Research On UAV's Autonomous Surveying And Mapping System For Shallow Water Depth Based On Dispersion Relation

In marine engineering research,one of the most challenging subjects so far is the depth measurement of extremely shallow waters in the coastal zone.The measurement method of inverting the depth based on the characteristics of the offshore waves is an effective means of intelligent mapping of the seabed topography in the coastal zone.The wave model has different expression forms according to different modeling theories,and the water depth/wave spectrum model based on the dispersion relationship is simple in form and has good effects in practical engineering applications.Based on the theory of the dispersion relation,this paper designs an actual flight surveying system to estimate the characteristic parameters of the waves through video images,so as to obtain topographic maps of extremely shallow waters by inversion.This paper mainly completes the realization of camera calibration,image perspective transformation,time delay estimation and spectrum estimation,and designs a set of measurement system that meets the theoretical requirements.Through comparative analysis of field tests,a large number of tests show that the average root mean square error between this system and the sonar mapping system is 0.13 m.The main research contents of this paper is as follows:(1)Assuming that the waves in extremely shallow seas meet the requirements of ideal fluids,according to the two-dimensional boundary conditions of the coastal waters,the boundary condition equations of the wave model are analyzed and established.After linearization and approximation,the solution of the model is obtained by the method of separation of variables and derived Excellent dispersion relation.According to the dispersion relation,how to use the crosscorrelation function and power spectrum to estimate the wave speed and period respectively according to the image pixel time series,and then invert the depth of the sea.(2)Aiming at the problem of obtaining orthophotos,this paper deeply discusses the principle of perspective transformation algorithm based on the principle of single-image space rear intersection,and proposes an algorithm for obtaining the camera's external parameter matrix and obtaining the orthophoto by using ground control points.Aiming at the problem of image shaking caused by drone shaking,a ground control point re-identification algorithm is proposed,which dynamically adjusts the image pixel coordinates of ground control points and improves the accuracy of the camera's external parameter matrix.(3)According to the principle of the dispersion relation inversion algorithm,the UAV surveying and mapping system is designed and realized from the aspects of hardware selection and software design process.The system can complete functions such as aircraft pose acquisition,ocean wave video acquisition,and ground control point position measurement.At the same time,a set of ground workstation software system was designed and implemented to complete the functions of image perspective transformation,image pixel time series acquisition,wave parameter estimation and water depth inversion.The UAV surveying and mapping system designed in this paper has been applied in practical projects,and its measurement accuracy also meets the needs of the project.