함 요동에 따른 함정 RCS 분석 및 산란점 위치 식별 연구

Abstract

RCS (radar cross section) level for maritime ship is closely related to the ship survivability because it can have a direct impact on the detection range from an anti-ship guided missile or enemy surveillance radar. For this reason, the RCS value of the ship should be identified by measurement in the maritime environment. If the value of the RCS exceeds a specific standard level, RCS patterns in radar aspect angle to the ship should be analyzed. After aspect angles of high RCS level are identified, two-dimensional scattering point analysis techniques such as hourglass and ISAR (inverse synthetic aperture radar) image give the identification of scattering point positions in the target at the the aspect angles, and then appropriate RCS reduction measures of the target are established. In this dissertation, the range-Doppler processing method is applied as the ISAR image analysis method. The range-Doppler processing technique assumes that there is no down-range movement of the target, and if the target rotates at a constant speed around the rotation axis, the position of the target scattering point in the down-range direction can be obtained at a resolution according to the bandwidth of the radar transmission frequency. In cross-range direction, a difference in rotation speed of the scattering point in the direction of the radar line of sight (RLOS) occurs in proportion to the distance between the target rotation axis and the scattering point within the target, and thus a Doppler shift occurs. Accordingly, a two-dimensional ISAR image can be obtained by mapping the scattering points for each distance in the radar line of sight and the Doppler signal formed in the vertical direction of the radar line of sight. If the above-mentioned ISAR image processing is applied to the radar signal reflected by a maneuvering target, however, the focus of ISAR image is blurred. The cause of the blurring in the ISAR image is due to the change in the translational component with the movement of the target in the direction of the radar line of sight and the nonlinear change in the observation angle by the non-uniform rotational movement. For this reason, translational and rotational compensations are required for ISAR image processing. The translational motion compensation can be divided into range alignment and phase adjustment processes. Range alignment is the process of moving the range cells of each HRRP (high resolution range profile) such that signals reflected from the same scattering source in the target during the radar observation time are located in the same range bin within the HRRP. The phase error of the scattering point within the range bin between HRRPs due to range alignment occurs as the range moves. Phase adjustment is the process of removing the phase errors caused by range alignment. Rotational motion compensation is the process of estimating and compensating the phase error between HRRPs caused by nonlinear rotational motion combined with translational motion to form a constant Doppler shift for the same scattering point. In this dissertation, hourglass and ISAR analysis are performed with the measured data obtained from the test vessel in the marine environment, and using the above-mentioned compensation techniques, the locations of the scattering points in the target structure are identified. Hourglass and ISAR image analysis are performed with RCS analysis software that is often used in the design phase of new naval ship’s RCS reduction, which is compared with measurement results. In order to establish a method to reduce the RCS of maritime ship, it is necessary to analyze the RCS in the elevation angle direction considering the steering angle of the enemy guided missile as well as the RCS for the target's radar aspect azimuth angle. RCS information for each azimuth and elevation angle is essential for naval tactical maneuvering to evade anti-ship guided missiles. In this dissertation, the self-motion of the ship caused by the fluctuation of sea wave is collected using the attitude measurement equipment, and the radar aspect azimuth and elevation angle information for the target are analyzed by transforming the coordinate axis of target plane into image projection plane using the motion data. The ISAR image is formed by projecting the scattering point of the target onto the image projection plane. However, due to the target fluctuation according to the maritime condition, the mismatch between the image projection plane and the actual target plane constantly occurs, which causes the ISAR image to lose focus. The focusing of ISAR image is directly affected in proportion to target fluctuation. In this dissertation, the quality of ISAR image according to target fluctuation is quantitatively analyzed using two-dimensional ISAR image entropy, and the temporal change characteristics between target plane and image projection plane was analyzed using target aspect azimuth and elevation angle calculated from the fluctuation information of the ship. In order to improve a technique for ISAR image formation in a situation where target fluctuation occurs, this dissertation demonstrated a method to automatically find the section where misalignment between the target plane and image projection plane is minimum during the radar observation time. The search method, time-winding optimization, is applied, and ISAR image analysis and image quality are compared for simulation and real data.