Study On Key Technologies Of Beidou Navigation Satellite System In High-precision Deformation Monitoring

Beidou navigation satellite system(BDS)is an independently constructed in China.Its development has experienced three stages: navigation satellite demonstration system(BDS1),regional navigation satellite system(BDS2)and global navigation satellite system(BDS3).Before the completion of BDS,GNSS,represented by GPS of the United States,had played a widely recognized important role in the research of millimeter precision crustal deformation and tectonic movement monitoring.As a rising star of the GNSS family,can China’s BDS play the same role in high-precision crustal deformation and tectonic movement monitoring? Compared with the sophisticated GPS,what other key technologies need to be improved,perfected and broken through in the millimeter level high-precision observation and application of BDS? Around the above two aspects,based on the BDS/GPS observation data produced by the National BDS Ground Augmentation System(NBGAS),Crustal Movement Observation Network of China(CMONOC)and The International GNSS Service(IGS)global framework core station network,this paper adopts internationally advanced data processing methods and strategies to solve the station coordinate time series,baseline time series and velocity field,and then objectively and comprehensively evaluates the precision positioning accuracy,stability and the systematic deviation from GPS.After that,the causes are analyzed and studied,and the solutions to eliminate the deviation and jointly carry out high-precision deformation monitoring are given.Specific to the scientific and technical problems related to the application of BDS in high-precision crustal deformation monitoring,the following aspects are studied:(1)BDS and GPS data are processed by the precise point positioning and precise differential positioning respectively,and the positioning accuracy of the two observation systems under the two positioning modes is compared and analyzed,so as to evaluate the actual effect of high-precision positioning of BDS;(2)Based on the results of station coordinate time series,baseline time series and velocity field,the improvement process and change characteristics of positioning accuracy in the development of BDS are analyzed in detail;On this basis,the differences between BDS and GPS positioning results and their causes are analyzed and discussed,and The Eulerian rotation and Frame Transformation(HELMET)methods are used to eliminate the differences between them,so as to achieve better positioning of the integration of the two systems;(3)How to use the existing base station to realize the results of BDS differential positioning under the stable framework,and what are the consistency and difference characteristics between this framework and GPS framework;(4)In the way of actual combat verification,the co-seismic deformation and seismic waveform characteristics of Maduo earthquake on May 22,2021 are obtained using BDS,the differences between BDS and GPS are analyzed,and the application effect of BDS in sliding fracture inversion and magnitude acquisition of large earthquake is discussed.Through the research,this paper mainly obtains the following understandings and conclusions:(1)There are many factors that affect the positioning accuracy of BDS geodetic survey,among which the accuracy of basic public data products such as satellite precision orbit and precision clock error is the most significant.Over the years,many institutions led and coordinated by the IGS have released public data products such as GNSS multi-system satellite precision orbit and clock error as the basis for high-precision data processing of GNSS geodesy.At present,the accuracy of all satellite products is basically the same,showing that the accuracy of BDS precision orbit and clock error products is slightly lower than that of GPS.Using the precise single point positioning method,we found that the positioning accuracy of BDS2 is lower than that of GPS.In particular,the vertical positioning accuracy of BDS2 is relatively weak,and it is unable to distinguish the seasonal fluctuation signals usually revealed by GPS.In addition,the vertical velocity field obtained by BDS2 also has an integral offset with the GPS results.By Using the helmet method,it is found that the reference frame of the time series results of BDS2 and GPS has an obvious trend turning around 2014.6.We believe that the main reason for this phenomenon is that the BDS2 ground tracking stations are only distributed in the Asia Pacific region,and the relevant correction models such as the BDS satellite antenna phase center are imperfect,resulting in the deviation of the frame scale factor or the frame origin.(2)When Using the precise differential positioning method and the positioning results of GPS under the global reference framework of ITRF14,We have realized the positioning results of BDS2 under the framework of ITRF14.Comparing the 7 parameters of BDS2 and GPS framework,it is found that the fluctuation of 7 parameters of BDS2 framework is significantly higher than that of GPS,indicating that the stability of BDS2 framework is weaker than that of GPS.The statistical analysis of BDS2 positioning results shows that its dispersion is greater than that of GPS: the horizontal accuracy of BDS2 is about 6 ～ 10 mm,the vertical accuracy is about 15～35mm,the horizontal accuracy of GPS is about 2 ～ 3mm,and the vertical accuracy is about 4～8mm.The vertical accuracy of BDS2 is obviously weak,and the seasonal fluctuation signals usually revealed by GPS can no longer be distinguished by BDS2.(3)The precision differential positioning results show that both the crustal movement velocity field obtained from BDS2 and GPS results can show the characteristics of the overall horizontal crustal movement of the Chinese mainland.However,compared with GPS results,the horizontal velocity field of BDS2 has rigid and non-rigid offsets,especially the southeast overall offset,which is about 1mm/yr;The relative uplift of vertical velocity field 1.8mm/yr.The analysis results of removing rigid movement difference based on Eulerian rotation show that the difference in crustal movement velocity field between BDS2 and GPS is not only caused by different frames,but also may be affected by the uneven spatial distribution of positioning accuracy of BDS2.This difference shows a certain linear change trend in space,which may be caused by the imperfection of BDS satellite antenna phase center model and the inconsistency of positioning accuracy in space.(4)The BDS2 regional framework based on CMONOC sites with uniform spatial range can reduce the overall difference with the global framework to a certain extent,so as to reduce the difference between the velocity field obtained by BDS2 and GPS;At the same time,the regional framework can eliminate the common mode error,thereby improve the stability of position time series.Therefore,we suggest that when using BDS2 for precision differential positioning,reference stations with uniform coverage should be selected as much as possible,so as to reduce the network effect of regional framework and obtain results consistent with the global framework.(5)Based on the analysis of precise point positioning results of 40 GNSS multi-system observation data around the world,it is found that the current BDS3 is consistent with GPS in horizontal positioning accuracy,especially in vertical positioning accuracy,which has been significantly improved and able to monitor vertical seasonal fluctuation signals.Specifically,the positioning accuracy of BDS3 in the three directions of NEU is 4.0mm,6.1mm and 9.9mm respectively,which has gradually approached the 2.9mm,3.4mm and 6.8mm of GPS.(6)Based on the analysis of BDS code deviation characteristics,it is found that BDS2 has code deviation effect related to satellite altitude angle,while BDS3 has not similar code deviation effect.The differential positioning accuracy of BDS3 is further improved than precise point positioning.The accuracy in three directions of NEU is 3.8mm,3.7mm and9.0mm respectively.Compared with the precise point positioning result of GPS,this result is equivalent in the horizontal direction and slightly weaker in the vertical direction.(7)Based on the observation data of BDS and GPS,the co-seismic deformation of the Ms7.4 earthquake struck in Maduo,Qinghai Province on May 21,2021 is obtained respectively.By comparing and analyzing the difference and error distribution characteristics of the two sets of horizontal co-seismic deformation results,it is found that BDS and GPS have basically the same ability to obtain horizontal co-seismic deformation,and the maximum difference between the two results is within 5mm;However,the vertical co-seismic deformation difference between BDS and GPS is large,and the maximum difference is 8mm.Based on the comparative analysis of the distribution of underground sliding fracture retrieved from BDS and GPS observation data,it is found that the co-seismic deformation results obtained by BDS are completely consistent with GPS in distinguishing the distribution characteristics of sliding fracture.(8)For dynamic high-frequency GNSS single epoch positioning,BDS and GPS have the same accuracy level in three directions of NEU.The horizontal accuracy of differential positioning is 10 mm,and the vertical accuracy is about 20～40 mm.The precise point positioning results show that the vertical accuracy of the two is improved to 15-20 mm.Based on differential positioning method,the 1Hz horizontal dynamic displacement waveform within 400 km of the Maduo earthquake is obtained from BDS,which is very similar to that obtained by GPS,which proves that BDS can obtain the surface vibration image with the same accuracy as GPS.In addition,after deducting the linear migration characteristics,the seismic wave shape obtained by BDS is more stable than GPS,which may be due to the more visible satellites of BDS than GPS in the region.(9)Based on the existing empirical relationship between PGD and moment magnitude,we verify and calculate the moment magnitude and its error of the Maduo earthquake using the high-frequency seismic waveform obtained by BDS.It is found that the magnitude of this earthquake can be obtained quickly and accurately by using the empirical relationship.This result can be used as a supplement to check the early-warning magnitude of large earthquakes quickly determined by the traditional seismological method,and effectively avoid the problem of low early-warning magnitude caused by magnitude saturation and amplitude limitation of seismograph.