Nonlinear influences on ocean waves observed by X-band radar
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ORIGINAL RESEARCH PAPER
Nonlinear influences on ocean waves observed by X-band radar Li-Chung Wu Æ Dong-Jiing Doong Æ Beng-Chun Lee Æ Laurence Zsu-Hsin Chuang Æ Chia Chuen Kao
Received: 1 January 2007 / Accepted: 27 March 2008 / Published online: 16 April 2008 Ó Springer Science+Business Media B.V. 2008
Abstract The purpose of this study is to discuss the influence of signal nonlinearity upon X-band radar observations. A method for estimating the degree of nonlinearity by bispectral analysis was applied and discussed. We found that bispectral analyses from spatial radar backscatter series are similar to results obtained from water level time series. In addition, the average nonlinear degree from radar backscatter is related to wind speed. The accuracy of wave observations derived by consideration of the nonlinear effect from radar backscatter was also investigated. The estimated error in wave height from the radar data is also related to the degree of nonlinearity. In order to improve accuracy, the modulation transfer function method was applied in order to eliminate the influence of nonlinearity. Keywords Radar backscatter Nonlinearity Bispectral analysis
L.-C. Wu (&) C. C. Kao Department of Hydraulic and Ocean Engineering, National Cheng Kung University, 1, Ta-Hsueh Rd., Tainan, Taiwan, ROC e-mail: [email protected] D.-J. Doong Department of Marine Environmental Informatics, National Taiwan Ocean University, 2, Beining Rd., Keelung, Taiwan, ROC B.-C. Lee Department of Environmental and Hazards-Resistant Design, Huafan University, Huafan Rd. Shihding Township, Taipei County, Taiwan, ROC L. Z.-H. Chuang Institute of Ocean Technology and Marine Affairs, National Cheng Kung University, 1, Ta-Hsueh Rd., Tainan, Taiwan, ROC
Introduction Ocean waves have attracted the attention of both the general public and scientists throughout history. In the present day, understanding of the mechanisms of wave formation and the way in which waves travel across the ocean is still not complete. Knowledge of wave characteristics is important in modern coastal technology and oceanographic studies. Wave measurement is a useful way to understand and describe wave characteristics. This process can be largely classified into two categories as in situ and remote sensing. While in situ instruments are used to reconstruct time variations of waves at a single point, remote sensing techniques give information over a broader area. Because waves are distributed over a large-scale region in the spatial domain, the spatial characteristics of ocean waves should also be studied in greater detail. Several studies about ocean waves using remote sensing have been performed since 1960s (Pidgeon 1968; Valenzuela and Laing 1970; Alpers and Rufenach 1981; Lee et al. 1996), and satellite images have often been used for detecting and studying waves. However, because most of the satellites travel along a predetermined track, it is difficult to track the same wave continuously. New technologies for obtaining continuous images of waves have been prop
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