## 3 Data Processing
### Quality of GNSS Observations  
To compare the performance of the Xiaomi Mi8, the ZED-F9P u-blox, the ZED-F9R u-blox and the Sepentrtio AsteRx-U receiver, several pre-analyses were performed, namely:  
* Satellite visibility during measurements;
* Multipath; and
* Cycle slips for the received signals. Figure 6 shows the available number of GNSS satellites and the metric quantification of the multipath effect on the L1 frequency of the total trajectory for the Sepentrtio AsteRx-U geodetic receiver. The figures illustrate the areas with worse measurement conditions, which refer to urban street canyons and areas with lush roadside vegetation, as shown in Figure 6. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **RTKLIB** \\ \hline Constellations & GPS/GPS + GLONASS + Galileo \\ Observations & code and carrier phase (L1/E1 + L2/E5b + L5/E5a) \\ Ambiguity & fix-and-hold/continuous \\ Ephemeris & broadcast \\ Elevation angle & \(15^{\circ}\) \\ Ionospheric delay & iono-free (LC) \\ Tropospheric delay & Saastamoinen \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Processing parameters used in RTKLIB. Figure 6: (**a**) Number of available satellites along the route, and (**b**) multipath in metres for the Sep tentrio AsteRx-U receiver. Initially, the multipath was estimated; however, furthermore in RTKLIB, a module was used to resolve LLI by using several algorithms, including integer least squares and LAMBDA (Least-Squares Ambiguity Decorrelation). For static sessions, the \"fix-and-hold-algorithm was used\", while for kinematic, the \"continuous mode\" was used (Table 1). As shown in Table 2, a smartphone was capable of acquiring observations only at the L1 and L5 frequencies, while the u-bloxes were capable only at the L1 and L2 frequencies. From the results, it can be seen that the Septentrio AsteRx-U geodetic receiver, which is equipped with an active geodetic GNSS antenna with built-in low-noise amplifiers that amplify the signals before further processing, and which has a higher gain, performed much better under difficult conditions. The root mean square error (RMSE) for the multipath along the entire track is 0.3554 m, 0.4320 m, and 0.3313 for frequencies L1, L2, and L5, respectively. The smartphone's multipath estimate is much higher on the L1 frequency band with respect to the Septentrio and two of the u-bloxes, but it reaches the u-bloxes' results from L1 and L2 on the L5 frequency band. It is also evident that the L5's Septentrio observations were much less affected by the multipath. The results confirm the fact that L5 signals are less susceptible to multipath because they rely on a higher frequency and wider bandwidth compared to other GNSS signals. The advantages of the Septentrio AsteRx-U receiver's active antenna are also evident in Figure 7, which shows the carrier-to-noise ratio (C/N0) in colours and the loss-of-lock indicator (LLI) phenomenon in red lines. LLI is an indicator that shows when the receiver has lost the link or signal to a satellite, resulting in a cycle slip event (Figure 7). This information is particularly useful for GNSS post-processing to improve position accuracy and repeatability. As can be seen in Figure 7, LLIs occur for all receivers during kinematic positioning (from 13:55 to 14:45), but they are less frequent for the Septentrio AsteRx-U antenna. LLIs are also common and quite frequent with the u-bloxes and the Xiaomi Mi8 smartphone in static mode. Overall, the measurements with the Septentrio AsteRx-U were the least affected by LLIs even in kinematic measurements; however, interestingly, the problem was greatest with the ZED-F9R u-blox. Moreover, it can be confirmed that the signal quality of the overall measurements with the Xiaomi Mi8 receiver is significantly worse compared to the u-bloxes, as previously demonstrated by other authors [50], albeit with different smartphones.