Research On Surface Freeze-thaw State Monitoring Method Based On GNSS-I

Surface freeze-thaw an important factor affecting hydrology,water cycle and carbon cycle,so effective monitoring of surface freeze-thaw state is very important in climate change and environmental research.GNSS-IR(Global Navigation Satellite System-Interference Reflectometry)technology is a new type of surface freeze-thaw state monitoring method,which has the advantages of high temporal and spatial resolution,large scale and low power consumption,and has a good application prospect in freeze-thaw monitoring.However,the accuracy and applicability of surface freeze-thaw monitoring are affected by the earth environmental parameters(snow depth,soil moisture,etc.)and the intensity and accuracy of signals.based on this,this paper uses GNSS-IR technology to monitor the surface freeze-thaw state,analyzes the impact of the changes of earth environmental parameters in complex cold regions on the monitoring accuracy,and increases the monitoring accuracy by improving the signal strength and accuracy of data quality control.The research contents and conclusions of this thesis are as follows:(1)The feasibility of monitoring surface freeze-thaw state by GNSS-IR technology is studied,and a star selection algorithm is proposed to improve the monitoring accuracy.The dielectric constant of the soil near the receiver of the PBO(The Plate Boundary Observatory)observation network is calculated,and the results show that the change of the dielectric constant will affect the surface reflectivity and lead to the change of the ground reflection signal,and the dielectric constant of each carrier is calculated,and the optimal signal carrier is selected according to the station data.The satellite selection processing algorithm is to improve the monitoring accuracy by analyzing the frequency spectrum oscillation amplitude of different satellite signals and selecting the satellites with high signal strength and accuracy.(2)The surface freeze-thaw monitoring accuracy of GNSS-IR technology under different earth environmental parameters is studied,and the applicability of this method under the fluctuation of environmental parameters is analyzed.The GNSS-IR technology is used to separate the reflected signals of the observed data of the AB33 station during the freeze-thaw conversion period from 2017 to 2022,and the data quality is improved.The satellite with poor signal is eliminated by the star selection algorithm,and the reflected component of the satellite signal with good signal is processed.The experimental results show that the accuracy is better when the environmental parameters change smoothly,and the highest is90.63%.In the case of great changes in environmental parameters,the accuracy decreased significantly,with a minimum of 78.72%,and the overall average accuracy reached 83.9%.Relatively speaking,GNSS-IR already has the ability to effectively monitor the freezing and thawing state of the surface.(3)The method of eliminating observation noise based on wavelet analysis is studied to improve the accuracy of freeze-thaw monitoring.Using the principle of fast wavelet decomposition and reconstruction,it is found that the sixth layer component of wavelet decomposition is the most suitable.The SNR reflection component reconstructed by wavelet analysis is fitted with the original data,and compared with the original data fitted by quadratic polynomials,it is found that the fitting data after wavelet decomposition and reconstruction have higher fit degree and higher data accuracy.Through the processing of GNSS observation data,the experimental results show that the surface freeze-thaw monitoring accuracy after wavelet analysis is more than 82.5%,the accuracy is improved by more than 3%,and the overall average accuracy is 86.29%.Compared with the freeze-thaw product monitoring accuracy after star selection algorithm,the monitoring accuracy is improved by 2.39%.Therefore,the combination of wavelet analysis method to control data quality is more accurate than star selection algorithm,which can significantly improve the monitoring accuracy of GNSS-IR technology,which is a new idea of surface freeze-thaw state monitoring with high temporal and spatial resolution.