# Effects of Equatorial Plasma Bubbles on Multi-GNSS Signals: A Case Study over South China
## 4 Signal Quality Assessment
The constellation is marked in the upper left corner of each subpanel, and the upper right corner indicates the average number of satellites observed at each epoch. To better assess the impact of EPBs on each GNSS constellation and frequency, we defined a parameter, the error epoch occurrence rate. A scintillation event is thought to be a signal path crossing the EPB, and the occurrence rate is defined as the total number of error epochs divided by the number of total epochs during a scintillation event in 1 min. Figure 7 shows the occurrence rates of the error epochs for the four GNSS constellations. In general, the traditional L1 frequency is better than the L2 and L5 frequencies. The scintillation behavior of L1, L2, and L5 is generally consistent with strong scattering theories, which indicate that low-frequency scintillation saturates earlier than high-frequency scintillation [45; 46; 47]. Studies have shown that the average deep fading values of L2 and L5 are almost twice that of L1, which also indicates that L1 has better stability in a strong scintillation scenario [29]. However, the sensitivity to scintillations is also dependent on the signal modulation features. We can also see that the L5 (L5Q, L7Q, L8Q) of Galileo was better than L1 (L1C) and L2 (L6C), which may be due to the different coding and AltBOC signal modulation used in Galileo. The signal tracking of GPS-L2L was superior to that of GPS-L2W, even though they have the same frequency. Among the four constellations, Galileo performed significantly better than the others, while BDS showed an unusually high incidence of error epochs. For GLONASS, the occurrence rates were relatively lower, which is consistent with the findings of previous studies indicating that GLONASS has a lower hourly occurrence probability of cycle slip than do GPS and BDS [48]. Figure 6: Comparison of the number of error epochs, number of scintillation events, and corresponding mean S\({}_{4}\) intensity for three orbits (MEO, GEO, IGSO) of the BDS at L1 frequency (L2I, 1561.098 MHz). This figure is similar to Figure 5.  
Figure 8 shows the occurrence rates of the error epochs for BDS-2 and BDS-3 and for different satellite orbits. According to Figure 8a,b, from the results of the common observation codes L2I and L6I, BDS-3 was significantly better than BDS-2. The unique observation codes (L1P, L7D) of BDS-3 were also better than the BDS-2/3 common observation codes (L2I, L6I). For satellites with different orbits, Figure 8c-e show that the error epoch occurrence rate of IGSO was significantly greater than those of MEO and GEO. Therefore, the main contribution to the higher occurrence rate of BDS error epochs in Figure 7 is from the IGSO satellites. As mentioned in Figure 6, the orbital characteristics of IGSO determine wide latitude coverage and narrow longitude coverage for the signal path trajectory (please refer to Figure 10 in [43]). On the one hand, the signal path reaches latitudes closer to the equator and could suffer significant EPB depletion. On the other hand, the signal path of IGSO is influenced by varying elevations, while the wide latitude coverage makes it more susceptible to scintillations because the signal path could be nearly parallel to the geomagnetic field line [44, 49]. During EPB events, data from IGSO satellites need to be used carefully for positioning purposes. However, the new BDS-3 signals L1P and L5P on IGSO satellites still provide great control of signal tracking (Figure 8e). Overall, the performance of BDS signal quality is dependent on both the signal modulation and satellite orbits. Figure 7: Occurrence rate of error epochs for (**a**) GPS, (**b**) Galileo, (**c**) GLONASS, and (**d**) BDS during 12–20 UT on 3 April 2022. The color of the bar indicates the frequency band and the signal types are marked in x-label. When the S\({}_{4}\) index exceeds the threshold, the signal is considered to have passed through the EPB at this time.