## 4 SF-PPP Performance Analysis of Android Mobile Phones
### GNSS Data Quality Analysis of Mobile Devices  
The C/N0 values obtained by the receiver indicate the signal gains and losses along the transmitting chain [40]. In general, the higher the C/N0 value, the stronger signal and the better observation quality that can be expected. Figure 4 shows the average C/N0 values of all satellites tracked by the P40 with external antenna (P40-EXT) and internal antenna (P40-INT) at different elevation bins to demonstrate the characteristics of C/N0 values for P40. For P40-EXT, the C/N0 values showed stable increments from about 30 dB-Hz to 45 dB-Hz with respect to satellite elevations; in addition, when satellite elevation was higher than 30 degrees, the P40-EXT C/N0 values were quite stable at around 45 dB-Hz. This was identical to the elevation-dependent stochastic model in Equation (2), where the observation weight was set to an equal weight model for the elevations higher than 30 degrees. In comparison, the P40-INT C/N0 values were overall smaller than P40-EXT and their difference was close to 10 dB-Hz. Especially, the P40-INT C/N0 showed large variations between 35 dB-Hz and 40 dB-Hz for elevations higher than 30 degrees and no significant correlations could be observed. This should be because the received signal strength of P40-INT was more susceptible to the environment with a linearly polarized non-uniform gain antenna [1,41]. Figure 4: Mean C/N0 values of all satellites tracked by different smartphone antenna devices at different elevation bins. The elevation interval is 2 degrees. The noise level of the pseudorange and carrier phase of P40 GNSS observations were evaluated by third-order between-epoch differences [3]. In Figure 5, the noise of P40-INT and P40-EXT pseudorange and carrier-phase are depicted for different GNSS systems over 4 h; the C/N0 are also illustrated to investigate their correlations. It should be noted that the selected periods are the same for different satellites, and the BII and B1C signals for C36 are both illustrated as an insight on BDS-3 new signal evaluation with smartphones. As seen in Figure 5, the P40-INT data experienced large C/N0 variations from 25 dB-Hz to 40 dB-Hz. The G01 satellite from P40-INT showed the largest C/N0 variations, followed by E21. They exhibited a large C/N0 decrease over 60-80 min from approximately 40 dB-Hz to about 30 dB-Hz with a reduction as large as 12 and 10 dB-Hz, respectively. This should be attributed to signal acquisition incidents of the P40-INT devices, as such a decrease was not seen for P40-EXT. The E21 C/N0 also showed another minor decrease of 5 dB-Hz at around 40 min. However, the two BDS signals showed much smaller C/N0 fluctuations. Their C/N0 stayed at a rather lower C/N0 level (35 dB-Hz) during the first 60 min and then gradually grew to larger than 40 dB-Hz. Comparatively, the B11 C/N0 values were, overall, 2 dB-Hz larger than B1C. The P40-INT code triple-difference series showed strong correlations with respect to the C/N0 variations. The E21 and C36 code variations were within \(+/-3\) m when their C/N0 stayed above 40 dB-Hz, while G01 were larger, at around \(+/-5\) m. However, when C/N0 dropped below 30 dB-Hz, their variations amplified to about 10 m. The carrier-phase triple-differences mainly fluctuated around \(+/-0.5\) cycles and barely showed any correlations with respect to C/N0. The P40-EXT C/N0 values manifested gradually variations, which were typically above 40 dB-Hz for elevations higher than 30 degrees. This was consistent with Figure 4. As result, the P40-EXT code triple-differences stayed stably within \(+/-3\) m, while the carrier-phase was around \(+/-0.5\) cycles. To further analyze the relationship between the P40 measurement noise and elevation or the C/N0 value, Figure 6 shows the RMS values of triple-differences of pseudorange and carrier phase with different ranges of elevation and the C/N0 value for P40-EXT and P40-INT.