## 4 Conclusions and Discussions  
In this study, we use more than four weeks of data from 20 MGEX, BETS, and GA stations to assess the different contribution of BDS GEO, IGSO, MEO, and GLONASS observations to the convergence time and positioning accuracy in different cases. For BDS only static PPP, the PPP performances demonstrate that the IGSO satellites make the greatest contribution to the reduction of the convergence time and improvement of positioning accuracy of static PPP among the three kinds of BDS satellites for the sites with better IGSO tracking condition, whereas MEO observations are most essential for the sites with poor geometry. Various numerical results show that the PPP by the combination of GPS and other GNSS observations can significantly reduce the convergence time for both ambiguity-float and -fixed PPP. However, the contribution of BDS to the reduction of convergence time is still not as large as that of GLONASS, this could be partly due to relatively lower quality of BDS orbit and clock products for the time being. Regarding the positioning accuracy of GPS based combined PPP solutions, GLONASS satellites could improve the accuracy in the vertical more than BDS ones, while in the horizontal component, the same accuracy improvement can be achieved by inclusion of either BDS IGSO/MEO or GLONASS satellites. As to the contribution of different BDS satellites in general, the positioning performance is improved after adding any single type of BDS satellites to combine with the GPS ones. The IGSO satellites make the largest contribution to GPS based combined static and kinematic ambiguity-float \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**TYPE**} & \multicolumn{3}{c}{**STD (cm)**} & \multicolumn{3}{c}{**RMS (cm)**} \\ \cline{2-7}  & **E** & **N** & **U** & **E** & **N** & **U** \\ \hline GPS & 1.57 & 0.98 & 2.40 & 1.64 & 1.02 & 2.54 \\ GPS + B & 1.02 & 0.71 & 2.15 & 1.22 & 0.84 & 2.43 \\ GPS + R & 0.85 & 0.65 & 1.81 & 1.08 & 0.78 & 2.07 \\ GPS + RIM & 0.69 & 0.57 & 1.65 & 0.98 & 0.75 & 2.03 \\ GPS + G & 1.46 & 0.89 & 2.44 & 1.56 & 0.95 & 2.61 \\ GPS + I & 0.89 & 0.70 & 1.91 & 1.10 & 0.82 & 2.24 \\ GPS + M & 1.42 & 0.92 & 2.29 & 1.53 & 0.97 & 2.46 \\ GPS + GI & 1.06 & 0.74 & 2.20 & 1.26 & 0.86 & 2.49 \\ GPS + GM & 1.40 & 0.86 & 2.40 & 1.52 & 0.93 & 2.58 \\ GPS + IM & 0.84 & 0.67 & 1.86 & 1.07 & 0.80 & 2.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Position error statistics of convergent kinematic ambiguity-fixed PPP solutions for all 20 sites. Figure 7: Distribution of convergence time in each 10 min and the averaged convergence time of the kinematic GPS ambiguity-fixed PPP solutions for all sites. or -fixed PPP. However, the statistical results demonstrate the inclusion of BeiDou GEO satellites could slightly degrade the position accuracy and slow the convergence only in the case that IGSO satellites are also used in combined solution. The negative contribution of GEO satellites in this case could be possibly caused by the multipath or potential satellite deduced systematic errors that are not optimally handled when combined with other observations [15, 16, 17]. In addition, the relative lower quality of orbits and clocks of GEO satellites may have some impacts that are difficult to be described by the stochastic model. In order to improve the situation, further investigation on refinement of stochastic model and better orbit/clock determination are needed. For the time being, the BDS GEO observations should be cautiously used in the Multi-GNSS constellation combined PPP solutions. However, we should realize that the BDS GEO still make positive contributions in some extreme observation situation (like site TUVA), which often happens in dynamic environment. It is noticed that one of the latest results demonstrated in [18] shows that the GEO BDS satellites slightly reduced convergence time and horizontal positioning errors. The results were achieved by simulated obstruction of signal tracking.