School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan 430079, China.
Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, Wuhan University, 129 Luoyu Road, Wuhan 430079, China.
Sensors (Basel). 2019 Mar 6;19(5):1138. doi: 10.3390/s19051138.
The prevalence of real-time, low-cost, single-frequency, decimeter-level positioning has increased with the development of global navigation satellite systems (GNSSs). Ionospheric delay accounts for most errors in real-time single-frequency GNSS positioning. To eliminate ionospheric interference in real-time single-frequency precise point positioning (RT-SF-PPP), global ionospheric vertical total electron content (VTEC) product is designed in the next stage of the International GNSS Service (IGS) real-time service (RTS). In this study, real-time generation of a global ionospheric map (GIM) based on IGS RTS is proposed and assessed. There are three crucial steps in the process of generating a real-time global ionospheric map (RTGIM): estimating station differential code bias (DCB) using the precise point positioning (PPP) method, deriving slant total electron content (STEC) from PPP with raw observations, and modeling global vertical total electron content (VTEC). Experiments were carried out to validate the algorithm's effectiveness. First, one month's data from 16 globally distributed IGS stations were used to validate the performance of DCB estimation with the PPP method. Second, 30 IGS stations were used to verify the accuracy of static PPP with raw observations. Third, the modeling of residuals was assessed in high and quiet ionospheric activity periods. Afterwards, the quality of RTGIM products was assessed from two aspects: (1) comparison with the Center for Orbit Determination in Europe (CODE) global ionospheric map (GIM) products and (2) determination of the performance of RT-SF-PPP with the RTGIM. Experimental results show that DCB estimation using the PPP method can realize an average accuracy of 0.2 ns; static PPP with raw observations can achieve an accuracy of 0.7, 1.2, and 2.1 cm in the north, east, and up components, respectively. The average standard deviations (STDs) of the model residuals are 2.07 and 2.17 TEC units (TECU) for moderate and high ionospheric activity periods. Moreover, the average root-mean-square (RMS) error of RTGIM products is 2.4 TECU for the one-month moderate ionospheric period. Nevertheless, for the high ionospheric period, the RMS is greater than the RMS in the moderate period. A sub-meter-level horizontal accuracy and meter-level vertical accuracy can be achieved when the RTGIM is employed in RT-SF-PPP.
随着全球导航卫星系统 (GNSS) 的发展，实时、低成本、单频、分米级定位的普及度越来越高。在实时单频 GNSS 定位中，电离层延迟占实时单频精密单点定位 (RT-SF-PPP) 中大多数误差的主要部分。为了消除实时单频精确点定位 (RT-SF-PPP) 中的电离层干扰，国际 GNSS 服务 (IGS) 实时服务 (RTS) 下一阶段设计了全球电离层垂直总电子含量 (VTEC) 产品。本研究提出并评估了基于 IGS RTS 的实时全球电离层图 (GIM) 的生成。生成实时全球电离层图 (RTGIM) 有三个关键步骤：使用精确点定位 (PPP) 方法估算站差分码偏差 (DCB)、从 PPP 原始观测值中推导出斜向总电子含量 (STEC) 以及建模全球垂直总电子含量 (VTEC)。进行了实验来验证算法的有效性。首先，使用来自 16 个全球分布的 IGS 站的一个月数据验证 PPP 方法的 DCB 估计性能。其次，使用 30 个 IGS 站验证了原始观测值静态 PPP 的精度。第三，在高和安静的电离层活动期间评估残差的建模。之后，从两个方面评估 RTGIM 产品的质量：（1）与欧洲轨道确定中心 (CODE) 全球电离层图 (GIM) 产品进行比较，（2）使用 RTGIM 确定 RT-SF-PPP 的性能。实验结果表明，使用 PPP 方法的 DCB 估计可以实现平均精度 0.2 ns；原始观测值的静态 PPP 可以分别在北、东和向上分量达到 0.7、1.2 和 2.1 cm 的精度。中等和高电离层活动期间模型残差的平均标准偏差 (STD) 分别为 2.07 和 2.17 TEC 单位 (TECU)。此外，一个月中等电离层期间 RTGIM 产品的平均均方根 (RMS) 误差为 2.4 TECU。然而，在高电离层期间，RMS 大于中等期间的 RMS。在 RT-SF-PPP 中使用 RTGIM 可以实现亚米级水平精度和米级垂直精度。
