Abstract
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.
Highlights
The use of global navigation satellite systems (GNSSs) has widely increased in fields such as geodesy, navigation, precision agriculture, and hazard monitoring [1,2,3,4]
Experiments of differential code bias (DCB) estimation, static precise point positioning (PPP) with raw observations, and global ionospheric modeling are presented to verify the effectiveness of the real-time global ionospheric map (RTGIM) generation method
Observation data from 16 global distribution International GNSS Service (IGS) stations were collected from day of year (DOY) 121 (1 May) to DOY 151 (31 May), 2018
Summary
The use of global navigation satellite systems (GNSSs) has widely increased in fields such as geodesy, navigation, precision agriculture, and hazard monitoring [1,2,3,4]. Sensors 2019, 19, 1138 these applications primarily use dual-frequency positioning, which provides millimeter to centimeter accuracy. Because dual-frequency GNSS receivers are more costly than single-frequency devices, single-frequency applications are more feasible for ordinary users. To satisfy real-time, low-cost, single-frequency GNSS positioning, precise satellite orbit and clock corrections, as well as ionospheric delay corrections, are necessary. The International GNSS Service (IGS) real-time service (RTS), which provides precise orbit and clock corrections via the Internet, was officially launched on 1 April 2013 [5,6]. Official IGS RTS products are generated by combining
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