Study On The Environmental And Communication Channel Characteristics Of Oceanic Evaporation Duct

The environmental and channel characteristics of evaporation duct over sea surface cover a wide range of research topics.This dissertation mainly studies several important problems,such as the influence of wind speed on channel fading and correlation characteristics,the influence of special meteorological conditions such as cold air on different evaporation duct models and corresponding propagation losses,and new method of modeling modified refractive index profile of evaporation duct.The phenomenon of evaporation duct above the ocean has a significant effect on the propagation of electromagnetic waves.The in-depth study of the relationship between evaporation duct model and environmental conditions,as well as the evaporation duct channel model,is also of great significance for the design and application of maritime electronic countermeasure systems,radars and communication systems.Utilizing evaporation duct,the effective range of microwave communication system can be greatly extended,and high-speed transmission of data over sea surface can be realized.However,the existing theoretical research on the mechanism of the evaporation duct channel is not clear enough,and the specific analysis under several weather conditions is not enough.The modeling of the evaporation duct model is based on Monin-Obukhov similarity theory presently,which results in the insufficiency in the mining of the data itself.This dissertation introduces up-to-date meteorological reanalysis product,a fractal sea surface model and artificial neural network model.Based on these,the relationship between evaporation duct model and environmental conditions,as well as the evaporation duct channel model,are analyzed comprehensively.This article mainly includes these research contents: the influence of sea surface wind speed on the evaporation duct channel through the sea surface model,the comparison of different evaporation duct models under cold air conditions,and the modeling of the evaporation duct model based on neural network method.Specific research results and innovations of the studies are as follows:1.The relationship between wind speed and channel characteristics under evaporation duct conditionA method of analyzing the relationship between wind speed and the fading,vertical correlation characteristics of evaporation duct channel is proposed by introducing the onedimensional fractal sea surface model and parabolic equation model.Using the NCEP CFSR meteorological reanalysis data set and the similar functions in the NPS model,wind speeds at19.5 m height required by the fractal sea surface model are calculated,and the temporal and spatial distributions of 19.5 m high wind speed at several points in the South China Sea are analyzed.Using a one-dimensional fractal sea surface model,the root mean square of sea surface fluctuations under different wind speeds conditions is calculated,and the sea surfaces as the boundary conditions of parabolic equation model are generated.The sea surfaces produced by the one-dimensional fractal sea surface model and other microwave and evaporation duct parameters were repeatedly simulated and utilized to analyze the effect of wind speed on the spatial diversity characteristics and channel fading characteristics of the evaporation duct channel.2.The analysis of performance of different evaporation duct models under cold air conditionsIn view of the frequent cold air weather phenomenon in the South China Sea in the winter half year,the applicability of four kinds of evaporation duct models under cold air conditions is studied.Through the sensitivity analysis of the models and the use of the experimental data,the law of performance change of different models under cold air conditions is revealed.For four evaporation duct models based on the Monin-Obukhov similarity theory,based on typical meteorological parameters under cold air conditions in the South China Sea,the sensitivity of the evaporation duct height and microwave propagation path loss to these parameters is analyzed.In a microwave transmission experiment in the South China Sea under cold air conditions,the changes of various meteorological parameters at different stages are discussed.Based on the conclusion of the sensitivity analysis,the reasons for different evaporation duct heights calculated by using each evaporation duct model are pointed out.The measured path loss data are compared with the simulation results of each evaporation duct model,and performance of each model at different stages of cold weather are summarized.3.A calculation method of modified refractivity profile of evaporation duct based on artificial neural networkArtificial neural networks can be used to approximate complex non-linear functions.In view of the lack of universality of the original evaporation duct models based on the MoninObukhov similarity theory,a data-driven method of calculating the modified refractive index profile of evaporation duct based on artificial neural network is proposed.Based on the NCEP CFSR data set and the NPS evaporation duct model,a training and verification data set of the neural network is obtained,and the corresponding neural network model is worked out.By comparison with the evaporation duct height calculated by the NPS model,and the corresponding path loss calculated using the parabolic equation method,the effectiveness of the neural network method for modeling the modified refractivity index profiles in boundary layer is verified.According to the stability of the atmospheric boundary layer,the training and verification data sets are grouped through air-sea temperature difference,and different neural network models are trained and compared with NPS model to analyze the differences under different boundary layer conditions.