Retrieval For High Arch Dam Water Level Using GNSS-R Signals And Its Application

With the gradual improvement of the BeiDou Navigation Satellite System(BDS)in China,Beginning on December 27,2018,the BDS system became the fourth system to achieve global positioning service capabilities.In addition to providing services such as timing,communication and positioning,BDS,like other GNSS(Global Navigation Satellite System)systems,can also be used to monitor the surface environment.At present,GNSS-R(Global Navigation Satellite System Reflectometry)technology has been widely used in research fields such as snow depth change,sea level change and soil moisture change detection.Water level change is one of the important monitoring tasks of power station dams,which helps to monitor and analyze the operating status of power station dams.This paper attempts to apply GNSS-R technology to water level dam inversion.Different from the traditional water level acquisition method of power station dams,this method can improve the low degree of automation and low reliability of traditional water level collection methods,and achieve high-efficiency water level change monitoring.Therefore,the water level inversion study combined with the signal-to-noise ratio signal of the GPS/BDS system satellite is of great significance for the expansion of GNSS-R technology and the water level monitoring of hydropower dams.In this paper,Based on the multi-path signal characteristics of water surface reflection,this paper analyzes the influence of BDS/GPS system signal frequency,altitude angle range and arc length on the water level inversion results of power station dams.Realized the use of GNSS receiver to obtain water level change results of power station dam.This paper further investigates the correlation between water level and deformation of power stations.In order to accurately analyze the relationship between water level and other factors and GNSS monitoring displacement,this paper also analyzes and verifies the reliability of GNSS monitoring displacement based on the vertical displacement of power station dam.The research contents and achievements are presented as follows:(1)This paper compares and analyzes the BDS B1/B2 frequency and GPS L1/L2 frequency.The experimental results show that the BDS B1 frequency and GPS L1 frequency will ensure the maximum satellite utilization and inversion success rate.Due to the occlusion of the environment near the power station dam,the range of available low altitude angles is limited.The influence of signal-to-noise ratio(SNR)data on water level inversion results is analyzed experimentally.The experiment found that the SNR data water level inversion results of 20°~35° can guarantee the lower gross error rejection rate and higher water level inversion accuracy.As for the satellite signals with different arc lengths,the accuracy of the water level inversion results has little effect.(2)The GPS/BDS-SNR water level retrieval results show good consistency with the actual water level record,and the correlation between the SNR retrieval result and the measured water level data can reach 0.9 or more.The water level retrieval accuracy of the GPS system is 0.1-0.15 m,the water level retrieval accuracy of the BDS system is 0.35-0.45 m,and the combined inversion accuracy of the GPS and BDS systems is 0.1-0.13 m,which is better than the single system retrieval precision.At present,there is a big gap between the number and spatial distribution of visible satellites between BDS and GPS.Among them,the GEO satellite of BDS system has limited altitude angle and cannot use this method for water level retrieval.The water level retrieval accuracy of GPS system is higher than that of BDS system.(3)Based on the vertical displacement of the power station dam,this paper analyzes and verifies the reliability of GNSS monitoring displacement.The experimental results show that the GNSS system and the vertical line system show good consistency between the monitoring displacements,and the deviation between the two is within 2mm.The correlation between water level and other factors and the deformation of power station dams were analyzed by means of regression analysis.The experimental results show that the GNSS radial displacement of the power station dam has the largest deformation range compared with other directions,and the correlation between the results and the water level is large.The correlation between tangential displacement and temperature of GNSS is large,especially in the case of long period.Obviously;the correlation between water level and temperature and GNSS elevation direction is not obvious.