Ocean Wave Information Extracted From X-Band Radar Images

The ocean wave is a very familiar phenomenon and an important field in physical oceanography. In recent years, considerable interests have been shown in the application of remote sensing techniques to the measurement of the ocean waves by X-band radar. When the electromagnetic waves of the X-band radar reach the sea surface, they would be interacted with the capillary waves caused by wind. Bragg scattering criterion is fulfilled by those waves having a wavelength equal to one half of the electromagnetic wavelength. The radar backscatter is incepted by radar antenna and shown on the radar screen. This phenomenon is called sea clutter. The longer waves become visible in the radar images, because they modulate the short Bragg scattering waves. The modulate mechanisms are consisted of shadowing, orbital modulation, hydrodynamic modulation and tilt modulation. Thus, the radar images contain luxuriant sea state information. It is important to study how to obtain the wave information from X-band radar images accurately and effectively for the research of the ocean wave.Beginning with the imaging theory of the X-band radar, the theory of Bragg resonance and long wave modulation and their effects on the radar imaging were introduced firstly. The mechanisms of Bragg waves scattering electromagnetic waves were analyzed, and the modulations of the longer wave on the Bragg waves by shadowing, orbital modulation, hydrodynamic modulation and tilt modulation is also introduced.Based on the analysis of the radar imaging theory, the algorithm for obtaining the wave information from radar images was studied. Firstly, by applying a 3D-FFT algorithm to the time series of radar images the 3D radar image spectrum was obtained. Furthermore, the surface current velocity was gained from the dispersion relation from the radar images. Then, the wave spectral components were filtered, taking into account the dispersion relation with the current velocity. Finally, the wave directional spectrum was acquired according the experiential transfer function between the radar image spectrum and the wave directional spectrum. The other parameters included by the wave information were obtained, such as peak period, peak frequency, wave direction, etc.Lastly, the radar data gained in the offshore experiment which was carried out on the Small Mai island was calculated and compared with the corresponding buoy data. The compared result is good, showing that the wave information can be gained through the algorithm for obtaining the wave information from radar images effectively. Especially in the rough sea state, it is significant to use this way to monitor the ocean wave.