## 5 Discussion  
In this study, we mainly evaluated the RTK positioning accuracy and ambiguity resolution using the built-in antenna of a smartphone for GNSS observations. The results show  
Figure 15: Results of simulated dynamic deformation monitoring using smartphone. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Moving Time (GPST)** & **North Displacement (mm)** & **East Displacement (mm)** \\ \hline
08:00 (start) & 0 & 0 \\
08:30 & 0 & 0 \\
09:00 & \(-\)20 & \(-\)20 \\
09:30 & \(-\)20 & \(-\)20 \\
10:00 & 20 & 20 \\
10:30 & 20 & 20 \\
11:00 (end) & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Schedule of simulated deformation. that cycle slips are very frequent in smartphones, considerably undermining high-precision positioning. In addition to accurate cycle-slip detection, the frequency of cycle slips in smartphones should be mitigated. Although adopting appropriate strategies can increase the fixing rate, it remains low, and correct fixation takes a long time initially. Moreover, the highest accuracy can reach 1 cm, even if the ambiguity is fixed correctly. Observation noise and changes in the antenna phase centre may hinder additional improvements in the positioning performance through data processing. The low fixing rate and low positioning accuracy can be explained by the use of low-cost antennas that have low gain and low multipath resistance, resulting in low-accuracy pseudo-range and carrier-phase observations. Improving the antenna design is essential to improve the fixing-rate reliability and reduce the convergence time. To illustrate this concept, we used a choke coil for the geodetic antenna in auxiliary experiments. The smartphone was placed upright on the choke coil to reduce the effect of multipath signals, as shown in Figure 16. Approximately 1.5 h of data were collected from 1:50 to 3:25 (GPS Time). Since the data were sufficiently long, we partitioned the data into two subsets. Similar to the previous analysis, the first subset (before 2:45) was utilized to examine the long-term stability of fixed solutions, while the second subset (after 2:45) was used for convergence time analysis in tiny segments. The observation selection and calculation parameter settings were the same as those in Section 3.4. The results of long-term RTK positioning using the choke coil are shown in Figure 17, with a 92.4% fixing rate. It can be observed that the error is less than 2 m in the initial stage, which is better than when the choke coil is not used. Moreover, the solution can remain fixed for about 10 min after the start, and the error remains stable. Following that, we analyzed the segmented RTK positioning in detail. Because the ambiguity can be resolved within 10 min in the initial stage, the calculation segment interval was likewise set to 10 min. The results are shown in Figure 18 and Table 10. It can be seen that the time to fix ambiguity was shortened to less than 5 min, with the fastest fixation taking only 1.5 min. Even though it took nearly 10 min to be fixed in  
Figure 16: Experiments using choke coil to suppress multipath interference. Figure 17: Results of long-term RTK positioning in choke-coil experiment. much shorter than the time when using the built-in smartphone antenna. In addition, the positioning accuracy after convergence was higher than when the choke coil was not used. In segment 4, the RMS errors along the east, north, and up directions were 0.003, 0.004, and 0.007 m, respectively, showing millimetre-level accuracy. After using the choke coil, the mean number of candidate ambiguities was relatively large, and there were almost no cycle slips. In addition, the pseudo-range accuracy was approximately 2 m, representing an increase of more than 50%. The choke coil experiment confirms that the antenna design is essential to improve the fixing rate, convergence speed, and positioning accuracy because it increases the observation quality.