## 4 Results and Discussion  
With the aim of providing an assessment on the accuracy of different PPP solutions, the reference trajectory (or known coordinates) should be obtained. In order to achieve this purpose, a nearby single reference station with precisely known ITRF coordinates was used and the coordinates of each measurement epoch were obtained with the PPK (Post-Processed Kinematic) differential method using GrafNav Software by applying \"Both\" processing strategy. In this approach, forward and reverse solutions were processed independently, and then automatically combined. This combination maximizes the solution accuracy and assists in quality control [33]. At the end of these stages, the coordinates of each measurement epoch were calculated within the precision of 15 mm and 25 mm for 2D position and height coordinates on average, respectively. The differential solution was used as reference coordinates for later comparison of the PPP solutions. The PPP-derived coordinates obtained from all mentioned processing scenarios were compared to the known coordinates epoch-by-epoch. The corresponding coordinate differences  
Through the histograms, mean values and outliers can easily be identified in the datasets. When Figures 8 and 9 were interpreted, it was noticed that there were many outliers for the differences. If these outliers and mean values were excluded from the dataset, the differences given in Figure 8 decreased to the level of \(\pm\)10 cm for both easting and northing components, respectively. When the differences for height coordinates were considered, it was seen that the differences decreased to the level of \(\pm\)20 cm approximately (in 95% confidence interval). When looked at Figure 9 for the processing scenarios from 6 to 9, the distributions (except G+R+E+C solution using Wuhan University's precise products) are between intervals approximately \(-\)5 cm and +5 cm for both easting and northing components and \(-\)10 cm and +10 cm for height component. In order to make an accuracy estimate of the kinematic PPP solutions, mean values, and Root Mean Square Error (RMSE) of differences for the 2D position and height components were calculated and given in the following Tables 4-6 for all solutions. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Scenarios**} & \multicolumn{2}{c}{**2D Position (cm)**} & \multicolumn{2}{c}{**Height (cm)**} \\ \cline{2-5}  & _Mean_ & _RMSE_ & _Mean_ & _RMSE_ \\ \hline
**Scenario 1 (G)** & & & & \\ _using CODE’s COM ID precise products_ & 6 & \(\pm\)6 & 9 & \(\pm\)12 \\
**Scenario 2 (G+R)** & & & & \\ _using CODE’s COM ID precise products_ & 6 & \(\pm\)7 & 11 & \(\pm\)14 \\
**Scenario 3 (G+R+C)** & & & & \\ _using CODE’s COM ID precise products_ & 4 & \(\pm\)5 & 1 & \(\pm\)7 \\
**Scenario 4 (G+R+E)** & & & & \\ _using CODE’s COM ID precise products_ & 5 & \(\pm\)6 & 12 & \(\pm\)14 \\
**Scenario 5 (G+R+E+C)** & 4 & \(\pm\)5 & 2 & \(\pm\)8 \\ _using CODE’s COM ID precise products_ & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistical analysis of the differences in 2D position and height components for different satellite combinations. Figure 9: Histograms of the calculated differences for Easting (**a**), Northing (**b**) and Height (**c**) components, obtained from processing Scenarios 6–9. As given in Figure 6, Figure 8 and Table 4, the mean values reached 6 cm in the 2D position and 9 cm in height component for the GPS-only PPP processing results. The RMSE for the GPS-only are \(\pm\)6 cm and \(\pm\)12 cm for 2D position and height components, respectively (Scenario 1). The combination of GPS and GLONASS (Scenario 2) also produced similar results with the mean values being 6 cm in the 2D position and 11 cm in height component. When the results were examined by means of RMSE, the values obtained were \(\pm\)7 cm for 2D position and \(\pm\)14 cm for height components. These results imply that the solutions produce almost the same level of accuracy for both GPS-only and GPS and GLONASS constellation solutions.