Research On Ocean Current And Wave Inversion Method Based On UAV Optical Remote Sensin

Ocean current and wave are important parameters of ocean dynamics.Traditional in-situ and remote sensing observation methods are difficult to meet the requirements of real-time and large coverage observation.The unmanned aerial vehicle(UAV)platform has the characteristics of flexibility,low cost and high resolution,which provides a new way for ocean current and wave observation.However,due to the impact of the flying attitude of small UAVs,sun glint and other factors,the methods to retrieve ocean current and wave parameters are still need to be improved.In this study,the algorithms of retrieving ocean current and wave from UAV optical sea surface image are studied,and analyzes the impact of different factors on the method.The main contents are as follows.(1)A method of the inversion of ocean current parameters using UAV optical images is proposed,and the influence of different factors is analyzed.First,after geometric correction,image enhancement and other pre-processing steps,the sea image sequence is obtained.Then,the velocity and direction are retrieved based on the three-dimensional fast Fourier transform and wave dispersion relationship,and the inversion results are compared with the measured current data of buoys to evaluate the method.Secondly,combined with the experimental data,the influences of different factors on ocean current inversion are analyzed thoroughly: Under the experimental conditions of this study(effective wave height 0.2～1.5 m,main wave period3.8～5.7 s),the accuracy of ocean current inversion is higher when the time resolution of the optical images is 2.3～3.4 s,and the spatial resolution and size of the studied subregion are0.01～0.04 times and 2～3 times of the actual main wave length,respectively.When the altitude of UAV is 3～4 times the size of the subgraph,it is more suitable for ocean current observation.In order to reduce the effect of sun glint,a quadratic function fitting method based on image gray value is proposed.The experimental results show that the accuracy of ocean current inversion is improved by image correction.(2)In order to extract the inhomogeneous wave field effectively,a method to retrieve wave parameters from optical sea surface images is proposed based on two-dimensional continuous wavelet transform.Firstly,the two-dimensional continuous wavelet transform is applied to the optical sea surface images from UAV to extract wave parameters at different positions based on the peak of the wavelet spectrum.The effectiveness of the method is verified by comparing the inversion results with buoy-measured wave data.Additionally,the non-uniformity of the nearshore wave field is analyzed based on the observation results.Secondly,the influence of the selection of key parameters on wave inversion is analyzed,including wavelet function parameters,image parameters,and wavelet transform parameters.The values of each parameter that result in better ocean wave inversion were determined based on experimental data: the central wave number of the wavelet function and the anisotropic parameter is between 1 and 2;the spatial resolution and image size of the inversion subgraph should be 0.01～0.04 times and 5～10 times of the actual main wave length;among the wavelet transform parameters,the scale parameter is divided into 64-144 scales.The results of observation experiments show that the proposed method of ocean current and wave inversion from UAV optical images is reasonable,which provides a useful preparation for practical application.