| As an important space for people’s survival and development,ocean is rich in material resources.Accurate acquisition of ocean wave information is of great significance to marine scientific research.Microwave radar can use the interaction mechanism of microwave electromagnetic waves and ocean capillary waves,and cooperate with the corresponding inversion algorithm to achieve ocean wave telemetry.Now,most of the current microwave radar platforms,limited by the small number of stations and close detection distance,these platforms are far from meeting the needs of large-scale,high-density environment monitoring and early warning.The RORSE(Radio Ocean Remote Sensing Laboratory of Wuhan University),independently developed and completed the shipboard coherent S-band wave radar system with high accuracy,little external interference,easy to carry and all-weather operation.Based on the Doppler Effect,this radar system continuously measures the orbit velocities of water particles in different positions on the ocean surface and then obtains the wavenumber spectrum and wave parameters by using the linear wave theory.It can be installed on kinds of mobile platforms and overcome shortcomings of the fixed-based radar such as close detection distance,small space density and low observation frequency.It is a new way to achieve navigation observation of ocean waves.The research of this issue is carried out to solve the essential problems of wave parameters extraction to establish the set of wave inversion algorithm of shipboard coherent S-band wave radar.Combined with this theme,this paper mainly discusses the basic theory of wave inversion technology,ocean dynamics theory,ship motion attitude calibration model,sea echo simulation.These research contents are used to apply this algorithm in the radar system.Finally,the results of several sea trials are analyzed and compared to verify the correctness of this inversion algorithm.Firstly,the basic knowledge of microwave coherent wave radar is introduced.The principles of distance and velocity measurement from the radar equations are analyzed.Then,two equivalent methods of fluid motion description are compared.The ocean dynamics theory related to microwave radar wave telemetry technology is described,including wave parameters and linear wave theory.The general expression of ocean wave is also described and the equivalence of wavenumber spectrum and frequency spectrum is proved.Besides that,this thesis describes current commonly used wave spectrum models and direction distribution functions,introduces frequency and wavenumber representations of various wave spectrum models,discusses and analyzes the scattering mechanism of microwave Doppler radar on the sea surface.The Bragg scattering principle and the composite surface scattering theory are also discussed in the end.Based on the Lagrangian idea of the spatial dimension,this thesis analyzes the various components of orbital velocities of the water particles in different positions at ocean surface on the moving platform such as a ship.According to the composite surface scattering theory,the linear wave theory,the interaction of three-scale waves,their different contributions to the Doppler spectrum,the conversion function between the wavenumber spectrum and the water particles of each element of the sea surface is deduces.The parameter inversion algorithm of the shipboard coherent S-band wave radar are proposed.In this process,for the echo Doppler spectrum under low SNR,a center frequency estimation method named successive approximation is proposed to improve inversion accuracy;for the wavenumber spectrum under low sea conditions,a wavenumber spectrum reconstruction method based on the wave steepness function is proposed to effectively extract the components of wind waves.Finally,the process of obtaining wave parameters such as wave height and period by moment analysis method is introduced.The calculation method of average wave direction under the condition of suspension and the judgment method of wave direction ambiguity under the condition of navigation are given.The characteristics of ship shaking will affect the antenna attitude and the radar echo signal.By analyzing the timing characteristics of 6-DOF shaking based on the strip theory,the influence of 6-DOF on radar echo is evaluated.In order to improve the efficiency of engineering calculation,a 3-DOF ship swing model based on spatial displacement is proposed.By analyzing the dynamic geometric relationship of 3-DOF,the instantaneous comprehensive speed caused by the ship shaking could be deduced to eliminate the impact of ship motion on the accuracy of wave inversion.In this paper,the attitude changes such as the azimuth and elevation of radar antenna are analyzed when the ship shakes.It is confirmed that the slight change of detection area has little influence on the wave inversion results.On this basis,in order to further explore the applicability of inversion algorithm under high ship speed,the simulation methods of Doppler spectrum of radar echo considering only forward linear motion(navigation)and only roll and pitch motions(stop)are studied.The changes of center frequency and bandwidth of echoes under different ship motions are discussed.Then the simulation method of single range element is extended according to the wave dissipation relationship and wave propagation law.The models of Doppler spectrum quantization with multi range and multi time elements are studied,and the simulation results are obtained.Finally,the inversion algorithm is used to process the simulated data.By comparing inversion results with the theoretical values,the correctness of the wave inversion algorithm is preliminarily verified.Finally,through the analysis and comparison of measured results,the reliability and accuracy of the wave inversion algorithm of the shipboard coherent wave radar are verified.This part analyzes three shared voyage tests,including the comparison test of artificial observation in the Pearl River Estuary in 2017,the sea trial in the middle of the Southern China Sea in June 2017 and the comparison test of buoy observation in the middle of the Southern China Sea in September 2018.The wave inversion processing for the measured data is carried out.This thesis introduces the data acquisition methods of MTI’g attitude sensor and water depth data,shows the data of ship shaking motion and echo signal.In the first comparison experiment of artificial observation in the Pearl River Estuary,the trend of radar results are very consistent with that of artificial record,and the correlation coefficient is high.In the third 2018 Central Experiment of Southern China Sea,the waverider buoy was used to participate in the comparison.The results show that the correlation coefficients of the siginificant wave height and the average wave period measured by the radar and the buoy are higher than 0.92,and the root mean square errors are respectively 0.1m and 0.3s.This proves that the wave inversion algorithm proposed in this thesis has good performance,indicating that the shipborard radar system has the great ability to monitor the ocean waves while traveling. |
PDF Full Text Request