Retrieval Of Chlorophyll-a Concentration In South China Sea By Remote Sensing Based On Radiation Transfer Model

In the past few decades,with the rapid economic development of our country,some environmental problems have also been brought about.The water environment of our country,especially the inland lakes,the inlets of rivers and the coastal waters are more and more seriously polluted by the influence of human activities.The sea near Sanya Bay in the Nanhai Sea,which is close to Sanya City in Hainan Island,is affected by activities such as human tourism,life and marine aquaculture,so the eutrophication and pollution of the sea water has become more and more serious.Therefore,monitoring the water quality of Sanya Bay in the South China Sea is of great significance for understanding the ecological environment of the South China Sea and exploiting and utilizing the resources of the South China Sea.Chlorophyll is an important indicator to reflect the ecological environment of water body.Therefore,the content of phytoplankton in seawater is a key parameter to reflect the biomass of phytoplankton and its variation.In this paper,the chlorophyll-a concentration of phytoplankton in the waters of Sanya Bay of the South China Sea was taken as the object of study.In March 2018,the chlorophyll-a concentration and other water component concentration parameters were measured in the South China Sea,and the intrinsic optical quantity and the apparent optical quantity of the water body were simultaneously measured.The optical properties of the water body in the study area are analyzed and studied.The remote sensing data in this paper are derived from the GF-4 satellite image.GF-4 is a high-resolution optical remote sensing satellite in geostationary orbit.It has the characteristics of relatively fixed Earth observation position,high time resolution and wide observation range.Ideal for long-term continuous monitoring and quick access to the ground.In this paper,we pre-process the remote sensing images of GF-4 satellite,including radiometric calibration,geometric correction,water extraction,atmospheric correction,and so on.The key link of atmospheric correction is mainly introduced.The atmospheric correction parameters under different parameters and different bands are calculated by using 6S model,and the lookup table is established for atmospheric correction of GF-4 satellite images.According to the characteristics of GF-4,a complete atmospheric correction process was designed,and the atmospheric correction of GF-4 satellite image was completed.Based on fully absorbing the advantages of the original standard atmospheric correction method,according to the characteristics of GF-4,a new lookup table structure and retrieval method are designed.A hierarchical method is proposed to establish the lookup table and a multi-dimensional data retrieval method based on weight.The structure and storage form of the lookup table are optimized to meet the processing flow of GF-4 satellite data.Based on the spectral characteristics of the water body in the South China Sea,a remote sensing inversion model of chlorophyll-a concentration suitable for the characteristics of the water body in the South China Sea was established by using the satellite image of the GF-4 and the measured chlorophyll-a concentration.By analyzing the correlation between remote sensing image band and chlorophyll a concentration of water body,the band range sensitive to the change of chlorophyll a concentration is selected,and the optimal wavelength is determined by iterative optimization method.A universal model for retrieving chlorophyll-a concentration in the South China Sea was constructed by using a three-band model.