Development Of An Oceanic Lidar System And Its Typical Detection Results

Perceiving ocean environmental information is the basis of ensuring the safety of ocean environment,when the ecological problems has become more prominent,such as the increasing occurrence of extreme weather events nowadays.Multi-perspective observation of ocean environment with high precision in an all-round way meets urgent demand.Lidar has proved its potential to be an important tool for ocean observation.In this thesis,a high-performance shipborne oceanic lidar system with multiple detection abilities of polarization,multi-field-of-views(MFOVs),fluorescence,and Raman is developed,together with signal simulation and system validation.Field experiments were carried out in typical waters,such as Qiandao Lake and coastal waters of China.This work integrates the model mechanism,instrument development,retrieval algorithm and application analysis of shipborne oceanic lidar,providing complete solutions for ocean observation.It is the basis of spaceborne oceanic lidar in the future and plays an important role in estimating ocean carbon fluxes,clarifying the dynamic process and mechanism of marine environmental changes,and predicting the trend of the earth's climate system.The main contents of this thesis are summarized as following:A shipborne oceanic lidar with multiple functions was developed and showed stable performance and convenient operation.The design principles of oceanic lidar were introduced thoroughly,including the transmitting system,receiving system and control system.So far,to meet diverse detection demands,a multi-functional lidar including functions of Mie scattering lidar,polarization lidar,fluorescence lidar,Raman lidar,and MFOVs lidar was developed after three generations of renewal.A design scheme compatible with various oceanic lidars was given according to lidar principles.The retrieval algorithms for the inherent optical parameters(IOPs)and biological parameters of the water were clarified starting from the pre-processing of the lidar data.Oceanic lidar can be deployed on the vessel conveniently.By observing the depth-resolved profiles of the waters,the optical and biological properties of the water can be obtained such as the diffuse attenuation coefficient,particle backscattering coefficient,depolarization ratio and chlorophyll concentration,etc.A forward model of oceanic lidar with multiple parameters and functions is established using the analytical model and Monte Carlo(MC)simulation.Multiple scattering effect exists in laser transfer in water.Compared with the lidar equation under the approximation of single scattering,the analytical model and MC simulation take the multiple scattering into account to construct an efficient and accurate forward model of the oceanic lidar signals.The former method has high efficiency,while the latter can simulate the most real lidar signals.Using above methods,the influence of various factors,including operating height,receiving field-of-view(FOV),water quality,and water stratification,were discussed for Mie lidar signals.The signals in co-polarized and cross-polarized channels were analyzed.The feasibility of retrieving the chlorophyll concentration from the ratio of fluorescence signal to Raman signal was demonstrated.The procedure of calibration and the validation with merging of in situ data and forward model is established for oceanic lidar signals The necessity and feasibility of the calibration were introduced from aspects of system response,background bias,time offset,gain nonlinearity,etc.Signal distortion caused by inherent properties of lasers and detectors,as well as ambient light were calibrated.Various methods including lidar equation,MC simulation,and analytical model were adopted to validate lidar signals and retrieved properties quantitatively under different water types and different FOVs with input of in situ data.Muptiple retrieval methods of optical and biological properties are merged and applied to analyze typical waters,such as Qiandao Lake,the East China Sea and the South China Sea.Due to the complexity of water,one algorithm is hard to retrieval the target characteristics accurately.Multiple retrieval methods were combined,including the slope method,perturbation method,Fernald method,bio-optical model method,and Raman corrected fluorescence method.A comprehensive comparison,verification and analysis of the bio-optical properties of typical waters have been carried out with in situ data and ocean color remote sensing data.Navigating observations were carried out in Qiandao Lake in summer,indicating the process that the inland water was affected by local meteorological events and overland runoff.Long-distance navigation was carried out in the East China Sea and the South China Sea,recording continuous data of three major areas including coastal waters near Zhejiang-Fujian-Guangdong,the sea area near Pearl River Estuary and the sea area in the east of Hainan.The characteristics of the scattering layer were discussed in detailIn summary,the thesis introduces the development of a multi-functional shipborne oceanic lidar from the system design,simulation,validation and experiment,etc.The lidar has demonstrated accuracy,reliability and stability in the observations of the Qiandao Lake,the East China Sea,and the South China Sea.This research is of great significance for promoting the oceanic lidar for application in building a three-dimensional observation system for the upper ocean in the future.