Research On GNSS Propagation Model And Its Application In Ground Altimetry And Soil Moisture Estimation

Global Navigation Satellite System Reflectometry(GNSS-R)is an emerging approach to carry out microwave remote sensing that uses GNSS multipath signals reflected from the surface of the Earth,which are conventionally considered to be error signals.Multipath propagation occurs when reflected signals caused by environmental characteristics reaches the receiving antenna.These multipath signals can be used for ocean and land surface detection because they contain physical parameters of reflective surface.Altimetry and soil moisture estimation are important applications of GNSS-R technology.Multipath signals reflected from the ground can be directly used for soil moisture estimation,and the interference signals which are the sum of the direct and multipath signal components can also be used for altimetry and soil moisture estimation.The traditional geometric scattering model is not suitable for analyzing the interaction between GNSS signals and complex terrain.The GNSS multipath propagation model was established from the perspective of radio wave propagation,which can accurately predict the propagation of complex surface and GNSS multipath signals were apply to the antenna height measurement and soil moisture estimation.In this paper,the Parabolic Equation(PE)is used to analyze the multipath propagation behavior of GNSS signals on the land surface.The terrain features are combined into the traditional PE model to fully describe the multipath propagation behavior and provide visual results.Through field experiments,the influence of local terrain on height measurement and the feasibility of new data types and data from new source for soil moisture estimation are analyzed.The main work and research results are as follows:Firstly,based on the traditional PE method,the reflection coefficient and the ground roughness are combined into the lower boundary conditions to establish a model which can describe the propagation of GNSS multipath signals.The GNSS multipath interference signal extracted from the simulation results has good consistency with the measured signal,which verifies the correctness of the simulation results.Next,based on the method of altimetry by interference model,the influence of arbitrary terrain on height measurement is studied.The boundary shift method is used to deal with terrain boundaries,and the GNSS propagation model on arbitrary terrain is established to simulate the influence of different terrains on the interference signals.The field experiments for ground altimetry in three typical scenes are carried out,the results indicate that the soil block near the antenna will bring errors to the height measurement,and the zenith-looking antenna can suppress the influence of the near terrain and get more accurate estimation than horizontal-looking antenna.Then,a new data,signal strength indicators(SSI)signal is used to estimate the soil moisture.The 80-day SSI signals are collected by field experiments for soil moisture estimation.The results show that the correlation coefficient between the phase of SSI signal and the in-situ soil moisture is 0.72,and the root-mean-square error of estimated soil moisture is less than 9.9%.The feasibility of using SSI data instead of raw SNR data to estimate soil moisture is preliminarily verified.Finally,the GEO multipath reflection signal is used to observe the continuous long-term soil moisture in the fixed area,focusing on the effects of the small movement of the GEO satellite,the antenna gain mode and the zero drift of the hygrometer on the reflection coefficient.Using the reflection coefficient of GEO satellite to obtain the continuous 14-day soil moisture estimation value of the fixed area.The experimental results verify the feasibility of using the GEO satellite reflected signal power to estimate soil moisture in the fixed area for a long time.