eSWua: Phase Scintillation Index (Sigma-phi) computed from 50 Hz GNSS signal samples

The slant Sigma-Phi index (sigmaphi_slant) is calculated as the standard deviation, in radians, of the 50 Hz detrended carrier phase samples averaged over intervals of 3, 10, 30 and 60 seconds and provided for each available GNSS satellite link and signal frequencies (depending on the acquisition capabilities of the receiver). The detrending is performed by filtering the raw carrier phase measurements by a high-pass sixth order Butterworth filter having a cutoff frequency of 0.1Hz. Vertical values of the Sigma-Phi index (sigmaphi_vert) are estimated from the 1-min sigmphi_slant according to formula (3) 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.