# Mitigating the Scintillation Effect on GNSS Signals Using MP and ROTI
## 4 Discussion
First, it was highly possible that the observations from the satellite with the maximum parameter value encountered cycle slips, which might have been detected and repaired by the methods namely Hatch-Melbourne-Wubbena and the Geometry-Free Combination in the PPH preprocessing, before application of the positioning algorithm. Alternatively, the outlier detection algorithm in PPPH had identified and removed the noisy observations from the scintillation satellite in advance [28]. As a consequence, removing this scintillation satellite scarcely impacted the output. Moreover, it was possible that multiple satellites were affected by scintillation, hence removing a single satellite could not effectively mitigate the scintillation effect. However, it would not be appropriate to remove multiple satellites simultaneously as the number of available satellites might drop below 4, or the DOP values could be severely affected. The improvement achieved with the observation removal strategy was more substantial than the satellite removal strategy. As shown in Table 4, the 3D error was improved on 11 out of 14 days based on MT and scintillation parameters, and the largest improvement in height error was 91.7% based on MT and standard parameters. Hence, it is possible that scintillation parameters are more likely to lead to improvement in positioning whereas standard parameters tend to lead to larger improvements in the observation removal strategy. However, the combination of standard parameters can be more effective. For instance, the combination of MP1 and ROTI could improve the height error more substantially than scintillation parameters using MT. In this case, standard parameters were able to replace scintillation parameters. Compared with the satellite removal strategy, the observation removal strategy was able to remove scintillation-affected observations from multiple satellites for an epoch once the corresponding parameter value exceeded the threshold. Furthermore, MT generally led to a higher success rate of improvement and improvement percentage. As MT was lower than ET, MT removed more observations, which led to fewer measurements with errors. Therefore, the scintillation effect was more thoroughly mitigated, which explained the higher effectiveness of applying MT in the observation removal strategy. However, more observations removed could also lead to a worse satellite geometry and larger GDP, resulting in the instability of positioning quality [42]. Thus, the observation removal strategy was less reliable than the weight strategy especially when MT was applied. There is a tradeoff between choosing MT for a larger improvement and a higher success rate of improvement, and choosing ET for a better satellite geometry. As shown in Tables 5-7, though the proportion of days with improvement obtained with the weight strategy was relatively lower as compared with the observation removal strategy, the weight strategy was capable of acquiring an even higher improvement, that is 93.1% in height error. However, the errors considerably increased on 8 September 2017, 13 September 2017 at SJCU and 6 June 2016, 28 July 2016 at SNA0P. The same situation also occurred for the observation removal strategy as shown in Tables S6, S7 and S12-S14, which did not occur for the satellite removal strategy. This is partially because the original RMSEs were already small on several days, such as 2 April 2016, 28 July 2016 and 13 October 2016 at SNA0P, where the RMSEs did not exceed 0.01 m. This meant that the positioning performance of PPP was not substantially affected by scintillation on these days. Hence the presence of scintillation does not always mean that positioning will be affected and removing or weighting multiple observations simultaneously may even degrade the positioning performance of PPP. To clarify this, the number of satellites affected by cycle slip was investigated. Take SAOOP and SJCU, stations close to each other, as an example. The errors on 8 September 2017 substantially decreased at SAOOP, but substantially increased at SJCU.