneous imaging algorithm is proposed: one method is based on S-Method and the other is based on time-frequency domain filtering. The second algorithm category is based on parameter estimation. Adaptive Chirplet decomposition is adopted to estimate signal parameters such as initial frequency, chirp rate, centeral time and signal width. Then imstaneous image is obtained according to results of Chirplet.Multi-path is one problem which influences imaging of ship targets. We analyze the mechanism of multi-path and obtain signal model under the condition of multi-path. Then we propose one method with the procedure of"initial images-separation-second time imaging".Since the complexity of ship movement, we investigate movement estimation based on received signal. We proposed one method according to which initial images series are obtained firstly. Then we estimate the ship movement based on image processing and feature extration. Data are choosed based on the results and simulation results are gaven.