# Preliminary Results on Tropospheric ZTD Estimation by Smartphone
## 3 Results
### Case Study 2  
As previously stated, for this case study, the analyses have been made considering the use of both CSRS-PPP and RTKLIB software for ZTD estimation. In this case, the  
Figure 12: ZTD difference between TORI considering CSRS-PPP and RTKLIB. Figure 13: ZTD residuals estimated on TORI by CSRS-PPP and RTKLIB. two considered stations are GENU and GESM, the geodetic CORS and the smartphone, respectively. As highlighted in Figure 14, the behavior of the two types of software is analogous to what already emerged from the previous case study: CSRS-PPP produces modelled ZTD for GESM whereas it correctly estimates ZTD for GENU. Similar to Case study 1, GENU and GESM data were processed also using RTKLIB. The ZTD estimations obtained with this elaboration are shown in Figure 15. The positioning outputs for this elaboration are shown in Figure 16, while Figure 17 highlights the differences between the estimated and reference coordinates for GENU and GESM estimated with RTKLIB. Figure 14: ZTD values for GENU (estimated, blue) and GESM (Xiaomi Miß—modelled, orange) obtained with CSRS-PPP software. Figure 15: ZTD estimates for GENU (blue) and GESM (Xiaomi Miß—orange) obtained with RTKLIB. Table 4 shows the statistics of the solution in Figure 17. For the two solutions, the statistics are computed in respect of the reference position, i.e., the convergence solution, and excluding the convergence time (i.e., from 8:15 UTC onwards). The residuals of the three positioning components of GENU and GESM with respect to the corresponding reference positions are reported in Figures 18 and 19, respectively. Finally, Figure 20 shows the ZTD differences between GENU and GESM as computed with RTKLIB software  
Figure 16: Differences between estimated and reference coordinates for GENU (blue) and GESM (Xiaomi Mi 8—orange) receivers, obtained with RTKLIB. Figure 17: Differences between estimated and reference coordinates for GENU and GESM estimated with RTKLIB. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **Average (m)** & **Standard Deviation (m)** & **Root Mean Square (m)** \\ \hline  & E-W & \(-0.03\) & 0.02 & 0.03 \\ GENU & N-S & 0.02 & 0.01 & 0.02 \\  & U-D & \(-0.08\) & 0.04 & 0.09 \\  & E-W & 0.23 & 0.35 & 0.42 \\ GESM & N-S & 0.07 & 0.23 & 0.24 \\  & U-D & 0.02 & 0.18 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistical parameters for the positions output for GENU and GESM with respect to the corresponding reference positions. Figure 18: Residuals of positioning components for GENU with respect to the reference position, as obtained with RTKLIB. Figure 19: Residuals of positioning components for GESM with respect to the reference position, as obtained with RTKLIB. Figure 20: ZTD difference between GENU and GESM considering RTKLIB software. In order to check the coherence of the ZTD values with official products, the obtained values, both estimated and modelled, were compared with those evaluated from EUREF [53] for the GENO CORS [54], which is a few kilometers away from the considered test site. These results are shown in Figure 21, where it is possible to notice that the estimations made using CSRS-PPP are more consistent with the EUREF solutions than with the ones obtained with the modified version of RTKLIB, although this last one provides promising results. The same comparison with official estimates has been performed considering GESM. Provided that the previous tests demonstrated the lack of performances of CSRS-PPP in estimating ZTD from smartphone data, only the comparison between the estimates computed with RTKLIB and EUREF is presented in Figure 22.  
Figure 21: Comparison of ZTD values for GENU considering CSRS-PPP (orange, modelled) and RTKLIB (blue, estimated) software. The green dots represent GENO CORS ZTD estimates provided by EUREF. Figure 22: Comparison of ZTD estimates for GESM considering RTKLIB (blue). The green dots represent GENO CORS ZTD estimates provided by EUREF.