Present-day movement characteristics of the Qinghai Nanshan fault and its surrounding area from GPS observation

The Qinghai Nanshan fault is a larger fault in the Northeastern Xizang Plateau.In previous studies,its movement characteristics are mainly investigated with geological and seismic observations,and the tectonic transformation role of the fault on its east is not yet clear.This study uses data fusion to obtain denser GPS observations near the Qinghai Nanshan fault.Based on tectonic characteristics,we establish a block model to investigate the fault slip rate,locking degree,and slip deficit.The results show that the Qinghai Nanshan fault slip rate is characterized by sinistral and convergent movement.Both the sinistral and convergent rates display a decreasing trend from west to east.The locking degree and slip deficit are higher in the western segment(with an average of about 0.74 and 1.1 mm/a)and lower in the eastern segment.Then,we construct a strain rate field using GPS observations to analyze the regional strain characteristics.The results indicate that along the fault,the western segment shows a larger shear strain rate and negative dilation rate.Regional earthquake records show that the frequency of earthquakes is lower near the fault.The joint results suggest that the western segment may have a higher earthquake risk.In addition,the insignificant fault slip rate in the eastern segment may indicate that it does not participate in the tectonic transformation among the Riyueshan,Lajishan,and West Qinling faults.

The Progress of IGS Analysis Center at Wuhan University

As one of the Analysis Centers(AC)of the International GNSS Service(IGS),Wuhan University(WHU)has been contributing to the IGS by providing ultra-rapid as well as rapid orbit and clock solutions for the established GPS and GLONASS since 2012.In the same year,the IGS initiated the Multi-GNSS Experiment(MGEX)to support the analysis of the emerging GNSS systems and prepare the IGS for Multi-GNSS,which includes GPS,GLONASS,the European Galileo system,the Chinese Beidou Navigation Satellite System(BDS),the Japanese Quasi-Zenith Satellite System(QZSS)and the Indian Regional Navigation Satellite System(IRNSS/NaVIC).The major products,i.e.,orbits,Earth Orientation Parameters(EOPs),satellite clock as well as attitude have also been provided by WHU since 2012.More recently,WHU has engaged the third reprocessing of IGS for generating the highly accurate station coordinates as inputs for establishment of the International Terrestrial Reference Frame(ITRF)2020 during 2019—2020.This article presents the recent major advancements of the IGS AC at Wuhan University,including precise products,real-time products,bias products,antenna phase center calibration,and the non-linear motion modeling for GNSS Reference Stations.

根据峰值地动位移标度无饱和地计算地震震级

全球定位系统测量仪是非惯性的,它直接测量相对于全球参考框架的位移）而惯性传感器受到系统出格（主要因倾斜）的影响,对得到位移的积分有不利影响.我们根据分布在近源到区域距离（约10-1000km）上的高速全球定位系统台网记录,研究了Mw6-9地震的峰值地动位移（PGD）的震级标度性质,得出的结论是：实时全球定位系统的地震波形可用来快速测定震级）通常在破裂开始的第一分钟内,许多情况是在破裂结束前,可定出震级.虽然比用P波初始几秒钟波形的地震预警方法要慢一些,但我们的方法不会遇到地震传感器在大震级时出现的饱和效应.快速震级估算可用于快速生成地震的震源模型、海啸预测和需要长周期位移准确信息的地震动研究.

Precise orbit determination of Haiyang‑2D using onboard BDS‑3 B1C/B2a observations with ambiguity resolution

The Haiyang-2D altimetry mission of China is one of the first Low Earth Orbit(LEO)satellites that can receive new B1C/B2a signals from the BeiDou-3 Navigation Satellite System(BDS-3)for Precise Orbit Determination(POD).In this work,the achievable accuracy of the single-receiver ambiguity resolution for onboard LEO satellites is studied based on the real measurements of new BDS-3 frequencies.Under normal conditions,six BDS-3 satellites on average are visible.However,the multipath of the B1C/B2a code observations presents some patchy patterns that cause near-field variations with an amplitude of approximately 40 cm and deteriorate the ambiguity-fixed rate.By modeling those errors,for the B2a code,a remarkable reduction of 53%in the Root Mean Square(RMS)is achieved at high elevations,along with an increase of 8%in the ambiguity-fixed rates.Additionally,an analysis of the onboard antenna’s phase center offsets reveals that when compared to the solutions with float ambiguities,the estimated values in the antenna’s Z direction in the solutions with fixed ambiguities are notably smaller.The independent validation of the resulting POD using satellite laser ranging at 16 selected high-performance stations shows that the residuals are reduced by a minimum of 15.4%for ambiguity-fixed solutions with an RMS consistency of approximately 2.2 cm.Furthermore,when compared to the DORIS-derived orbits,a 4.3 cm 3D RMS consistency is achieved for the BDS-3-derived orbits,and the along-track bias is reduced from 2.9 to 0.4 cm using ambiguity fixing.

Observable-specific phase biases of Wuhan multi-GNSS experiment analysis center’s rapid satellite products

Precise Point Positioning(PPP)with Ambiguity Resolution(AR)is an important high-precision positioning technique that is gaining popularity in geodetic and geophysical applications.The implementation of PPP-AR requires precise products such as orbits,clocks,code,and phase biases.As one of the analysis centers of the International Global Navigation Satellite System(GNSS)Service(IGS),the Wuhan University Multi-GNSS experiment(WUM)Analysis Center(AC)has provided multi-GNSS Observable-Specific Bias(OSB)products with the associated orbit and clock products.In this article,we first introduce the models and generation strategies of WUM rapid phase clock/bias products and orbit-related products(with a latency of less than 16 h).Then,we assess the performance of these products by comparing them with those of other ACs and by testing the PPP-AR positioning precision,using data from Day of the Year(DOY)047 to DOY 078 in 2022.It is found that the peak-to-peak value of phase OSBs is within 2 ns,and their fluctuations are caused by the clock day boundary discontinuities.The associated Global Positioning System(GPS)orbits have the best consistency with European Space Agency(ESA)products,and those of other systems rank in the medium place.GLObal NAvigation Satellite System(GLONASS)clocks show slightly inconsistency with other ACs’due to the antenna thrust power adopted,while the phase clocks of other GNSSs show no distortion compared with legacy clocks.With well-estimated phase products for Precise Orbit Determination(POD),the intrinsic precision is improved by 14%,17%,and 24%for GPS,Galileo navigation satellite system(Galileo),and BeiDou-3 Navigation Satellite System(BDS-3),respectively.The root mean square of PPP-AR using our products in static mode with respect to IGS weekly solutions can reach 0.16 cm,0.16 cm,and 0.44 cm in the east,north,and up directions,respectively.The multi-GNSS wide-lane ambiguity fixing rates are all above 90%,while the narrow-lane fixing rates above 80%.In conclusion,the phase OSB products at WUM have good precision and performance,which will benefit multi-GNSS PPP-AR and POD.

Quasi-4-dimension ionospheric modeling and its application in PPP

Ionospheric delay modeling is not only important for Global Navigation Satellite System(GNSS)based space weather study and monitoring,but also an efficient tool to speed up the convergence time of Precise Point Positioning(PPP).In this study,a novel model,denoted as Quasi-4-Dimension Ionospheric Modeling(Q4DIM)is proposed for wide-area high precision ionospheric delay correction.In Q4DIM,the Line Of Sight(LOS)ionospheric delays from a GNSS station network are divided into different clusters according to not only the location of latitude and longitude,but also satellite elevation and azimuth.Both Global Ionosphere Map(GIM)and Slant Ionospheric Delay(SID)models that are traditionally used for wide-area and regional ionospheric delay modeling,respectively,can be regarded as the special cases of Q4DIM by defining proper grids in latitude,longitude,elevation,and azimuth.Thus,Q4DIM presents a resilient model that is capable for both wide-area coverage and high precision.Four different sets of clusters are defined to illustrate the properties of Q4DIM based on 200 EUREF Permanent Network(EPN)stations.The results indicate that Q4DIM is compatible with the GIM products.Moreover,it is proved that by inducting the elevation and azimuth angle dependent residuals,the precision of the 2-dimensional GIM-like model,i.e.,Q4DIM 2-Dimensional(Q4DIM-2D),is improved from around 1.5 Total Electron Content Units(TECU)to better than 0.5 TECU.In addition,treating Q4DIM as a 4-dimensional matrix in latitude,longitude,elevation,and azimuth,whose sparsity is less than 5%,can result in its feasibility in a bandwidth-sensitive applications,e.g.,satellite-based Precising Point Positioning Real-Time Kinematic(PPP-RTK)service.Finally,the advantages of Q4DIM in PPP over the 2-dimensional models are demonstrated with the one month's data from 30 EPN stations in both high solar activity year 2014 and low solar activity year 2020.

Assessing IGS GPS/Galileo/BDS-2/BDS-3 phase bias products with PRIDE PPP-AR

Ambiguity Resolution in Precise Point Positioning (PPP-AR) is important to achieving high-precision positioning in wide areas. The International GNSS (Global Navigation Satellite System) Service (IGS) and some other academic organizations have begun to provide phase bias products to enable PPP-AR, such as the integer-clock like products by Centre National d’Etudes Spatials (CNES), Wuhan University (WUM) and the Center for Orbit Determination in Europe (CODE), as well as the Uncalibrated Phase Delay (UPD) products by School of Geodesy and Geomatics (SGG). To evaluate these disparate products, we carry out Global Positioning System (GPS)/Galileo Navigation Satellite System (Galileo) and BeiDou Navigation Satellite System (BDS-only) PPP-AR using 30 days of data in 2019. In general, over 70% and 80% of GPS and Galileo ambiguity residuals after wide-lane phase bias corrections fall in ± 0.1 cycles, in contrast to less than 50% for BeiDou Navigation Satellite (Regional) System (BDS-2);moreover, around 90% of GPS/Galileo narrow-lane ambiguity residuals are within ± 0.1 cycles, while the percentage drops to about 55% in the case of BDS products. GPS/Galileo daily PPP-AR can usually achieve a positioning precision of 2, 2 and 6 mm for the east, north and up components, respectively, for all phase bias products except those based on German Research Centre for Geosciences (GBM) rapid satellite orbits and clocks. Due to the insufficient number of BDS satellites during 2019, the BDS phase bias products perform worse than the GPS/Galileo products in terms of ambiguity fixing rates and daily positioning precisions. BDS-2 daily positions can only reach a precision of about 10 mm in the horizontal and 20 mm in the vertical components, which can be slightly improved after PPP-AR. However, for the year of 2020, BDS-2/BDS-3 (BDS-3 Navigation Satellite System) PPP-AR achieves about 50% better precisions for all three coordinate components.