## 5 Conclusions  
With the rapid development of GNSSs, especially for BDS (BDS-3/BDS-2) and Galileo, the multisystem combination has been an emerging trend in recent years. The SPP technology, which is the open position service of GNSSs and is widely used in satellite navigation and low-accuracy positioning, can also benefit from the multi-GNSS integration. The compatibility among different GNSSs is a key issue in the multisystem integrated data processing. To solve this issue, the additional ISB parameters should be introduced into the observation model. In this study, we carefully characterize the ISB estimates for GLONASS, Galileo, BDS-2, and BDS-3 with respect to GPS in the four-system integrated SPP, and the performance enhancement of predicted ISBs as a priori constraints for multisystem SPP position solutions with a prediction period from a day to a week under a cutoff elevation angle from 10\({}^{\circ}\) to 50\({}^{\circ}\) (to simply simulate the harsh observation environments by increasing the elevation masks) is rigorously evaluated. The data sets from 120 globally distributed MGEX stations (covering seven different receiver types from three manufacturers) spanning a month from DOY 41 to 70 of 2020 are employed. The results indicate that the epochwise ISB estimates are relatively stable with a varying range of approximately 10 ns, and those of BDS-3 and BDS-2 show obvious differences by several nanoseconds. The daily ISBs do not present obvious fluctuations when the receiver firmware versions are changed, but the step changes can be observed \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Devices**} & \multirow{2}{*}{**Strategy**} & \multirow{2}{*}{**Items**} & \multicolumn{5}{c}{**Elevation Mask Angles**} \\ \cline{4-7}  & & & \(\mathbf{10^{\circ}}\) & \(\mathbf{20^{\circ}}\) & \(\mathbf{30^{\circ}}\) & \(\mathbf{40^{\circ}}\) & \(\mathbf{50^{\circ}}\) \\ \hline \multirow{6}{*}{Xiaomi Mi8 (1)} & \multirow{2}{*}{Without A Priori ISB} & Availability (\%) & 100.0 & 100.0 & 100.0 & 95.7 & 62.7 \\  & & & Num. of Sats. & 20.0 & 17.8 & 14.8 & 11.8 & 8.7 \\  & & & PDOP & 1.3 & 1.7 & 2.5 & 3.9 & 8.4 \\ \cline{2-7}  & \multirow{2}{*}{With A Priori ISB (1 day)} & Availability (\%) & 100.0 & 100.0 & 100.0 & 100.0 & 82.9 \\  & & & 20.0 & 17.8 & 14.8 & 11.5 & 8.2 \\  & & & PDOP & 1.3 & 1.7 & 2.5 & 4.0 & 9.0 \\ \hline \multirow{6}{*}{Xiaomi Mi8 (2)} & \multirow{2}{*}{Without A Priori ISB (1 day)} & Availability (\%) & 100.0 & 100.0 & 100.0 & 98.3 & 69.9 \\  & & & 22.4 & 20.1 & 16.6 & 13.0 & 9.5 \\ \cline{1-1}  & & & 1.5 & 2.3 & 3.7 & 7.2 \\ \cline{1-1} \cline{2-7}  & \multirow{2}{*}{With A Priori ISB (1 day)} & Availability (\%) & 100.0 & 100.0 & 100.0 & 100.0 & 93.0 \\ \cline{1-1}  & & & 22.4 & 20.1 & 16.6 & 12.9 & 8.8 \\ \cline{1-1}  & & & 1.5 & 2.3 & 3.7 & 7.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Availability, satellite number, and PDOP value of four-system integrated SPP for two smartphones with and without a priori ISB constraints under different elevation masks. for the daily ISBs when encountering the replacement of receiver types. The short-term stability of epochwise ISBs for GLONASS, Galileo, BDS-2, and BDS-3 with respect to GPS can be 2.335, 1.262, 1.741, and 1.532 ns, respectively, whereas the corresponding long-term stability for daily ISBs can be 1.258, 1.288, 2.713, and 2.566 ns, respectively. The effects of receiver types on the short-term and long-term stability of ISBs are not significant. The daily ISBs are roughly consistent for the stations equipped with the same type of receivers. The numerical values range from 20 to 45 ns for the daily ISBs between GLONASS and GPS, from 30 to 70 ns for the daily ISBs between BDS-2 and GPS, and from 25 to 65 ns for the daily ISBs between BDS-3 and GPS, respectively, while the numerical values of daily ISBs between Galileo and GPS range from \(-\)13 to \(-\)7 ns for the stations with the TRIMBLE ALLOY receiver and from \(-\)5 to 3 ns for the stations with the other six types of receivers, respectively.