# Estimating and Analyzing Long-Term Multi-GNSS Inter-System Bias Based on Uncombined PPP
## 3 Experimental Data and Processing Strategies  
To verify the model and analyse the ISB parameter characteristics, the data of eight MGEX stations for 28 days were selected (150 th to 177 th day in 2018). The criteria of the selection of the station is based on the following: (1) the station should be able to observe four-system satellites simultaneously. That is, the number of satellites observed by each system must be greater than 1 at the same time; (2) the reference coordinates of the station can be found in the file (gbm.clk). The Beidou second-generation satellites show poor visibility in the worldwide at present (Figure 1); thus, to satisfy the first requirement, most stations selected in this experiment were distributed in the Asia-Pacific region (Figure 2) to ensure that the tracking station could observe a sufficient number of BDS satellites. The observation data required pre-processing prior to the experiment. The precise reference coordinates of each station in the file (gbm.clk) were extracted to replace the prior coordinates in the observation file, and the coordinate bias in three directions (i.e., east, north and up) were obtained by filtering, which can be considered as positioning accuracy [28].  
Figure 1: Number of Beidou system coverage satellites worldwide on day 150 in 2018.  
The receiver types in this experiment were primarily divided into two types, i.e., Leica and Trimble, while the antenna types were divided into three groups, i.e., leica25.r3, trim59,800.00 and javringant_dm, as shown in Table 1. A Kalman filter was used to estimate parameters day by day and station by station in the PPP data processing. The specific error correction and parameter estimation schemes are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Station** & **Types of Receiver** & **Types of Receiver Antenna** \\ \hline alic & leica gr25 & leica25.R3 \\ karr & trimble netr9 & trim59,800.00 \\ kiri & trimble netr9 & trim59,800.00 \\ kzn2 & trimble netr9 & trim59,800.00 \\ laut & trimble netr9 & javringant_dm \\ lhaz & leica gr25 & leica25.R4 \\ pngm & trimble netr9 & trim59,800.00 \\ xmis & trimble netr9 & javringant_dm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Receiver and receiver antenna types of stations.  
\begin{table}
\begin{tabular}{c l} \hline \hline
**Project** & \multicolumn{2}{c}{**Processing Strategy**} \\ \hline Observed value & \begin{tabular}{c} BDS, GLO, GPA, GAL pseudorange and carrier phase raw \\ observations \\ \end{tabular} \\ Sampling interval & \begin{tabular}{c} 30s \\ BDS:B1,B2 \\ GLO:L1,L2 \\ GPS:L1,L2 \\ GAL:E1,E5a \\ Kalman filtering \\ \end{tabular} \\ Parameter estimation & \
\begin{tabular}{c} Kalman filtering \\ 8° \\ Weight distribution of \\ observed values \\ \end{tabular} &
\begin{tabular}{c} Height angle model \\ (ocean tide, earth tide, polar tide) model correction \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Precise point positioning (PPP) parameter estimation and related error term processing strategies.  
Figure 2: Station distribution.  
Taking the XMIS station as an example, Figure 5 shows the number of available satellites of the single GPS and G + C + R + E systems on the same day. As can be seen, the number of satellites of the four systems on the same day reached 17-32, and the average number of available satellites was 26.1. However, the number of satellites in the single GPS system was 6-12, and the average number of usable satellites was 9, which is approximately one-third the number of usable satellites in the four systems. Figure 6 shows that the GDOP, PDOP, HDOP and VDOP values of the four systems, which are much less than that of the single GPS system. The average value decreased from 2.22, 2.97, 1.85 and 0.64 to 1.2, 0.77, 0.97 and 0.38, respectively. The increased number of available satellites effectively enhances the reliability and accuracy of PPP positioning.  
Figure 4: Satellite coverage of G + R + E + C system.  
Figure 5: Available number of satellites for single GPS and G + C + R + E systems.