# Mitigating the Scintillation Effect on GNSS Signals Using MP and ROTI
## 4 Discussion  
Data from 75 days at three stations, SAOOP, SJCU and SNA0P, were used in the PPP improvement experiments using the freely available MATLAB-based software PPPH. Data from 14 days with scintillation were investigated using three mitigation strategies for improving positioning quality. In Section 3.1.1, we evaluated the convergence time for PPPH at the three stations and concluded that it was around 30 min on a day without scintillation (Figure 3). Then, a visualization example was given based on the data on 13 September 2017 at SAOOP. In the first strategy, G10 and G21 were found to be the satellites with the most intense scintillation according to MP1, MP2, S4, \(\sigma_{\phi}\) and ROTI, respectively. As shown in Figures 4 and 5, removing G10 scarcely decreased the RMSE for both height and 3D errors while removing G21 improved the height error by 5.5% height and 3D error by 4.6%, which is not effective compared to improvements obtained in other strategies. However, future research could investigate the properties of the removed satellites to identify which conditions could lead to improvements in PPP. It should also be noted that, when the number of satellites was high and their geometry was good, the change of GDOP was not substantial when removing a single satellite, as shown in Figure 6.  
More substantial improvements were obtained with the observation removal strategy as seen in Figures 7 and 8. When using a single parameter as the threshold reference to remove observations, the improvement based on ROTI was higher than that based on MP1 and MP2; it exceeded 80% and was substantially more than the improvement through the satellite removal strategy. One possible reason for this phenomenon was that ROTI could identify more scintillation-affected observations than MP. Furthermore, when using a combination of MP1, MP2 and ROTI to remove observations, the height improvement was larger than that of MP1 and MP2, but smaller than that of ROTI. However, this combination improved the 3D error more than ROTI alone. A possible reason for this was that multipath effects are typically more severe at lower elevation. Thus, it is more likely that multipath contributes to the horizontal error rather than the height error, where the former is contained in the 3D error. Further, MP can be used to characterize multipath effects whereas ROTI cannot. Therefore, the combination of MP1, MP2 and ROTI was able to mitigate, not only scintillation, but also multipath in 3D. In addition, the GDOP was  
considerably influenced in the second strategy, especially when applying the combination of MP1, MP2 and ROTI, where several GDOP values exceeded 30 during first three hours as shown in Figure 9. This could be one of reasons why the height improvement using the combination was lower than that using ROTI. Compared with the observation and satellite removal strategies, the weight strategy had no effect on the GDOP, which is one of its advantages. As shown in Figure 10, the improvements based on scintillation and standard parameters were comparable, which were also comparable to most of improvements based on the observation removal strategy. Thus, the weight strategy was more reliable and stable than the other two strategies with consistently high improvements based on this example. All the results on 14 days were statistically summarized in Tables 1-7. Compared with the other two strategies, the satellite removal strategy was relatively ineffective in improving the positioning error. As shown in Tables 1-3, the highest proportion of days with improvement was 8 out of 14 days based on MP2. However, most of the improvements were less than 0.05 m or even 0.01 m, which was not substantial. There were two possible reasons for this phenomenon.