A curvelet-based method to determine wave directions from nautical X-band radar images
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A curvelet-based method to determine wave directions from nautical X-band radar images ZHA Guozhen1, HE Qingyou3, GUAN Changlong1, 2*, CHEN Jitao4 1 College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao 266100, China 2 Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China 3 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of
Science, Guangzhou 510301, China 4 Hydrology Bureau of Yellow River Conservancy Commission, Zhengzhou 450004, China
Received 10 August 2017; accepted 13 September 2017 ©The Chinese Society of Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
A new method to determine wave directions from nautical X-band images is proposed. The signatures of ocean waves show obvious scale and directional characteristics in nautical X-band radar images. Curvelet transform (CT) possesses very high scale and directional sensitivities. Therefore, it has good capability to analyze ocean wave fields. The radar images are decomposed at different scales, in different directions, and at different positions by CT, and curvelet coefficients are obtained. Given to the scale and directional characteristics of surface waves, the information of ocean waves is centralized in the curvelet coefficients of certain directions and at certain scales. Therefore, the wave orientations can be determined. The 180 ambiguity is removed by calculating crosscorrelation coefficients (CCCs) between continuous collected images. The proposed method is verified by the dataset collected on the Northwest coast of the Zhangzi Island in the Yellow Sea of China from March to April 2009. Key words: X-band radar, wave direction, surface wave, curvelet transform Citation: Zha Guozhen, He Qingyou, Guan Changlong, Chen Jitao. 2018. A curvelet-based method to determine wave directions from nautical X-band radar images. Acta Oceanologica Sinica, 37(1): 11–19, doi: 10.1007/s13131-018-1154-z
1 Introduction The ordinary nautical X-band radar is an active imaging radar system working in microwave frequency band. It transmits electromagnetic waves to the sea surface and generates images by receiving electromagnetic waves reflecting from the sea surface. Generally, the gray value of radar images represents the backscattering strength of echoes from the sea surface. The backscattering is mainly caused by centimeter-scale surface roughness elements. The long surface waves will modulate the small-scale scattering elements and affect the radar backscattering. At moderate incidence angles, tilt and hydrodynamic modulations are well-established imaging mechanisms, while at grazing incidence, the modulation stems also from the shadowing of the radar beam due to the ocean waves (Alpers et al., 1981; Seemann et al., 1997). In addition, the non-Bragg scattering will play an important role when the incidence angle is large (≥70°) (Plant and Keller, 1990). Furthermore, wave breaking is believed to have contributions to radar back
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