## 3 Experiments and Analysis
### The Influence of STD on the Parallel PPP Receiver  
It was the unmodeled tropospheric errors, rather than multipath or observational noise in STD models, that improved the PPP accuracy, therefore three parallel stations were selected for data processing. Figure 5 illustrates the location of the stations (CUAA, CUT0 and CUTC), among which CUTC and CUT0 used the same receiver. The ZTD difference for the three stations was ignorable due to their close location. The observation from CUT0 was used for STD modeling, then its ZTD and STD products were used to augment CUTC and CUAA for float and fixed PPP. Furthermore, the STD products calculated from the same receiver were also used for comparison. Figure 4: The positioning error series of the CFRM station (2013-05-20) calculated by different tropospheric correction strategies. (**a**) North. (**b**) East. (**c**) Up. Figure 5: Station distribution and corresponding receivers. Figure 6 presents the positioning error series of the PPP float solution based on four different tropospheric corrections on Day of Year (DOY) 253 of 2017 for the CUTC station located in Curtin University. Minor positioning accuracy improvement was observed when we adopted ZTD corrections. In comparison, the positioning accuracy in the E direction around 20:00 and the U direction near 4:00 were significantly improved when adopting the STD correction of the external CUT0 station (the \"slant (cut0)\" strategy). The positioning error series were rather smooth for the \"slant (cutc)\" strategy when the same station STD products were used. Table 3 presents the statistical RMS of the two stations (CUTC and CUAA) calculated by different tropospheric correction models. The \"fix-STD\" strategy represents PPP using the same station STD products. In comparison to the \"fixZTD\" strategy, the accuracy improvement for the \"fix-STD(CUT0)\" strategy was 14%/28%/31% in the N/E/U direction, respectively. The positioning accuracy could be further improved when the same station STD correction was used, and the positioning accuracy of the N/E/U direction was improved by 51%/51%/60% compared to the \"fix-ZTD\" strategy. As it is illustrated, the proposed STD correction could improve the PPP float accuracy compared to the traditional ZTD correction. We then analyzed the influence of STD correction on the PPP ambiguity-fixed solutions. The positioning error series of the fixed PPP solutions using different  
Figure 6: Positioning errors of float the precise point positioning (PPP) solutions of CUTC using different tropospheric corrections. (**a**) North. (**b**) South. (**c**) Up. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**RMS/m** & \multicolumn{3}{c}{**CUAA**} & \multicolumn{3}{c}{**CUTC**} \\ \hline  & **North** & **East** & **Up** & **North** & **East** & **Up** \\ \hline est-ZTD & 0.0107 & 0.0183 & 0.0351 & 0.0111 & 0.0195 & 0.0372 \\ fix-ZTD & 0.0106 & 0.0160 & 0.0222 & 0.0124 & 0.0174 & 0.0239 \\ fix-STD(CUT0) & 0.0091 & 0.0115 & 0.0174 & 0.0102 & 0.0116 & 0.0187 \\ fix-STD & 0.0052 & 0.0077 & 0.0074 & 0.0052 & 0.0089 & 0.0094 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: RMS values of kinematic PPP using different modeling strategies. correction strategies are given in Figure 7. The estimated parameters are re-initialized every 4 h in the experiment. It can be seen that the ambiguity-fixed PPP accuracy was not decreased using STD correction products. Moreover, the fixing rate used the \"fixSTD\" strategy in the E direction was improved during 16-20 h. The statistical results demonstrate the accuracy for the \"fixSTD\" strategy could improve by 14%/28%/21% in the N/E/U direction, and the fixing rate of PPP ambiguity was increased by 30%. The comparisons of PPP ambiguity-float and fixed solutions obtained by different STD models are given in Figure 8. It was observed that the ambiguity-fixed PPP could improve the positioning accuracy, especially in the E direction.