Ocean Wave Inversion Technology Using Microwave Doppler Radar

Ocean wave is composed by water particles fluctuating on the sea suface. It is important and practical to build an effective measurement of ocean wave for coastal engineering construction, exploration of marine resources and marine disaster prevention. In recent years, with the development of computer and electronic technology, radar remote sensing has been gradually applied to the field of ocean wave measurement.Microwave Doppler radar, whose measurement is based on Doppler principle, can measure orbital velocity of water particle and sea-echo intensity continuously in all directions. With the linear wave theory, ocean wave spectrum and wave parameters can be deduced. The ocean wave measurement can be carried out by the microwave radar with small radar antenna, high precision and less environmental interference. Meanwhile, microwave Doppler radar has such a high spacial resolution that makes it possible to accurately get detail information of sea surface. As a result it has an important value for marine scientific research. Therefore, as an important part of ocean observing system, microwave Doppler radar wave measurement technology is now developing fast in many countries. An S-band Microwave Ocean Remote SEnsor (MORSE) was developed by Radio Ocean Remote Sensing Laboratory of Wuhan University to detect the nearshore ocean dynamics.Based on the MORSE system, this research is to study and solve the key problem of ocean wave inversion, investigate the wave measuring theory and get a complete wave inversion algorithm for microwave Doppler radar. This paper will mainly discuss five parts including the sea state measurement theory using microwave Doppler radar, the ocean wave inversion algorithm, microwave Doppler spectrum simulation, the interference elimination and comparison between radar result and buoy. These five parts not only constitute a complete research of ocean wave measurement by microwave Doppler radar, but also includes the key technologies of the inversion algorithm.According to the composite surface scatter theory, microwave radar backscatter is composed by three different scale waves. Based on the contributions of large, intermediate and small scale waves, ocean wave spectrum can be deduced from the radial velocity series which are calculated by center frequencies of Doppler spectrum. Ocean wave parameters such as significant wave height and wave period can be calculated by moment investigation to wave spectrum.Simulation technology has always been an important and effective reserch method. To verify the ocean wave inversion algorithm, the simulation of microwave Doppler spectrum from sea surface is discussed. Firstly a simulation method of sea echo Doppler spectrum for single distance is introduced based on the different Doppler contributions of three wave scales including small, media and large. The microwave Doppler spectra are effectively simulated. And then this method is expanded according to the ocean wave dissipation relation and wave propagation theory and the algorithm for simulating multi-distance Doppler spectrum is established. Finally, the ocean wave inversion algorithm is used to deal with the simulated data. By comparing the inversion results and the theoretical value, the inversion algorithm was verified.The main clutter for microwave Doppler radar are breaking wave and ship target which two will disrupt the wave components in Doppler spectra, so as to directly affect the measurement of the radial velocity. This part firstly discusses the influence to wave inversion from breaking wave and target, and then introduces their impacts on the Doppler spectrum respectively. Based on the different paremeters of Doppler spectrum, a three-level Constant False Alarm Rate (CFAR) detection for breaking wave method is introduced on account of the frequency, bandwidth and energy of Doppler spectrum. After that, target CFAR detection based on time Doppler spectrum are introduced. Finally, the detection results are applied to the ocean wave inversion algorithm and the results shows clearly that the precision of wave inversion is significantly improved after eliminating the influences of breaking wave and target.The last part is the analysis of measured results. The experimental data acquired by two nearshore observation experiment using MORSE systems developed by Wuhan University were processed using the wave inversion algorithm. By comparing the results with buoy, the correctness of the inversion algorithm was verified. The paper firstly introduces the composition of MORSE system and its wave inversion process. And then two experiments and their results of wave inversion are discussed in detail. In the first experiment, the system was applied to investigate the correlation between coastal wave and tide for a seven-day experiment. The measured significant wave heights are presented with tide displacement. Cross-spectra and correlation coefficient are calculated to investigate the variation. The results suggest that significant wave height has a synchronous variation with tide at near-shore areas. During the second experiment, the measured results are presented and compared with buoy. The comparison results show that the correlation coefficient, average error and root mean square error of the significant wave height are all at high levels. The variation of the significant wave hight and period is consistent with buoy results. The error of significant wave period is relatively larger than the significant wave height, but still at a high level. The two experimental results show that the wave inversion algorithm proposed in this paper has a good performance on obtaining the wave spectrum and wave parameters.