eSWua: Amplitude Scintillation Index (S4) computed from 50 Hz GNSS signal samples

The slant Amplitude Scintillation Index (s4_slant) is calculated as the standard deviation of the 50 Hz raw signal power samples normalized to the average signal power over an interval of 60 seconds for each available GNSS satellite link and signal frequencies (depending on the acquisition capabilities of the receiver). The 1-min resolution s4_slant is corrected taking into account the thermal noise contribution of the receiver and the vertical Amplitude Scintillation Index (s4_vert) are estimated according to formula (4) of Spogli et al. (2014, http://dx.doi.org/10.1016/j.jastp.2013.10.003), by assuming the Ionospheric Pierce Point at an altitude of 350 km.