Scintillation is referred the phenomenon whereby the satellite signal fluctuates due to ionosphere. This is in particular evident in satellite signals below 2 GHz. An important index used to describe the severity of scintillation is S4 index (varying between 0 and 1). The GPS satellite signal with high S4 index is likely to experience loss-of-lock situation, therefore, it cannot be used for positioning estimation or navigation purpose.
Below is an example of S4 index measured at Phuket station, Thailand.