## 4 SF-PPP Performance Analysis of Android Mobile Phones
### Positioning Performance Analysis of Android Mobile Phones
The C/N0-dependent weighting model was represented in an exponential form related to the instantaneous C/N0 of the observations, a maximum C/N0 value for cutoff, and the corresponding pseudorange noise at maximum C/N0. Its detailed expression could be found in [42]. Considering the P40-INT noise analysis results in Section 4.1, the C/N0-dependent weighting model was adapted by setting the maximum C/N0 cutoff as 40 dB-Hz and the corresponding pseudorange noise 1 m. The positioning errors are shown in Figure 9a,b. Generally, the positioning errors of P40-INT varied abruptly and needed a longer convergence time for both SF-PPP approaches and for both weight models than P40-EXT. Considering the elevation-dependent weighting model, both the GRAPHIC and uncombined approaches manifested large variations around \(\pm\)2 m in the horizontal component and \(\pm\)6 m in the vertical component even after convergence; very similar systematic errors could be observed for both approaches, which might be due to some systematic observation errors and remains under investigation. Comparatively, the uncombined approach showed smaller positioning errors, especially at the vertical component. With the C/N0-dependent weighting model, the P40-INT positioning results showed a significant improvement over the elevation-dependent model for both SF-PPP approaches with smaller variations. Especially, the vertical error variations were within \(\pm\)4 m. However, the systematic errors were still found. The histograms of P40-EXT and P40-INT SF-PPP positioning errors during the last 120 min are shown in Figure 10. In Figure 10a, it could be seen that the GRAPHIC positioning errors of the P40-EXT case achieved comparable performance with the uncombined method. Their horizontal fluctuations varied within \(\pm\)0.5 m and their RMS values varied around 0.15 m. However, the uncombined approach showed much smaller errors in the vertical component with an improvement of about 30%, consistent with Figure 8. Figure 10b,c depict the P40-INT SF-PPP errors using the elevation-dependent weighting model and the corresponding \(\pm\)6 m uncertainty. The corresponding error values for the P40-INT SF-PPP and the corresponding \(\pm\)6 m uncertainty are shown in Figure 10a. The corresponding error  
the C/N0-dependent weighting model, respectively. Generally, the uncombined method overperformed the GRAPHIC method in the vertical component with dramatic improvements; for the elevation-dependent weighting model, the RMS reduction was about 1.40 m, while for C/N0-dependent model it was about 0.24 m. Their horizontal precisions were at a similar level below 1.0 m. Compared to the elevation-dependent weighting model, the C/N0-dependent model showed much better vertical precision at 0.66 m.  
Figure 11 summarizes the SF-PPP positioning results in each coordinate component of P40-EXT (left panel bars) and P40-INT. For P40-INT, the results of both the elevation-dependent weighting method (middle panel bars) and the C/N0-dependent weighting (right panel bars) are shown. The statistical values are based on the positioning results after 120 min, when the series vary stably. In the case of P40-EXT, the GRAPHIC and uncombined models of the smartphone reached comparable accuracies of about 0.15 m in horizontal components, while the uncombined model achieved 0.33 m in the vertical component that was better than the GRAPHIC model (0.48 m), indicating an improvement of about 31%. In addition, biases at a few centimeters level in the horizontal components and several decimeters level at the vertical component could be observed for both approaches, which was different compared to the results of MGEX stations. This again indicated that there were systematic errors in the P40 measurements. For the P40-INT case, the positioning errors of the GRAPHIC model with the elevation-dependent weighting strategy were 0.74 m, 0.42 m, and 2.35 m in the east, north, and up components, respectively.