Research On Retrieving Wave Height Based On X-band Radar Images

Sea wave information is crucial for various on-and off-shore activities such as ship navigation,coastal construction,and marine resource development.Since the X-band marine radar can image both the spatial and temporal variations of the sea surface with high resolutions,it has been gradually developed as an ocean remote sensor.In addition,the costs associated with traditional in-situ sensors such as anemometers and buoys can be significantly reduced,when the wind information and wave parameters are obtained from X-band marine radar.Thus,the research on retrieving sea wave information based on X-band marine radar image has been widely concerned.The research on retrieving sea wave parameters such as wave direction,wave period and significant wave height based on the X-band marine radar image is one of the hot issues at home and abroad.Especially,the research on retrieving wave height is always the difficulty issue.Based on the native defense scientific research project of�the ocean wave inversion technology by shipborne X-band radar�,the research results relating to the significant wave height inversion from the X-band marine radar image are summarized.The imaging mechanism of sea clutter in radar image and the basic principle of the significant wave height inversion are introduced.Meanwhile,In this paper,rainfall detection technology in the process of retrieving significant wave height and wave height inversion method are deeply studied.The main contents are as follows:The X-band marine radar images are commonly utilized to retrieve sea wave parameters.However,the X-band radar is sensitive to the rainfall signal.And the marine radar image usually contains rainfall interference noise,which lead to the acquired marine radar image usually contains rainfall interference noise and influences the inversion accuracy of the wave height when using X-band marine radar image to retrieve ocean wave parameters.In order to improve the image quality in the process of inversion and reliability of inversion results,the research on whether the acquired radar image contains rainfall information is investigated in this paper.Based on the correlation characteristics of the sea clutter and the difference in the correlation coefficients between sea wave images with and without rain,a novel method of rainfall detection from the acquired radar image is proposed.In addition,the detection threshold of the proposed method is determined.To verify the effectiveness of the proposed method,the X-band marine radar data collected are utilized in this paper.The experimental results demonstrate that the proposed method can effectively detect rainfall from X-band marine radar images.Because the wave is a typical non-stationary random process,the marine environment is very complex,especially in the nearshore shallow water area.Influenced by the seabed topography and coastline,the wave field shows significant non-stationary and non-uniform characteristics in time and space.To solve the problem that the existing spectrum analysis method of the three-dimensional Fourier transform can not accurately extract the wave parameters in the nearshore area,based on the synchrosqueezed wavelet transform,a method for retrieving the wave parameters in the nearshore area is proposed in this paper.The effectiveness of the proposed method is verified based on the simulated X-band radar image.The experimental results show that the proposed method can improve the inversion accuracy of wave parameters,compared with the results based on the traditional three-dimensional Fourier transform and wavelet transform.When the effective wave height is retrieved by using the shadow statistics,the echo intensity of the sea clutter in azimuth is changed as the change of the wind angle and exists large differences in azimuth.Thus,large error exists in the estimated surface slope of RMS(root mean square)and the significant wave height.In addition,the ideal analysis area is not always easy be selected in the upwind direction because of the influence of land in nearshore area.To solve these problems,the relationship between the echo intensity and the wind angle is investigated in this paper.The echo intensity of radar images is corrected in azimuth based on the relationship.Then,the significant wave height is retrieved by utilizing the corrected radar images based on the shadow statistics method.The data acquired were used to certify the validity of the improved method.The experimental results illuminate that the absolute error of the effective wave height obtained by using the corrected radar image is smaller and the correlation coefficient is greater compared to the original method,and the accuracy of the effective wave height obtained is improved.