## 3 Experiment and Results
### Evaluation Results of SBAS Ionospheric Corrections  
To evaluate the accuracy of the SBAS ionospheric corrections, 27 IGS stations are selected and the geographic distribution of the selected IGS stations is shown in Figure 13. For each SBAS system, 6 IGS stations located in or around service area are selected, except for GAGAN, for which only 3 IGS stations are found. The GNSS raw data with intervals of 30 seconds are downloaded, starting from 1 January to 30 June 2018, from GNSS data and products FTP server of the National Aeronautics and Space Administration (NASA) Crustal Dynamics Data Information System (CDDIS). Only GPS \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Type** & **WAAS** & **EGNOS** & **MSAS** & **GAGAN** & **SDCM** & **GPS Broadcast** \\ \hline PRN & 135 & 138 & 120 & 123 & 129 & 137 & 127 & 128 & 125 & 140 & \\ \hline SISRE & 0.954 & 0.954 & 0.646 & 0.645 & 1.931 & 1.931 & 1.336 & 1.313 & 0.634 & 0.666 & 0.695 \\ \hline \end{tabular}
\end{table} 
Table 2: The signal-in-space range error (SISRE) values of SBAS-derived and GPS broadcast orbits& clocks (unit: meters). Figure 12: The distribution of orbit and clock errors for Global Positioning System (GPS) broadcast navigation data. measurements are used for deriving the ionospheric delays by using code-leveled phase ionosphere measurements as mentioned in Equation (23). The types of GPS code measurements used in each station are listed in Table 3. To calculate the SBAS ionospheric correction, the ionospheric messages from the SBAS satellite with the highest elevation are used at each station. These ionospheric corrections are calculated at the same time epochs with the downloaded GNSS raw data for each station. The SBAS ionospheric correction errors are calculated with ionospheric delays derived code-leveled phase ionosphere measurements as the references. In addition, ionospheric corrections are obtained with GPS broadcast ionospheric parameters by using the Klobuchar model ([PERSON], 1996), and GPS broadcast ionospheric correction errors are also calculated with respect to the same references. The evaluation period is near the solar minimum and thus the solar activities are relatively mild ([[https://www.spaceweatherlive.com/en/solar-activity/solar-cycle](https://www.spaceweatherlive.com/en/solar-activity/solar-cycle)]([https://www.spaceweatherlive.com/en/solar-activity/solar-cycle](https://www.spaceweatherlive.com/en/solar-activity/solar-cycle))). Two standard indices, the disturbance storm time (Dst) and Kp, are used to indicate the geomagnetic activity. Larger Dst index variation and larger Kp index, the intenser the geomagnetic storm. The daily average Dst and Kp indices during the evaluated period are shown in Figure 14. The Dst indices are all within \(\pm\)40 nT and the Kp indices are smaller than 5, which indicate the geomagnetic activities are relatively quiet ([PERSON], 1996; [PERSON], 1996). In general, it is under relatively mild ionospheric condition during the evaluation period. However, the service areas of WAAS, EGNOS and SDCM are mainly in mid- and high-latitude regions, while the service areas of GAGAN and MSAS are in low- or mid-latitude regions. Considering that the ionospheric condition varies in different latitude regions, it may affect the accuracy of ionospheric corrections from different SBAS systems. Figure 15 presents the biases and the STDs of SBAS ionospheric correction errors at each selected IGS station. The average biases are \(-\)0.226, \(-\)0.106, \(-\)0.531, \(-\)0.676 and 0.169 m over the stations in the WAAS, EGNOS, MSAS, GAGAN and SDCM service area, and the average STDs are 0.438, 0.456, 0.581, 0.521 and 0.490 m, respectively. It is found that the STD values are at similar level, but the bias  
Figure 13: The distribution of the selected IGS stations.