(A) study on the calibration and angle estimation of an active linear phased array for multi-beam search radar systems

Abstract

In this dissertation, online self-calibration of an active uniform linear array (ULA) for a multi-beam search radar systems and a technique to estimate the elevation angle of the detected targets accurately by compensating the error caused by multipath effect is proposed. Since the error between the array elements in the active ULA is the main cause of the degradation of the side lobe level and the angle estimation, it is important to effectively calibrate the array error during the radar operation. Also, for naval search radar, the elevation angle error is increased due to the multipath effect occurring in the marine environment. That is, it is difficult to estimate the angle of the direct path and the indirect path separately due to the interaction of the two signals coming from the target. In this paper, we propose the two methods of the online self-calibration and the accurate estimation of the target elevation angle, using the characteristics of the multiple receive beams. In the first part, a novel self-calibration method for an active uniform linear array (ULA) of a multi-beam surveillance radar is proposed. The approach utilizes the radar returns containing the clutter and the opportunity targets, which requires no additional hardware and time for the calibration. The array error is estimated from the phase gradient of the active ULAs and the Fourier property of ULA beamforming. To correct the estimation error, several range bins used as the estimation sources are selected according to entropy value. The linear phase component in the phase error is eliminated using both the center shifting and the matched filter, thus enhancing the calibration performance. The effectiveness of the proposed method against that of conventional ones is demonstrated with a computer simulation and a field experiment. Next, a hybrid compensation scheme for the elevation angle error of a low altitude target in a multipath radar environment is proposed. The proposed method can be applied to multi-beam radar systems. Among various low angle estimation algorithms, the beam space domain maximum likelihood (BDML) can simply find the direction of arrival (DOA) of a target from a quadratic equation. However, when the BDML-based method is applied to the target angle estimation of the search radar, it has a drawback in that the estimation error increases as the phase difference between the direct path and the indirect path approach a zero degree, and the performance deteriorate in the region where the specular reflection coefficient is large. To overcome these problems, the hybrid angle compensation method is proposed. If the multi-beam formed pattern is simi-lar to the ideal beam pattern, then the estimated target angle by monopulse processing is corrected using the ratio of the horizon beam and the lower beam, else the BDML-based method is applied. Simulation results performed with various sea states are provided to prove the angle estimation performance.