Disentangling ionospheric refraction and diffraction effects in GNSS raw phase through Fast Iterative Filtering technique

The presence of irregularities in ionospheric plasma density results in perturbations on trans-ionospheric radio-waves, impacting Global Navigation Satellite System (GNSS) positioning and satellite communication. The sudden and rapid fluctuations of phase and amplitude of the GNSS signals triggered by small-scale irregularities are commonly referred to as "ionospheric scintillations" and are due to the diffraction of the signal. Refractive variations are induced by large-scale size irregularities and by changes in the plasma velocity, resulting in low-frequency variations. Fast Iterative Filtering (FIF), a novel signal filtering technique, is used for analyzing GNSS raw data to identify and remove the refractive contribution from the actual ionospheric scintillation. The algorithm is fast and well-fitted for non-stationary and non-linear signals. In addition, the technique is fully based on mathematical theory which is more prominent with regard to other time-frequency analysis techniques. Scintillation is quantified through the phase and amplitude scintillation indices, σ_Φ and S4 respectively. S4 is the standard deviation of the received power normalized by its mean value, while σ_Φ is the standard deviation of the detrended carrier phase in radians. Leveraging on FIF, we disentangle ionospheric refraction and diffraction contributions and we calculate the cut-off frequency for phase detrending. The rationale behind this study lies on the potential ability of FIF to efficiently detect the stochastic effects producing scintillations. This ability of FIF may lead to a more accurate estimation of σ_Φ. The work presents a case-study based on the GNSS data acquired by the network of scintillation receivers managed by INGV at high-latitude during the September 2017 geomagnetic storm.

Figure 1. (Panel a) Spectra of the Galileo E1 (f1) and E5a (f2) signals and of the Ionosphere Free Linear Combination (IFLC) as recorded by the receiver in Concordia Station (Antarctica, 75.10°S, 123.33°E). Galileo E1 signal, from 14:45 to 15:15 UT, 8 Sep 2017. Blue dashed line indicates the cutoff frequency. (Panel b) Time profile of s_F for Galileo E1 signal as parsed by the receiver (blue), calculated thorough FIF with a cutoff frequency of 0.1 Hz (black) and with 0.8 Hz cutoff. (Panel c) Corresponding time profile of S4.