## 2 Experimental Setup and Raw Observations Analysis  
The equipment used in this experiment includes an OPPO Reno 10X Zoom smartphone, a Huawei P10 smartphone, two Xiaomi 8 smartphones, and a geodetic receiver (Hi-Target iRTK2). They all supported the acquisition of the GNSS raw observations from the four major satellite navigation systems: GPS, GLONASS, BDS, and GALILEO. To avoid the duty cycle affecting the quality of the experimental data, the four smartphones had the power-saving mode turned off. Among them, two Xiaomi 8 smartphones had different operating system versions; one system version was Android 8, which does not support mandatory tracking mode (which tracks all GNSS constellations and frequencies when the work cycle is disabled), and the other system version was For Android 9 (supported to enable mandatory tracking mode). The specific parameters of the experimental equipment are shown in Table 1.  
Two datasets were collected in the experiment, the sampling rate was set to 1 s, and the collection location was the north side of Li Wenzheng Library on the Jiulonghu Campus of Southeast University. The observation conditions of this site were good and it was in a low multipath environment. To ensure the accuracy and reliability of the experimental results, the network RTK technology was used to obtain the accurate coordinates of the control points before the experiment. The accuracy of the points can reach 2-3 cm in the horizontal and 5 cm in the elevation to meet the high-precision positioning requirements of smartphones. The first dataset was collected on 19 June 2020 and the observation time was 1 h. The equipment used was the above-mentioned four smartphones and a geodetic receiver (iRTK2), as shown in Figure 1a. This dataset is used for quality analysis of the GNSS raw observations. The second dataset was collected on 28 September 2020 and the observation time was 1 h. The device used was a Xiaomi 8 B smartphone, as shown in Figure 1b. This dataset was used to analyze the effect of the positioning experiment. We developed the SEU_GNSSLog for data collecting, pre-processing and sending. In addition, a Windows desktop program was developed to invert the regional ionospheric model, and an APP was developed for real-time positioning. To systematically analyze the quality of the GNSS raw observations of different smartphones, this article uses the experimental equipment mentioned in Table 1 to perform simultaneous observations in the same environment. Taking the observations of iRTK2 as a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Device** & **Version** & **Sampling Rate** & **Observation Time** & **Global System** \\ \hline OPPO Reno 10X Zoom & Android 9 & 1 Hz & 1 h & GPS/GLO/GAL/BDS/QZS \\ HUAWEI P10 & Android 9 & 1 Hz & 1 h & GPS/GLO/GAL/BDS \\ Xiaomi 8 A & Android 8 & 1 Hz & 1 h & GPS/GLO/GAL/BDS \\ Xiaomi 8 B & Android 9 & 1 Hz & 1 h & GPS/GLO/GAL/BDS \\ Hi-Target iRTK2 & & 1 Hz & 1 h & GPS/GLO/GAL/BDS/QZS \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Equipment related parameters and experimental settings. Figure 1: Schematic diagram of experimental. (**a**) Geodetic receivers and smartphones to obtain the GNSS raw observations (19 June 2020, 7 p.m.); (**b**) Positioning test on the smartphone (28 September 2020, 8 p.m.). standard to analyze the number of visible satellites, signal continuity, the C/N0 of satellite, and pseudorange and carrier observations. The Android system sets the satellite signals captured by the smartphone's GNSS receiver to three levels: visible, synchronized, and tracking. Under normal circumstances, only at the traceable level, the GNSS raw observations output by the GNSS chip are accurate and credible. Analyzing Figure 2a-d, it can be concluded that Xiaomi 8 B smartphone has a stronger ability to capture satellite signals than Xiaomi 8 A, this is because the Android 9 version is forced to enable full GNSS signal tracking so that the smartphone can track all GNSS constellations and frequencies when the work cycle is disabled.