# Effects of Equatorial Plasma Bubbles on Multi-GNSS Signals: A Case Study over South China
## 4 Signal Quality Assessment
The longitudes of the sub-satellite points for GEO satellites and the longitudes of the intersection of sub-satellite trajectories for IGSO satellites are shown in the third column. The above results showed a strong correlation between the S\({}_{4}\) index and EPB. Thus, the S\({}_{4}\) index with a certain threshold is used to identify scintillation events during which the satellite signal passes through the EPB, i.e., only observations during scintillation events are analyzed. Since the actual S\({}_{4}\) index is dependent on the observed elevation, PRN code features, and type of signal modulation, the S\({}_{4}\) threshold value should be set carefully. After excluding the 20\({}^{\circ}\) elevation data, the S\({}_{4}\) mean values of three GPS signal types on a day without scintillation events correspond to L1C equal to 0.12, L2W equal to 0.14, and L5Q equal to 0.08. The three signal types of Galileo L1C, L5Q, and L6C correspond to 0.13, 0.10, and 0.21 respectively. Due to the difference quiet time S\({}_{4}\) for different GNSS constellations and signal modulations, the threshold here is chosen to be 1.5 times the mean value of the S\({}_{4}\) index for each satellite and signal type instead of a specific value. Figure 5 shows the number of error epochs and scintillation events with a 1 min time bin at a single frequency of L1 (GPS, Galileo, GLONASS-1575.42 MHz, and BDS-1561.098 MHz). The BDS is divided into BDS-2 and BDS-3, since the signal features are different between these two generations. Each constellation has been subjected to a mass of scintillation events for more than four hours with a high number of error epochs. Apparently, there were almost no errors during non-scintillation or quiet periods (10-12 UT). According to the scintillation intensity (yellow bar), error epochs tended to occur during strong scintillation periods, which may be caused by large signal intensity fluctuations and attenuations. For the traditional L1 frequency band as shown in Figure 5, the Galileo and GLONASS signals provide better stability for signal tracking. Although there are differences in the number of visible satellites between each constellation, the BDS clearly  
Figure 4: Variations of signal quality parameters for L6I (1268.52 MHz) observations from the BDS-C03 GEO satellite during 12–20 UT on 3 April 2022. From top to bottom are the variations of (**a**) the S\({}_{4}\) index (L6I) and airglow depletion, (**b**) the deviation VTEC (L2I-L6I), (**c**) the signal-to-noise ratio (C/N0, S6I), (**d**) the number of cycle slips, and (**e**) the number of losses of lock as a function of UT. has more error epochs. In addition, the performance of BDS-2 was worse than that of BDS-3. Figure 6 shows the MEO, GEO, and IGSO results for the BDS. Clearly, IGSO exhibited the worst stability compared with MEO and GEO, which implies that IGSO signals may be interrupted in large numbers during EPB transit. This poor quality of IGSO signal might be related to the signal path trajectory of the IGSO satellite (please refer Figure 10 in [43]); the signal path reaches latitudes closer to the equator and could suffer significant EPB depletion. And the orientation of IGSO signal path is easily parallel to the geomagnetic field direction, which results in strong phase slips [44]. In addition, the performance of the GEO signals was comparable to that of MEO, although there were more scintillation events in the GEO signals. Figure 5: Comparison of the number of error epochs (cycle slip or loss of phase observation, blue bar), number of scintillation events (S\({}_{4}>\) 1.5\({}^{\circ}\) averaged S\({}_{4}\), orange bar), and corresponding mean S\({}_{4}\) intensity (yellow bar) at L1 frequency (GPS L1C, Galileo L1C, GLONASS L1C: 1575.42 MHz, and BDS-2 and BDS-3 L2I: 1561.098 MHz) band for different constellations. For better comparison with the S\({}_{4}\) index, the error epochs are divided into 1 min time bins.