Refinement of global ionospheric coefficients for GNSS applications: Methodology and results

We developed the methodology for the optimal estimation of global ionospheric coefficients of the current Global Navigation Satellite Systems (GNSSs), including the eight- and ten-parameter Klobuchar-like as well as NeQuick models. The ionospheric coefficients of those correction models are calculated from two sets of globally distributed tracking stations of the International GNSS Services (IGS). Performance of the re-estimated Klobuchar-like and NeQuick coefficients are validated during 2002–2014 over the continental and oceanic areas, respectively. Over the continental areas, GPS TECs derived from 40 ground GPS receivers are selected as reference. The eight-, ten-parameter Klobuchar-like and NeQuick models can mitigate the ionospheric delay by 65.8, 67.3 and 75.0%, respectively. Over the global oceans, the independent TECs derived from Jason-1&2 altimeters are used as reference. The re-estimated ionospheric correction models can mitigate 56.1–66.7% of the delay errors. Compared to the original GPS Ionospheric Correction Algorithm (ICA), performance of those eight-, ten-parameter Klobuchar-like and NeQuick models has improved 3.4, 5.9 and 13.4% during the whole test period, respectively. The methodology developed here takes the advantage of high-quality ionospheric TECs derived from the global network of GNSS receivers. The re-estimated ionospheric coefficients can be used as precise ionospheric products to monitor and assess GNSS broadcast ionospheric parameters and to improve the performance of various single-frequency GNSS applications.     

基于极大验后推估理论的全球电离层地图预报

提出了一种基于极大验后估计理论的全球电离层预报方法，基于中国科学院电离层分析中心（CAS）提供的快速全球电离层地图（GIM），实现了1天、2天和5天GIM的预报。以国际GNSS服务组织（IGS）最终GIM、Jason测高卫星提供的电离层观测信息及全球GNSS基准站实测电离层总电子含量（TEC）为基准，评估了2008－2017年CAS电离层预报GIM在全球大陆及海洋区域的精度，并与欧洲定轨中心（CODE）、欧洲空间局（ESA）和西班牙加泰罗尼亚理工大学（UPC）的预报GIM进行对比。在评估时段内，与IGS-GIM相比，CAS预报GIM精度为2.4~3.1 TECU；与测高卫星TEC相比，CAS预报GIM的精度为5.1~6.6 TECU；与全球基准站实测TEC相比，CAS预报GIM的电离层延迟修正精度优于80%。总体来看，CAS预报GIM与CODE预报GIM精度相当，显著优于ESA和UPC预报GIM。      

Evaluation of COMPASS ionospheric model in GNSS positioning

As important products of GNSS navigation message, ionospheric delay model parameters are broadcasted for single-frequency users to improve their positioning accuracy. GPS provides daily Klobuchar ionospheric model parameters based on geomagnetic reference frame, while the regional satellite navigation system of China’s COMPASS broadcasts an eight-parameter ionospheric model, COMPASS Ionospheric Model(CIM), which was generated by processing data from continuous monitoring stations, with updating the parameters every 2 h. To evaluate its performance, CIM predictions are compared to ionospheric delay measurements, along with GPS positioning accuracy comparisons. Real observed data analysis indicates that CIM provides higher correction precision in middle-latitude regions, but relatively lower correction precision for low-latitude regions where the ionosphere has much higher variability. CIM errors for some users show a common bias for in-coming COMPASS signals from different satellites, and hence ionospheric model errors are somehow translated into the receivers’ clock error estimation. In addition, the CIM from the China regional monitoring network are further evaluated for global ionospheric corrections. Results show that in the Northern Hemisphere areas including Asia, Europe and North America, the three-dimensional positioning accuracy using the CIM for ionospheric delay corrections is improved by 7.8%–35.3% when compared to GPS single-frequency positioning ionospheric delay corrections using the Klobuchar model. However, the positioning accuracy in the Southern Hemisphere is degraded due apparently to the lack of monitoring stations there.     

基于CIT技术的广域增强系统电离层延迟修正算法

提出了一种基于电离层层析成像(CIT)技术的广域增强系统(WAAS)电离层延迟 修正算法. 该算法利用模式基函数与截断奇异值分解正则化组合的方式, 实现WAAS单频用户的电离层延迟误差修正. 基于中国区域23个广域基准站 和10个用户站的仿真结果分析表明, 传统的网格算法和基于CIT技术的电离 层延迟修正算法的电离层延迟修正精度与太阳活动、昼夜变化及地磁纬度 之间均存在明显的相关性. 基于CIT技术的电离层延迟修正算法精度优于网格算法, CIT算法的平均误差与标准差相比网格算法均有较大幅度的下降.      

WAAS系统中电离层折射校正的新方法及计算结果

电离层介质的色散性是影响电磁波信号进行卫星导航定位精度的重要因素之一.配合北斗二代分系绩研制任务,提出了一种新的电离层折射校正算法,并利用2000年7月1日到3日的双频GPS观测数据对6个用户站进行试算,进一步将试算所得均方根误差和电离层网格算法得到的误差进行比较.结果表明,对于中纬区域的用户站,估算的TEC误差约为0.5 m左右;而低纬用户误差相对增大,为1 m左右.文中给出的算法与电离层网格模型所提供的精度相差不多,在未来中国自主的卫星增强系统中采用新方法进行电离层进行修正是可行的及有效的.      

45°(N)纬度带的Klobuchar like电离层延迟季节修正模型与评估

电离层延迟是全球卫星导航系统（GNSS）的主要误差源之一。对于装配GNSS单频接收机的航空器,选择简单有效的Klobuchar广播电离层模型来改正电离层延迟误差,其修正率为50%～60%。针对45°（N）纬度带,提出了更高电离层修正需求。考虑到季节因素对中高纬度地区电离层的显著影响,利用GIMs（Global Ionospheric Maps）分析了昼夜中TEC（Total Electron Content）的峰值和谷值随季节（年积日）的变化,建立了一种适用于45°（N）纬度带的Klobuchar like电离层模型。该模型不增加广播模型系数,新模型的夜间和VTEC高峰时电离层修正率分别达到了82%和80%,表明在穿刺点集中的45°(N)纬度地区使用该模型可以更精确地描述该地区的电离层,帮助航空器实现更高精度的定位。     

全球电离层地图TEC数据的插值算法

由IGS工作组提供的全球电离层地图(GIM)是电离层重要的应用数据.卫星高度计能够提供全球实时的电离层延迟误差校正.利用GIM数据,以Jason-3时空分辨率进行电离层总电子含量(TEC)的时间维度插值和空间维度插值,其中空间维度插值采用了Kriging插值和双线性插值两种方法.针对两种插值方法得到的总电子含量,与平滑...     

Estimation of differential code biases with Jason-2/3 onboard GPS observations

With the continuous deployment of Low Earth Orbit (LEO) satellites, the estimation of differential code biases (DCBs) based on GNSS observations from LEO has gained increasing attention. Previous studies on LEO-based DCB estimation are usually using the spherical symmetry ionosphere assumption (SSIA), in which a uniform electron density is assumed in a thick shell. In this study, we propose an approach (named the SHLEO method) to simultaneously estimate the satellite and LEO onboard receiver DCBs by modeling the distribution of the global plasmaspheric total electron content (PTEC) above the satellite orbit with a spherical harmonic (SH) function. Compared to the commonly used SSIA method, the SHLEO model improves the GPS satellite DCB estimation accuracy by 13.46% and the stability by 22.34%, respectively. Compared to the GPS satellite DCBs estimated based on the Jason-3-only observations, the accuracy and monthly stability of the satellite DCBs can be improved by 14.42% and 26.8% when both Jason-2 and Jason-3 onboard observations are jointly processed. Compared with the Jason-2 solutions, the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations have an improved consistency of better than 18.26% and 9.71% with the products provided by the Center for Orbit Determination in Europe (CODE) and Chinese Academy of Sciences (CAS). Taking the DCB products provided by the German Aerospace Center (DLR) as references, there is no improvement in accuracy of the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations than the Jason-2 solutions alone. A periodic variation is found in the time series of both the Jason-3 and Jason-2 onboard receiver DCB estimates. Preliminary analysis of the PTEC distribution based on the estimated SH coefficients are also presented.     

基于NTCM-BC模型的全球卫星导航系统单频电离层延迟修正

选择NTCM-BC模型作为单频电离层延迟修正模型,通过非线性最小二乘拟合的方法,利用提前一天预测的电离层图（COPG文件）,计算得到NTCM-BC模型修正系数;利用Klobuchar模型和IGS发布的GIM数据对NTCM-BC模型进行比较和分析.对太阳活动高、中、低年实测数据的分析结果表明:全球平均水平上,NTCM-BC模型的电离层延迟修正性能明显优于Klobuchar模型,NTCM-BC模型的TEC平均误差和均方根误差比Klobuchar模型分别下降了41%和30%;模型的TEC计算误差与太阳活动剧烈程度成正相关,即太阳活动高年模型误差较大,太阳活动低年误差相对较低.相较于磁静日,磁扰日期间Klobuchar模型和NCTM模型的误差均有一定程度的增加.此外,模型的电离层修正误差同时存在明显的纬度、季节和地方时差异.      

Demonstration of the use of the Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS) measurements to validate GPS ionospheric imaging

There is a lack of independent ionospheric data that can be used to validate GPS imaging results at mid latitudes over severe storm times. Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS), a global network of dual-frequency ground to satellite observations, provides this missing data and here is employed as verification to show the accuracy of the ionospheric GPS images in terms of the total electron content (TEC). In this paper, the large-scale ionospheric structures that appeared during the strong geomagnetic storm of 20 November 2003 are reconstructed with a GPS tomographic algorithm, known as MIDAS, and validated with DORIS TEC measurements. The main trough shown in an extreme equatorward position in the ionospheric imaging over mainland Europe is confirmed by DORIS satellite measurements. Throughout the disturbed day, the variations of relative slant TECs between DORIS data and MIDAS results agree quite well, with the average of the mean differences about 2 TECu. We conclude that as a valuable supplement to GPS data, DORIS ionospheric measurements can be used to analyse TEC variations with a relatively high resolution, ∼10 s in time and tens of kilometres in space. This will be very helpful for identification of some highly dynamic structures in the ionosphere found at mid-latitudes, such as the main trough, TID (Travelling Ionospheric Disturbances) and SED (Storm Enhanced Density), and could be used as a valuable auxiliary data source in ionospheric imaging.     

Total electron content measurements in ionospheric physics

With the advent of modern global networks of dual-frequency Global Positioning System (GPS), total electron content (TEC) measurements along slant paths connecting GPS receivers and satellites at 22,000 km have become the largest data set available to ionospheric scientists. The TEC can be calculated from the time and phase delay in the GPS signal using the GPS Toolkit, but an unknown bias will remain. In addition, UHF/VHF radio beacons on board low-Earth-orbiting satellites can also be used to measure the electron content. However, the TEC measurements are obtained by integrating TEC differences between slant paths, but also contain biases. It is often necessary to use data assimilative algorithms like the Ionospheric Data Assimilation Three-Dimensional (IDA3D), and to treat both GPS- and LEO-beacon TEC measurements as relative data in order to conduct ionospheric studies.     

电离层不规则结构对GPS性能的影响

电离层不规则结构的存在可引起无线电信号的幅度和相位发生随机起伏, 这 种电离层闪烁现象会影响全球定位系统(Global Positioning System, GPS)的 性能, 降低定位精度, 严重时导致信号失锁. 电离层不规则结构对GPS性能的 影响涉及电离层物理、接收机设计和表征卫星几何分布的精度衰减因子(Dilution of Positioning, DOP)等多方面因素. 本文通过对表征电离层不规则结构参数 的分析, 根据GPS接收机跟踪环路和闪烁信号模型, 综合研究了电离层闪烁对 GPS接收机载波跟踪环和码跟踪环跟踪误差的影响; 结合实际观测, 评述 了电离层不规则结构对单频和双频GPS接收机定位性能的影响, 在此基础上 提出了有待深入研究的问题及具体建议.      

改进的WAAS电离层时延网格修正算法

在考虑电离层倾斜的基础上给出了一套改进的广域增强系统电离层时延网格算法，并通过模拟数据和GPS实测数据与原方法进行了分析比较。结果表明，同样条件下不考虑倾斜时产生的方差为考虑倾斜度时的2倍。     

地磁暴期间北半球高纬度地区电离层变化特征及对精密定位的影响

基于加拿大地区高纬度电离层观测网的电离层闪烁观测数据,分析了2018年8月26日地磁暴事件引发的北半球高纬度地区电离层总电子含量（TEC）异常变化、TEC变化率指数（ROTI）及电离层相位闪烁的变化特征.结果表明:加拿大地区最大异常值约6 TECU,磁暴引发全球电离层TEC异常峰值高达20 TECU;加拿大地区电离层相位闪烁发生率最大增至12.6%,而磁静日期间约为1%;强电离层闪烁期间,电离层相位闪烁指数与ROTI之间具有较强的一致性.对GPS双频精密单点定位（Precise Point Positioning,PPP）结果进行分析发现:无闪烁期间定位误差随测站纬度的增高呈现出增大趋势,但均方根误差小于0.4m;闪烁发生期间各测站的定位误差均显著增大,水平和垂直方向定位误差分别增至约0.9m及1.7m.      

Solar and magnetic control on night-time enhancement in TEC near the crest of the Equatorial Ionization Anomaly

The ionospheric Total Electron Content (TECs), derived by dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Bhopal (23.2°N, 77.4°E, Geomagnetic 14.2°N) were analyzed for the period of January, 2005 to February, 2008. The work deals with monthly, diurnal, solar and magnetic activity variations on night-time enhancement in TEC. From a total of 157 night-time enhancements, 75 occur during pre-midnight and 82 post-midnight hours. The occurrence of night-time enhancement in TEC is utmost during summer months, followed by equinox and winter months. The occurrence of night-time enhancement in TEC decreases with increase in solar and magnetic activities. We observed that peak size and half amplitude duration are positively correlated, while time of occurrence of night-time enhancement in TEC and time of peak enhancement are negatively correlated with solar activity. The peak size, half amplitude duration, time of peak enhancement and time of occurrence of night-time enhancement in TEC shows negative correlation with magnetic activity. The results have been compared with the earlier ones and discussed in terms of possible source mechanism responsible for the enhancement at anomaly crest region.     

GNSS电离层模型改正精度评估与分析

电离层时延误差是导航定位信号在空间传播路径上的主要误差源之一,因此全面了解GNSS电离层模型的改正精度具有一定现实意义.根据GPS,BDS和Galileo系统所采用的电离层修正模型,利用2014年电离层校正参数,以高精度全球电离层图为基准,评估分析了三大系统电离层时延的改正精度.结果表明:目前GNSS使用的几种电离层修正模型的改正率在65~75%左右;Galileo系统使用的第二版NeQuick模型与第一版NeQuick模型相比在修正精度上并无显著提高;GPS使用的Klobuchar 8参数模型在北半球25°-45°N的中纬度地区精度很高,但是在全球其他区域精度较低,分布性较差,而NeQuick模型全球改正率分布则较为平均且平滑.      

Trans-ionospheric GPS signal delay gradients observed over mid-latitude Europe during the geomagnetic storms of October–November 2003

Ionospheric disturbances are known to have adverse effects on the satellite-based communication and navigation. One particular type of ionospheric effects, observed during major geomagnetic storms and threatening the integrity performance of both ground-based and space-based GNSS augmentation systems, is the sharp increase/decrease in the ionospheric delay that propagates in horizontal direction, thus called for convenience ‘moving ionospheric wall’. This paper presents preliminary results from researching such anomalous ionospheric delay gradients at European middle latitudes during the storm events of 29 October 2003 and 20 November 2003. For the purpose, 30-s GPS data from the Belgian permanent network was used for calculating and analysing the slant ionospheric delay and total electron content values. It has been found that, during these two particular storm events, substantial gradients did occur in Europe although they were not so pronounced as in the American sector.     

Kalman滤波估算电离层延迟的一种优化方法

频间偏差（Inter Frequency Bias,IFB）通常会给电离层延迟的解算带来误差.目前从电离层延迟中消除频间偏差的方法是基于GPS双频观测数据建立垂直电离层模型,利用卡尔曼滤波实时估算电离层模型系数和频间偏差.然而滤波过程中的测量噪声协方差矩阵没有考虑系统观测量之间的相关性,这会导致滤波模型不准确,进而影响最后求解的电离层延迟的准确性.本文选取了美国19个参考站的GPS双频观测数据,利用卡尔曼滤波实时估算电离层模型系数以及频间偏差.在滤波过程中,通过将先验频间偏差的估计方差引入测量噪声方差,实现对测量噪声协方差矩阵的优化.计算结果表明,优化后得到的卫星频间偏差与欧洲定轨中心（Center for Orbit Determination in Europe,CODE）得到的频间偏差更接近.将优化后的电离层延迟代入伪距解算,得到的位置误差的标准差在东向和天顶向分别下降了12.5%和15.4%,天顶向误差平均值下降了17.6%,定位精度得到提高.      

SHAKING: Adjusted spherical harmonics adding KrigING method for near real-time ionospheric modeling with multi-GNSS observations

Precise positioning based on Global Navigation Satellite System (GNSS) technique requires high accuracy ionospheric total electron content (TEC) correction models to account for the ionospheric path delay errors. We present an adjusted Spherical Harmonics Adding KrigING method (SHAKING) approach for regional ionospheric vertical TEC (VTEC) modeling in real time. In the proposed SHAKING method, the VTEC information over the sparse observation data area is extrapolated by the Adjusted Spherical Harmonic (ASH) function, and the boundary distortion in regional VTEC modeling is corrected by the stochastic VTEC estimated using Kriging interpolation. Using real-time GPS, GLONASS and BDS-2/3 data streams of the Crust Movement Observation Network of China (CMONOC), the SHAKING-based regional ionospheric VTEC maps are re-constructed over China and its boundary regions. Compared to GNSS VTECs derived from the independent stations, the quality of SHAKING solution improves by 13–31% and 6–33% with respect to the ASH-only solution during high and low geomagnetic periods, respectively. Compared to the inverse distance weighting (IDW) generated result, significant quality improved of SHAKING-based VTEC maps is also observed, especially over the edge areas with an improvement of 60–80%. Overall, the proposed SHAKING method exhibits notable advantage over the existing regional VTEC modeling techniques, which can be used for regional TEC modeling and associated high-precision positioning applications.