Rotation Parameters Estimatoin in ISAR Imaging of Ship Targets

Abstract: Inverse Aperture Radar (ISAR) can image non-cooperative moving targets such as aircrafts, ships and celestial objects over a long distance under all weather and all day. It is of particular interest for strategic defense, anti-submarine warfare and radar astronomy. The ISAR imaging of ship targets depends on the ship’s rotation relative to the radar. Because the speed of the ship is very slow, this rotation mostly originates from the ship sway (roll, pitch and yaw) those comes from the waves. Moreover, the practical sea-state changes frequently and is unpredictable, and the ship’s rotation is complicated. All of these increase the difficulty in achieving high quality images. In this article, we have a deep study on the estimation of the ship’s rotation parameters, and it can become a base of achieving high quality ISAR images.First, this thesis researches on the motion of ship targets. The building of a ship’s motion model is difficult for complexity of the ship’s motion, but it is a prerequisite of researching. In the thesis, for a start, we determine the initial location of the ship's scatters, and approximate the ship rotation to sinusoidal form. Then according to rotation matrix theory, derive the locations of ship's scatters at other moments on the condition that the ship has not any other kinds of motion. From that we can achieve the model of ship’s scatters motion model approximately. It also elaborates a classic ISAR imaging algorithm, the Range- Doppler (RD) algorithm, and makes a detailed derivation of ship's RD imaging algorithm. Simulation includes different conditions with different motion.Then, the estimation of instantaneous Doppler frequency is discussed. In this part, the time-frequency analysis theory is introduced, and according to the theory of time-frequency analysis, instantaneous Doppler imaging method is elaborated. Respectively, two time-frequency analysis methods, the short-time Fourier transform (STFT) and the Wigner-Ville distribution (WVD) are used in the simulation. On this basis, the instantaneous Doppler frequency estimate theorem is explained detailed, and a suitable time-frequency analysis method STFT is chose for the instantaneous Doppler frequency estimation simulation.After the above work, this thesis studies the estimation of ship rotation parameters. The estimation of ship rotation parameters is based on the instantaneous Doppler frequency estimated above, and on the basis of using the relationship between instantaneous Doppler frequency and the ship’s rotation, the ship’s rotation parameters can be estimated by curve fitting method. In this thesis, rotation parameters estimation is decomposed into two parts. One part is to use the scatters on rolling axis to estimate pitch and yaw parameters, and the other is to use scatters on pitch and yaw axis to estimate roll parameters respectively. Meanwhile the feasibility of the method is detailed demonstrated. The simulation is completed at last.In the end, conclusions and future work are explained…
Key words: inverse aperture radar; ship targets; rotation parameters; instantaneous Doppler frequency