## 5 Conclusions and Discussion  
In this article, we have compared and analyzed the quality of the GNSS raw observations of different smartphones, and adopted the regional ionospheric correction model to improve the positioning accuracy of the smartphone. Taking the observations of the geodetic receiver as the standard value, analyzing the GNSS raw observations of OPPO Reno 10X Zoom, Huawei P10, Xiaomi 8 A and B smartphones, we found that the ability of smartphones to acquire GPS and GLONASS satellite signals is similar to that of geodetic receivers. For the BDS, the number of satellites that can be observed by the geodetic receiver is about 15, and the performance of each smartphone is different. Among them, the Xiaomi 8 B smartphone performs best, and the number of BDS that can be observed is stable at about 10. There is a positive correlation between the C/N0 and the continuity of the observations, but a very obvious signal loss-of-lock phenomenon occurs in each smartphone during the observation experiment. Through further analysis of the pseudorange and carrier, it can be concluded that the change rate of the combined pseudorange and carrier observations of each satellite is consistent with the results of the geodetic receiver in terms of numerical value and change law while, in stability, there is still a certain gap compared with geodetic receivers. The GIM file is used as the standard to interpolate the electron content of the pierce point and the ionospheric delay correction, comparing the results of the Klobuchar model and the regional ionospheric correction model with this standard. During the experiment, the electron content calculated by the regional ionospheric model changes slowly with time, and the accuracy of each system is less than 5 TECU. It can more accurately reflect the changes of the ionosphere than Klobuchar. In addition, the accuracy of the ionospheric delay correction calculated by the regional ionospheric model is up to 1 m. To further study the influence of the regional ionospheric model on positioning, this article performs carrier-smoothing pseudorange and single-frequency PPP positioning. Compared with the Klobuchar, the positioning accuracy of the regional model is improved, especially in the elevation direction. The accuracy of the regional ionospheric model is similar to the post-GIM product, which means that the regional model has a significant effect on ionospheric correction. Meanwhile, the use of the regional ionospheric model can shorten the convergence time of the elevation direction to less than 1 min, which can greatly improve the positioning performance of smartphones. Previous studies have found that the data quality of GPS and Galileo L5/E5 frequency is better than L1/E1 [22; 23; 24], and the use of L1/E1 and L5/E5 dual-frequency data can effectively weaken ionospheric delay. However, due to the fact that the number of dual-frequency data obtained by the smartphone is small, the utilization rate of the dual-frequency data is low. How to effectively use the dual-frequency data is the focus of the next research. During the experiment, we found that the carrier of the smartphone fluctuates greatly, and the lack of carrier often occurs, resulting in low carrier data utilization. After analysis, it can be seen that the data characteristics of Doppler observations are similar to those of carrier observations. Whether the Doppler data can replace the carrier to complete positioning appropriately when the smartphone cannot output the carrier, or whether the carrier has problems, are also the focus of future research. In addition, the antenna also affects the improvement of positioning performance [25] and we can try to establish a suitable model to correct it in the next research. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Klobuchar (s)** & **Regional (s)** \\ \hline \(1\)st & 94 & 59 \\ \(2\)nd & 99 & 42 \\ \(3\)rd & 83 & 47 \\ \(4\)th & 88 & 52 \\ \(5\)th & 91 & 45 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Convergence time in elevation direction of PPP.