Research On Oceanic Lidar For Detecting Water Optical Properties

The ocean covers 71%of the earth,which is an important strategic space for the world's economic and social development and an important field for the study of climate change and earth science.Lidar is an attractive tool due to its high spatial-temporal resolution,low dependence on observation conditions and the ability to detect the profile of subsurface water.The effects of the atmosphere and the solar altitude angle on the lidar are small,which effectively make up for the shortcomings of ocean color remote sensing,such as weak light detection,polar area detection,profile detection and so on.At present,there are still many key technologies and difficulties that need to be tackled in the oceanic lidar.The core issue is "how to evaluate and verify the detection accuracy of the oceanic lidar for the water optical properties".Therefore,a comprehensive and systematic study of the theory,system and experiment of oceanic lidar,as well as breakthrough in the key technology of detection accuracy evaluation and verification of oceanic lidar,can promote the application of oceanic lidar in the fields of ocean optical remote sensing,marine science,etc.In this paper,the general model of the oceanic lidar is improved and three key technologies are improved.The lidar radiative transfer model with multiple scattering are realized,the mechanism of phase function effects on lidar retrieval is studied,and the influence problem of system response on lidar retrieval is solved.The works indicate the direction for the detection accuracy evaluation and high accuracy system optimization.According to the theoretical breakthrough,the oceanic lidar system is developed.The theoretical and experimental results of the oceanic lidar are compared,which unifies the theory and experiment.The main contents are as follows:The general model of the oceanic lidar is improved to understand the actual lidar signal.The signal and signal-to-noise ratio under different environmental and system conditions are analyzed in detail.The key factors to improve the signal-to-noise ratio of lidar are pointed out,which provides references for the selection of lidar system parameters.Finally,three key problems of the oceanic lidar are proposed,which are connected to the following works.The standard Monte Carlo method,the semi-analytic Monte Carlo method and the analytical model considering the oblique incidence are established to simulate the lidar signals with multiple scattering.Their principles are illustrated systematically.Then,the lidar signals calculated by three models under different working conditions are compared.The influences of system parameters and water optical properties on the signals are discussed.The developed radiative transfer models of oceanic lidar have advantages in calculation efficiency and accuracy,respectively.The combination of the three simulation methods has important theoretical significance and practical values for the high-precision inversion of the seawater optical properties.The phase function effects on the lidar retrieval are studied.Based on the established lidar radiative transfer model,the particulate and molecular lidar signals under different phase functions are simulated,and 180 degree volume scattering function and lidar attenuation coefficient are studied independently.The relationships between 180 degree volume scattering function and lidar attenuation coefficient as well as the multiple scattering and phase function are explored.The inversion accuracy of 180 degree volume scattering function and lidar attenuation coefficient is evaluated.Furthermore,the high-precision inversion of lidar attenuation coefficient is studied.The secret behind the multiple scattering of oceanic lidar is deeply explored,which provides sufficient basis for the lidar retrieval.The influence of system response on lidar inversion is studied.The relationship between the system response and the internal components of lidar is discussed,including three system components:laser,detector and acquisition card.An approximate model for calculating the system response pulse width based on the performance of the three components is given.The research results will be able to guide the matching selection of the three components.Furthermore,the influence of lidar system response on the retrieval accuracy of diffuse attenuation coefficient in homogeneous and stratified water is evaluated.It shows the influence of the system response on lidar signals and its retrievals.Then,it gives the schemes of how to choose a suitable system response and how to correct the system response.The comparison between the theory and experiment of the oceanic lidar is carried out.The research results of inversion accuracy evaluation are applied to the design of lidar system.Then,the lidar signal and inversion results calculated by the theory are compared with the field experiment to verify the correctness of the theory and experiment.Then,the general inversion algorithms of lidar,such as disturbance method and Fernald method,are used to analyze the lidar data during the navigation.The results fully show the relationship between theory and experiment of lidar and improve the research system of the lidar theory and experiment.Finally,the key technology of evaluating and verifying the detection accuracy of lidar for water optical properties is broke through.