## 2 Data Collection and Experiment Design  
We evaluate the positioning performance and analyze the quality of observations collected by three recent smartphones, namely Xiaomi Mi 8, Xiaomi Mi 9, and Huawei P30 pro. All devices track dual-frequency multi-constellation (GPS, Galileo, BDS, and GLONASS) signals. We note that Xiaomi Mi 9 only collects code pseudoranges, whereas Huawei P30, Xiaomi Mi 8 allow an acquisition of phase and code measurements. Since phase observations are not employed in a standard solution with a single point positioning mode, we neither evaluate their quality nor use them in the positioning assessment.  
GNSS observations were collected on 11 December 2019 over a time span of 5 h (approximately 8-13 UTC) using Geo++ RINEX Logger ver. 2.1.6 application that runs on an Android system [23]. Since 3G and 4G signals may use the L-band and therefore can interfere with GNSS signals, SIM cards were not inserted in the devices during data collection.  
The smartphones were centered over the temporal sites using tribrachs and tripods as showed in Figure 1. The devices were separated by a distance of 20 cm; therefore, we may consider the observational conditions as the same for each device. The ground truth coordinates of the temporal sites were determined in a static relative positioning using observations collected by the geodetic receivers and processed with multi-purpose GNSS processing software [24].  
In Table 1, we show the average number of satellites that were tracked on consecutive frequency bands. A careful inspection of the table allows us to conclude that the most reasonable scenarios that can be used for the comparison of the smartphone positioning performance, are single-frequency (SF) GPS and SF multi-GNSS (GPS+Galileo+GLONASS+BDS). This is reasoned by the fact that other frequency bands offer a low number of acquired satellites. In addition, the classical dual-frequency ionosphere-free linear combination is unsuitable in this case due to few GPS satellites that provide signals on the L5 frequency band and a low number of acquired Galileo satellites on E5a band for Xiaomi Mi 8 and Huawei P30.  
The benefit from the application of L5/E5a code pseudoranges may be reliably verified only by using Galileo observations of the Xiaomi Mi 9 smartphone. In this case, the satellite tracking performance was comparable for both frequency bands, namely E1 and E5a. However, by observing Table 1, we can notice that the average number of acquired GPS satellites on L5 is much lower than that on L1 for all smartphones. For the Galileo constellation, we see how the Xiaomi Mi 8 has a low number of satellites tracked both on the E1 and on the E5a. For the Huawei P30, a positioning with the E1 may be regarded as even impossible since an average number of satellites over the session reached only 1.2.  
Despite the fact that Doresa and Milena--a pair of Galileo spacecrafts launched into highly eccentric orbital planes--have proved their applicability to standard [25] and precise GNSS positioning [26], the impact of these satellites may be considered as negligible, since only one of them was tracked for a few epochs by a single smartphone (Xiaomi Mi 8) during the experiment.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Smartphone**} & \multicolumn{2}{c}{**GPS**} & \multicolumn{2}{c}{**Galileo**} & \multicolumn{1}{c}{**BDS**} & \multicolumn{1}{c}{**GLONASS**} \\ \cline{2-7}  & **L1** & **L5** & **E1** & **E5a** & **B1** & **G1** \\ \hline Xiaomi Mi 8 & 9.5 & 3.6 & 4.0 & 4.0 & 7.1 & 5.6 \\ Xiaomi Mi 9 & 10.4 & 4.4 & 8.0 & 7.7 & 6.4 & 8.7 \\ Huawei P30 & 9.9 & 3.4 & 1.2 & 5.3 & 3.9 & 6.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Number of tracked satellites per frequency bands given as an average over the data collection session.  
Figure 1: Smartphones during GNSS data collection.