Study On Tropospheric Duct Propagation Characteristics Over Rough Ocean Surface Using GPS Signals

The earth has abundant ocean space.Due to the complex and variable environment of lowaltitude seas,the tropospheric atmospheric duct,which has an abnormal structure of atmospheric refractive index,often occurs.The atmospheric duct significantly affects the positioning and navigation of the GPS system and interferes with the normal operation of the radio equipment.Therefore,its research has important practical significance.In this paper,the electromagnetic wave with fixed transmission frequency and special signal structure emitted by GPS satellite is used to model the atmospheric duct propagation problem based on rough sea surface.The influence of sea surface roughness on the initial field and boundary conditions of parabolic equation is considered.At the same time,based on the sea surface electromagnetic scattering,the signal processing of the receiver,and the satellite position calculated by the satellite ephemeris data,the receiving power of the GPS sea surface scattering signal in the tropospheric duct environment is simulated and calculated,which has good fitting with the experimental results.The main results of this paper are as follows:The transmission characteristics in duct of GPS reflected multipath signals in rough sea environment are studied.Through the construction of the vector signal model,the coherent mirror reflection roughness attenuation factor and the incoherent diffuse reflection roughness attenuation factor are introduced to correct the Fresnel reflection coefficient.The non-uniform interference initial field model of the low elevation GPS multipath signal is proposed.Taking the Roughness,Dielectric and Spherical Earth into consideration,the spatial distribution of the GPS signal power is calculated and analyzed by polarization decomposition in the tropospheric duct environment above sea surface.The calculation results show that the initial field model is more in line with the actual field distribution,and there is a significant amplitude change in the peak of interference enhancement and attenuation.The curvature of the earth causes a non-uniform distribution of interference fringes.Based on the wave power spectrum model,the influence of the roughness of the sea surface on the lower boundary conditions in the calculation domain of the parabolic equation is discussed in detail.Using the Elfouhaily spectrum,the sea surface roughness is divided into two parts—large scale expressed by low-frequency long-band sea spectrum and small scale expressed by high-frequency short-band sea spectrum.For the large-scale sea surface roughness,one-dimensional random rough sea surface is generated by Monte-Carlo method and used as a duct underlying topographic factor.For the small-scale sea surface roughness,the Miller-Brown roughness attenuation factor is modified using a rms height calculation method by integrate the sea spectrum.Finally,combining with the continuous displacement topographic transformation algorithm,the GPS non-uniform surface processing method is used to complete duct propagation modeling of GPS under the boundary of hybrid correction roughness.The calculation results show that as the wind speed increases,the over-thehorizon transmission distance of the GPS signal in the evaporating duct and the non-base layer surface duct is shortened,the signal energy attenuation increases,and the area of the electromagnetic blind area above the duct increases.A high-sensitivity GPS dual-channel delay mapping receiver is introduced to capture and track the direct-wave module of weak GPS signals scattered by sea surface.Based on satellite broadcast ephemeris and precise ephemeris,the position of satellite is calculated.The PE propagation factor of the GPS sea surface scattering signal are modeled and polarization analysed in different duct environments.Then,based on the principle of sea surface electromagnetic scattering and the bistatic radar equation,the simulated power data at the receiver is obtained.The conversion relationship between the chip delay and propagation distance is analyzed.Finally,the experimental measurement data is compared with the theoretical modeling data.The results show a good consistency and the satellite constellation in the visible range of the receiver is drawn.