Regional ionosphere modeling in support of IRI and wavelet using GPS observations

Dual-frequency global navigation satellite systems (GNSS) observations provide most of the input data for development of global ionosphere map (GIM) of vertical total electron content (VTEC). The international GNSS service (IGS) develops different ionosphere products. The IGS tracking network stations are not homogeneously distributed around the world. The large gaps of this network in Middle East, e.g., Iran plateau, reduce the accuracy of the IGS GIMs over this region. Empirical ionosphere models, such as international reference ionosphere (IRI), also provide coarse forecasts of the VTEC values. This paper presents a new regional VTEC model based on the IRI 2007 and global positioning system (GPS) observations from Iranian Permanent GPS Network. The model consists of a given reference part from IRI model and an unknown correction term. Compactly supported base functions are more appropriate than spherical harmonics in regional ionosphere modeling. Therefore, an unknown correction term was expanded in terms of B-spline functions. The obtained results are validated through comparison with the observed VTEC derived from GPS observations.     

地基GPS VTEC约束的电离层掩星反演方法

本文在分析GPS电离层掩星Abel积分变换反演的基础上,介绍了一种顾及电离层水平梯度信息的反演新方法,即地基GPS VTEC约束的电离层掩星反演法,并将该方法应用于COSMIC低纬掩星观测资料的反演.初步的统计结果表明,与经典Abel变换反演相比,地基GPS VTEC约束反演的临界频率f_oF₂更接近于电离层测高仪观测的结果.     

汶川Ms8.0地震前电离层VTEC的异常变化

利用地基GPS观测资料反演的垂直电子总含量（VTEC）,本文考察了2008年5月12日汶川Ms8.0地震期间的电离层变化.在考察过程中,以前10天的VTEC均值和伴随的2倍标准偏差作为参考来识别电离层异常.最终的考察分析结果表明：（1）VTEC的异常增加出现在5月3日和5月9日,而VTEC的异常减小出现在4月29日和5月6日;（2）所有这4天的电离层VTEC异常持续时间均超过了4 h,且异常自东向西漂移;（3）相对于电离层的非扰动状态,在5次电离层异常的峰值时刻,震中附近VTEC增加或减小超过30％的空间区域沿经、纬向分别超过了4000 km和1500 km.鉴于地震期间的日－地空间环境和电离层VTEC异常特征,我们认为,5月3日下午和5月9日的VTEC正异常可能是本次汶川地震的电离层前兆.     

汶川M_s8.0地震前电离层VTEC的异常变化

利用地基GPS观测资料反演的垂直电子总含量(VTEC),本文考察了2008年5月12日汶川M_s8.0地震期间的电离层变化.在考察过程中,以前10天的VTEC均值和伴随的2倍标准偏差作为参考来识别电离层异常.最终的考察分析结果表明:(1)VTEC的异常增加出现在5月3日和5月9日,而VTEC的异常减小出现在4月29日和5月6日;(2)所有这4天的电离层VTEC异常持续时间均超过了4 h,且异常自东向西漂移;(3)相对于电离层的非扰动状态,在5次电离层异常的峰值时刻,震中附近VTEC增加或减小超过30%的空间区域沿经、纬向分别超过了4000 km和1500 km.鉴于地震期间的日-地空间环境和电离层VTEC异常特征,我们认为,5月3日下午和5月9日的VTEC正异常可能是本次汶川地震的电离层前兆.     

汶川地震前电离层VTEC的异常响应

2008年5月12日我国四川省汶川县发生了8.0级大地震,基于中国地壳运动观测网(CMONOC)的GPS观测数据,解算得到了震中上空附近的电离层VTEC;以VTEC的滑动均值为背景参考值,以2倍均方差作为误差限值,对临震一个月的VTEC 资料进行了处理分析．结果显示,在震前多次出现VTEC的异常扰动,其中4月29日、5月6——7日为异常减少,5月9日为异常增加,而且电离层VTEC的异常驼峰有向磁赤道漂移的趋势．由于距离发震时刻较近,排除其它因素后可能源于此次地震引起的电离层效应．      

芦山MS7.0地震前水平位移和同震水平位移研究

本文利用“中国地壳运动观测网络（二期）”多个GPS连续观测站观测数据处理结果,将2013年4月20日四川芦山MS7.0地震区域参考框架同震水平位移与全球参考框架同震水平位移进行比较,结果表明两组框架解一致,说明两种参考框架均可当作位错参考框架,也即全球参考框架同震水平位移也可视为区域参考框架同震水平位移．区域参考框架下GPS连续观测站地震前的水平位移和同震水平位移结果表明,震前数年,SCTQ站西侧的GPS站构造运动十分显著,而该站水平位移却很小,即出现反常的闭锁．但该站的同震水平位移使其弹性回跳至正常构造水平位移水平,因此SCTQ站震前的位移闭锁是水平位移空间分布中的异常,是芦山MS7.0地震的前兆．水平位移时空变化表明,该站震前和震时位移完全符合里德的弹性回跳理论．区域参考框架中位移时间系列和同震水平位移的综合研究有助于对芦山地震地壳运动前兆的认识和解释．尽管本文未能直接采用其它GPS连续观测站的资料,但结合本文和其它研究结果可以证实,震中附近其它站地震前后的变化与SCTQ站类似．基于芦山地震前水平位移和同震水平位移及其与前兆关系的研究,本文进一步讨论了GPS监测网的布设、 数据处理和分析等问题．      

Local Ionospheric Modeling Using the Localized Global Ionospheric Map and Terrestrial GPS

global ionosphere maps are generated on a daily basis at CODE using data from about 200 GPS/GLONASS sites of the IGS and other institutions. The vertical total electron content is modeled in a solargeomagnetic reference frame using a spherical harmonics expansion up to degree and order 15. The spherical Slepian basis is a set of bandlimited functions which have the majority of their energy concentrated by optimization inside an arbitrarily defined region, yet remain orthogonal within the spatial region of interest. Hence, they are suitable for decomposing the spherical harmonic models into the portions that have significant strength only in the selected areas. In this study, the converted spherical harmonics to the Slepian bases were updated by the terrestrial GPS observations by use of the least-squares estimation with weighted parameters for local ionospheric modeling. Validations show that the approach adopted in this study is highly capable of yielding reliable results.     

GNSS测站坐标非线性变化影响机制的研究

对影响地球参考框架稳定度与精确度的地球动力学因素进行深入研究,能更好地促进毫米级地球参考架的建立.而研究GNSS测站坐标的非线性变化及其相关机制,是实现毫米级地球参考框架的前提,也为提升GNSS坐标精度、研究地球物理因素等提供参考.首先研究了GNSS测站坐标非线性变化的影响机制与理论改正模型,给出了季节性温度效应、地表物质负荷效应、GNSS周年性系统误差和高阶电离层延迟这四种因素的研究进展,并提出了在机制和模型研究方面一些改进的方向.然后通过以高阶电离层延迟对GNSS测站坐标时间序列的影响为例,进行分组实验,研究了高阶电离层延迟对GNSS测站坐标非线性变化的影响,并进一步将影响进行了量化.研究结果有助于对测站坐标非线性变化的机制做出更精确合理的解释,同时也有助于削弱与限制高阶电离层延迟对建立高精度地球参考框架的影响;为我国北斗地球参考框架的建立与维持提供了可靠的理论基础,具有一定的参考价值.     

A three-dimensional nonlinear reduced-order predictive joint model

Mechanical joints can have significant effects on the dynamics of assembled structures. However, the lack of efficacious predictive dynamic models for joints hinders accurate prediction of their dynamic behavior. The goal of our work is to develop physics-based, reduced-order, finite element models that are capable of replicating the effects of joints on vibrating structures. The authors recently developed the so-called two-dimensional adjusted Iwan beam element (2-D AIBE) to simulate the hysteretic behavior of bolted joints in 2-D beam structures. In this paper, 2-D AIBE is extended to three-dimensional cases by formulating a three-dimensional adjusted Iwan beam element (3-D AIBE). Impulsive loading experiments are applied to a jointed frame structure and a beam structure containing the same joint. The frame is subjected to excitation out of plane so that the joint is under rotation and single axis bending. By assuming that the rotation in the joint is linear elastic, the parameters of the joint associated with bending in the frame are identified from acceleration responses of the jointed beam structure, using a multi-layer feed-forward neural network (MLFF). Numerical simulation is then performed on the frame structure using the identified parameters. The good agreement between the simulated and experimental impulsive acceleration responses of the frame structure validates the efficacy of the presented 3-D AIBE, and indicates that the model can potentially be applied to more complex structural systems with joint parameters identified from a relatively simple structure.     

Reconstructing Regional Ionospheric Electron Density: A Combined Spherical Slepian Function and Empirical Orthogonal Function Approach

The computerized ionospheric tomography is a method for imaging the Earth’s ionosphere using a sounding technique and computing the slant total electron content (STEC) values from data of the global positioning system (GPS). The most common approach for ionospheric tomography is the voxel-based model, in which (1) the ionosphere is divided into voxels, (2) the STEC is then measured along (many) satellite signal paths, and finally (3) an inversion procedure is applied to reconstruct the electron density distribution of the ionosphere. In this study, a computationally efficient approach is introduced, which improves the inversion procedure of step 3. Our proposed method combines the empirical orthogonal function and the spherical Slepian base functions to describe the vertical and horizontal distribution of electron density, respectively. Thus, it can be applied on regional and global case studies. Numerical application is demonstrated using the ground-based GPS data over South America. Our results are validated against ionospheric tomography obtained from the constellation observing system for meteorology, ionosphere, and climate (COSMIC) observations and the global ionosphere map estimated by international centers, as well as by comparison with STEC derived from independent GPS stations. Using the proposed approach, we find that while using 30 GPS measurements in South America, one can achieve comparable accuracy with those from COSMIC data within the reported accuracy (1 × 10¹¹ el/cm³) of the product. Comparisons with real observations of two GPS stations indicate an absolute difference is less than 2 TECU (where 1 total electron content unit, TECU, is 10¹⁶ electrons/m²).     

GPS-derived height changes in diurnal and sub-diurnal timescales

This paper describes the research concerning precise short-time GPS solutions conducted in the Centre of Applied Geomatics, Military University of Technology, Warsaw, Poland. The data from ASG-EUPOS (Polish Active Geodetic Network) was processed using Bernese 5.0 software and EPN (EUREF Permanent Network) standards and models. In this study, the adapted 3-hour observation window is shifted every hour for obtaining hourly geocentric coordinates in ITRF2005 reference frame. The adjusted network consisted of over 130 stations from Poland and the neighbouring countries, the period covered observations collected from June 2008 through June 2010. These two years of observations allowed for examining short-period oscillations which were found to be closely related to the tidal (dynamic) frequencies. The analysis of the residua from the IERS2003 tidal model was performed using the least squares method with the Eterna software. It confirmed the existence of significant (several millimetres) oscillations in the frequencies corresponding to S1, K1 and K2. The paper describes the idea of data processing and analysis, presents the results of vertical (the Up component) oscillations in main tidal frequency bands, and also includes discussion of possible explanation of the existence of short-period oscillations in the GPS precise solutions and the possibility of propagation of short-period oscillations into long-period spurious changes in the daily (standard) GPS solutions.     

Investigation of Lushan earthquake ionosphere VTEC anomalies based on GPS data

Vertical total electron content (VTEC) time series were obtained from 22 GPS stations near the epicenter of the Lushan earthquake. In this paper, we have adopted a sliding average method to detect and analyze anomalous VTEC associated with the earthquake. The results show that significant, negative ionosphere VTEC anomalies appeared over the 5 days before the earthquake, and on the day when earthquake occurred. The maximum value of VTEC anomalies that exceeded the lower bound reached 20 TECU. The spatial distribution of VTEC anomalies showed a conjugate structure, which shifted to the magnetic equator, and subsequently moved westwards.     

Ionospheric anomalies detected by ground-based GPS before the Mw7.9 Wenchuan earthquake of May 12, 2008, China

In this paper, we used the vertical total electron content (VTEC) derived from ground-based GPS observations to examine the ionospheric variations during the Mw7.9 Wenchuan earthquake which occurred on May 12, 2008, in China. The final results showed that the ionospheric VTEC around the epicenter increased obviously 9 days before the earthquake, decreased significantly 6 days before the earthquake, and increased strikingly 3 days before the earthquake. Relative to a normal non-perturbed state of the ionosphere, the maximum anomalies in the 3 days exceeded 40%, 40%, and 70%, respectively. The spatial sizes of the VTEC decrement and increment by more than 30% extended larger than 1500 km in latitude and 4000 km in longitude. Moreover, the maximum anomalies 6 days and 3 days before the earthquake moved towards the equator.     

Recent Geodetic Results in the Azores Triple Junction Region

—GPS (Global Positioning System) observations started to be carried out in the Azores region under the scope of the TANGO (TransAtlantic Network for Geodesy and Oceanography) project in 1988. The measurements carried out between 1993 and 2000 (five campaigns) on nine GPS sites (one per island) were reprocessed using two state–of–the-art software packages. Different methodologies were applied to compute each campaign solution and the derived velocity field. The velocity fields, including the motions of two permanent stations, recently installed in the Azores, were computed within the most recent geodetic reference frame, ITRF2000 (International Terrestrial Reference Frame, solution 2000). They are compared with the motions of the stable rigid tectonic plates using as reference DEOS2k, a global tectonic model developed using geodetic data. The relative motions between the Western and Central groups of islands yield to evaluate the opening rate of the Mid-Atlantic Ridge (boundary between the North American plate and the Eurasian and African plates). Concerning the boundary between the Eurasian and African plates, the motion of the TANGO sites in the Central and Eastern groups clearly identifies the transition pattern between those two plates. Two of the sites are considered to be located in the stable part of these plates, whereas the remaining five are within the deformation region of the Eurasia-Africa boundary. The conclusions are analyzed in view of the different deformation models, derived from geodynamic or geophysical data that have been proposed for the region.     

顾及多方向观测值权比反演地球重力场的动力积分法

考虑到不同坐标系下各个方向观测值对反演地球重力场的频谱贡献不同,建立了顾及多方向观测值权比的动力积分法,并利用该方法反演了高精度的GOCE HL-SST卫星重力场模型.首先,分析了不同坐标系下各个方向观测值与地球重力场信息的响应关系,其中惯性系(IRF)下X、Z方向的观测值分别对扇谐系数、带谐系数最为敏感,Z方向的解算精度在全频段均略高于X、Y方向;地固系(EFRF)下各个方向的独立解算精度均与能量守恒法的解算精度相当;局部指北坐标系(LNOF)下X、Z和Y三个方向的解算精度依次递减,且Y方向在47阶附近有明显"驼峰"现象.其次,比较了不同坐标系下顾及三个方向观测值权比的加权解算模型,其中加权联合解算模型精度在20至70阶次均明显优于等权解算模型,在带谐项和共振阶次精度提升明显,且LNOF下的加权联合解算精度要优于IRF和EFRF.最后,比较了GOCE和CHAMP卫星的模型解算精度,采用本文计算方法,仅利用2个月GOCE轨道观测值解算的模型精度优于包含更长观测时段信息的AIUB-CHAMP01S和EIGEN-CHAMP03S模型,且略优于ASU-GOCE-2months模型.     

GNSS(GPS)观测研究地壳运动的新进展

介绍了近几年来GNSS(GPS)系统、观测技术和数据处理方法和技术的发展,数据格式标准化的进展,全球参考框架现状,参考框架对地壳运动研究的影响和IGS的产品。介绍利用GNSS观测研究地壳运动的成果,包括大地震前后及震时的地壳运动结果,时间序列观测结果的研究进展等。     

Seasonal Water Storage Variations as Impacted by Water Abstractions: Comparing the Output of a Global Hydrological Model with GRACE and GPS Observations

Better quantification of continental water storage variations is expected to improve our understanding of water flows, including evapotranspiration, runoff and river discharge as well as human water abstractions. For the first time, total water storage (TWS) on the land area of the globe as computed by the global water model WaterGAP (Water Global Assessment and Prognosis) was compared to both gravity recovery and climate experiment (GRACE) and global positioning system (GPS) observations. The GRACE satellites sense the effect of TWS on the dynamic gravity field of the Earth. GPS reference points are displaced due to crustal deformation caused by time-varying TWS. Unfortunately, the worldwide coverage of the GPS tracking network is irregular, while GRACE provides global coverage albeit with low spatial resolution. Detrended TWS time series were analyzed by determining scaling factors for mean annual amplitude (f _GRACE) and time series of monthly TWS (f _GPS). Both GRACE and GPS indicate that WaterGAP underestimates seasonal variations of TWS on most of the land area of the globe. In addition, seasonal maximum TWS occurs 1 month earlier according to WaterGAP than according to GRACE on most land areas. While WaterGAP TWS is sensitive to the applied climate input data, none of the two data sets result in a clearly better fit to the observations. Due to the low number of GPS sites, GPS observations are less useful for validating global hydrological models than GRACE observations, but they serve to support the validity of GRACE TWS as observational target for hydrological modeling. For unknown reasons, WaterGAP appears to fit better to GPS than to GRACE. Both GPS and GRACE data, however, are rather uncertain due to a number of reasons, in particular in dry regions. It is not possible to benefit from either GPS or GRACE observations to monitor and quantify human water abstractions if only detrended (seasonal) TWS variations are considered. Regarding GRACE, this is mainly caused by the attenuation of the TWS differences between water abstraction variants due to the filtering required for GRACE TWS. Regarding GPS, station density is too low. Only if water abstractions lead to long-term changes in TWS by depletion or restoration of water storage in groundwater or large surface water bodies, GRACE may be used to support the quantification of human water abstractions.     

GPS地面台网和掩星观测结合的时变三维电离层层析

本文给出GPS地面台网和掩星观测结合的时变三维电离层层析的原理、算法和基于实测数据的反演结果.反演结果的比较表明,联合地基GPS与掩星观测数据进行重建,电子密度整体图像的重建质量特别是其垂直结构的重建质量得到了明显改善.在平静日和磁暴期间两种条件下利用实测数据的重建结果表明,GPS地面台网和掩星观测结合的电离层层析可以获得电离层电子密度在高度-纬度-经度-时间四维空间中的变化.重建结果清晰地显示了磁暴期间电离层负相暴效应,表明结合GPS地面台网和掩星观测的时变三维电离层层析可以有效地监测扰动条件下的大尺度电离层结构.     

区间B样条小波有限元GPR模拟双相随机混凝土介质

基于可分离小波理论,由一维区间B样条小波尺度函数的张量积构造二维B样条小波基,并将它作为GPR波动方程求解的插值函数,通过引入转换矩阵,实现小波系数空间与雷达电磁场之间的转换.应用Galerkin算法,推导了二维区间B样条小波有限元GPR波动方程离散格式,求出了2阶1尺度与2阶2尺度BSWI尺度函数的积分值及联系系数,给出了该算法的详细求解过程.编制了BSWI的Matlab模拟程序,应用该程序对两个典型实例进行了正演,结果表明:BSWI能采用较少的单元达到与FEM相似的精度,而BSWI算法尺度提升能提高解的精度,但耗时会急剧增加.最后,将BSWI算法应用于双相随机混凝土模型,说明随机介质模型理论能灵活、有效地描述实际混凝土介质的分布,正演剖面与实测剖面特征更相符,能更真实地模拟雷达波的传播过程,可为提高GPR的探测效果和解释准确性提供理论基础.     

GPS与PS-InSAR联网监测的台湾屏东地区三维地表形变场

联合高精度的GPS水平位移观测和高密度的PS-InSAR雷达视线位移测量,实现地表三维形变的精确反演.本文在准确计算卫星轨道方位角基础上,使用GPS观测位移与星载雷达LOS方向形变的投影转换模型,将雷达LOS方向形变转换为垂直方向位移,并基于地面GPS与SAR影像PS目标联合构建形变监测网,采用参数平差算法估计区域地表形变场.以地质构造活动极其活跃的台湾岛及其西南屏东高雄地区为例,联合屏东地区48个GPS监测台站与雷达PS目标,监测该地区从1995-1999年间由于板块构造挤压运动和地下水抽取导致的三维地表形变.结果表明,该地区年均水平位移量为向西30~50 mm/a,高雄沿海地区发生明显的逆时针西偏南的逐渐增大的水平位移;垂直位移为屏东平原南部呈现-10 mm/a~-15 mm/a的地面沉降,而平原北部和高雄地区呈现约+5 mm/a~+10 mm/a的地面抬升.