Study On Station Environment Effect In GNSS Deformation Monitoring

Large buildings(dams,extra large bridges,super high-rise buildings,etc.)and landslides are affected by external loads and gravity,and various disasters caused by internal and external activities will pose a great threat to human life and property security.Deformation monitoring of feature points such as large buildings and landslides to determine accurate deformation can provide basic data for safety state assessment and disaster prediction and warning.It is also a research hotspot at home and abroad in this field.With the increasing development of Global Navigation Satellite System(GNSS),its application in the field of deformation monitoring is also expanding.A large number of studies have shown that under good observation conditions,the accuracy of deformation monitoring by GNSS can reach millimeter level in horizontal direction,sub-centimeter level or even millimeter level in vertical direction,which can meet the accuracy requirements of relevant industry specifications for deformation monitoring.However,in practical application,some monitoring points are inevitably occluded by the surrounding environment and its own structure,or are subject to the multipath effect caused by the surrounding environment,which will reduce the accuracy of the observation value,and then affect the service level of GNSS.The impact of the station environment is collectively referred to as the station environmental error in this paper.Based on the above situation and problems,this paper analyses the influence of the change of satellite geometry structure on GNSS monitoring accuracy in the case of station occlusion.The results are helpful to evaluate the monitoring accuracy level of GNSS in different occlusion environments worldwide.The methods of mitigating the multipath effect in time and space domains are studied,which weakens the influence of station multipath effect on monitoring accuracy.The research findings in this paper lays a foundation for the application of GNSS in the environmental occlusion area.The main research contents and achievements of this paper are as follows:1)The GNSS data processing method in deformation monitoring are systematically researched and studied.This paper introduces the GNSS data processing methods used in this paper,including basic mathematical model,parameter estimation method and ambiguity resolution method.The causes,types and characteristics of multipath error,are described and the coordinate domain sidereal filtering,the observation domain sidereal filtering based on single difference and double difference residual and the spatial domain sidereal filtering method based on the single difference and the double difference residual are introduced.The mathematical relationship between the positioning error and the observation error and the concept of the precision factor are expounded.2)The distribution and change of global DOP in different occlusion environments are analyzed,and the accuracy evaluation model in global occlusion environment is established.The distribution characteristics of global EDOP,NDOP,VDOP and PDOP in the west,north and south directions and the cutoff height angles of 20 degrees,30 degrees and 40 degrees are analyzed,and the accuracy of the global occlusion environment is further established.The evaluation model can be used to give the theoretical precision at any position in the world under specific occlusion conditions,which provides a reference for the deformation monitoring.3)The global occlusion environment accuracy evaluation model is verified.This paper selects the mid-low latitude station(35 degrees north latitude)and the mid-high latitude station(55 degrees north latitude),and occludes the satellites below the 40 degree elevation angle on the west side,north side and south side of the station,respectively.The DOP value,ENU direction coordinate standard deviation and RMS value are used to evaluate the accuracy of the above model.The results show that the change characteristics of each precision index of the two stations before and after occlusion of satellites in all directions are consistent with those of the above-mentioned models,and the influence degree of each index of the stations in middle and high latitudes is greater than that of the stations in middle and low latitudes.The difference of the influence degree between the two stations basically conforms to the model characteristics.4)The time domain and spatial domain multipath effect mitigating methods are put forward.In this paper,the S071 station with severe multipath effects in the Xilongchi deformation monitoring network in Shanxi Province is used to weaken the multipath effect using sidereal filtering method.The sidereal filtering method in time domain and space domain is studied.The time domain is divided into the coordinate domain and the observation domain.The observation domain is further divided into a double-difference residual and a single-difference residual weakening method.The spatial domain sidereal filtering is divided into two methods that builts the multipath hemispherical map using double-difference residual and single-difference residual.The optimum orbital repetition period is determined to be 23h55min55 s for sidereal filtering in time domain.The all kinds of sidereal filtering methods is compared and analyzed.The results show that all sidereal filtering methods effectively mitigate the multipath effect of the station.The Ratio value of fixed ambiguity and the fixed rate of ambiguity are improved,and the RMS values of fixed solution and floating-point solution are reduced.Among them,the method based on double difference and the method based on single difference have the same effect on improving various evaluation indexes.The spatial domain sidereal works best for the medium and low sampling rate data used in this paper,and it is better than the time domain sidereal filtering method.