With the development of state-of-the-art low earth orbit observation satellites, polarimetric synthetic aperture radar(SAR) images have been widely utilized. However, polarimetric signals are inevitably distorted by channel imbalance(CI) and interference between multiple channels, resulting in rapidly degraded quality of polarimetric SAR images. Therefore, several polarimetric calibration methods using a polarimetric active radar calibrator(PARC), which can provide different scattering matrices, have been developed. Nevertheless, errors generated by the performance of imperfect PARCs are inevitable, leading to significant errors of estimated CIs. In this study, we propose a framework for calibrating polarimetric SAR images, which consists of two stages: 1) coarse estimation of distortion parameters through a conventional method using three PARCs, and 2) fine estimation of CIs using particle swarm optimization and a single PARC. In simulations using polarimetric SAR images, we observed that our proposed method can more accurately calibrate polarimetric SAR images as compared to conventional methods.
The difference of mooring tension by type of buoy was investigated in the circulating water channel and the wave tank for deducting the most stable buoy from the current and the wave condition. 5 types of buoy made up of short cylinder laid vertically (CL-V), short cylinder laid horizontally (CL-H), capsule (CS), sphere (SP) and long cylinder (CL-L) were used for experiments.A mooring line and a weight were connected with each buoy. A tensile gauge was installed between a mooring line and a weight. All buoy' s mooring tension was measured at the same time for the wave test with periods of 1.5-3.0 sec and wave heights of 0.1-0.3 m, and the current test with flow speeds of 0.2-1.0 m/sec. As a result, the order of tension value in the wave test was CL-H > CL-V > SP > CS > CL-L. In the current test CL-V and CL-H were recorded in the largest tension value, whereas SP has the smallest tension value. So it seems that SP buoy is the most effective in the location affected by fast current. CS is predicted to be suitable for a location that influence of wave is important more than that of current if practical use in the field is considered.And it was found that the difference of mooring tension among buoys in wave is related to the product of the cross sectional area and the drag coefficient for the buoy' s bottom side in high wave height. The factor for the current condition was not found. But it was supposed to be related to complex factors like a dimension and a shape by buoy' s posture to flow.
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