## 3 Results and Discussion
The S4 and \(\sigma_{\varphi}\) indices recorded by the receiver are also seen to be ranging from moderate to high-intensity levels, whose peaks follow the TEC depletion characteristic curves, suggesting a close relationship between the depletion and scintillation indices under the influence of EPBs. The maximum \(\Delta\)(sTEC), \(\Phi_{\text{sTEC}}\), and the corresponding delays of available frequencies in the respective constellations are given in Table 3. The GPS shows the highest depletion depth of \(\sim\)45 TECU within the maximum apparent duration of 48 min. The Galileo and GLONASS also show extended duration but with lesser depletion depths, suggesting that there may be less impact of EPBs on these constellations. The GPS exhibits a high ionospheric delay of 7.29 m in their L1 frequency followed by the GAGAN with a delay of  
Figure 4:0 The sTEC (black), elevation angle (pink) and their respective signal 1 scintillation indices S4 (blue) and \(\sigma_{\varphi}\) (orange) of the GLONASS PRNs 44, 58 and 59, showing the depleted TEC between 15:00 UT to 18:00 UT on the 23 April 2023 geomagnetic storm day. Figure 4:1 The sTEC (black), elevation angle (pink) and their respective Signal 1 scintillation indices S4 (blue) and \(\sigma_{\varphi}\) (orange) of the Galileo PRNs 82 and 89, showing depleted TEC between 15:00 UT and 18:00 UT on the 23 April 2023 geomagnetic storm day. 6.8 m in L1. BeiDou shows the least ionospheric delay in B1 (5.28 m) and the highest in B2 (8.83 m), unlike the GPS which exhibited the lowest in L1 (7.29 m) and the highest in L3 (13.09 m). Galileo shows the lowest ionospheric delay of 3.56 m out of all the constellations. To further understand the severity of scintillation activity, we classify S4 and \(\sigma_{\varphi}\) into weak (0.17 \(\leq\) S4 \(<\) 0.3; 0.15 \(\leq\)\(\sigma_{\varphi}\) \(<\) 0.5), moderate (0.3 \(\leq\) S4 \(\leq\) 0.5; 0.5 \(\leq\)\(\sigma_{\varphi}\) \(\leq\) 0.8), and strong (S4 \(>\) 0.5; \(\sigma_{\varphi}\) \(>\) 0.8) following the acceptable limits [88; 89]. Figure 14 shows a clear enhancement of scintillation indices (S4 in blue and \(\sigma_{\varphi}\) in orange) from 15:00 UT to 18:00 UT in Signal 1 under GAGAN (PRNs 127, 128, and 132). We note that all the above PRNs are geostationary satellites (GEOs) having a fixed elevation and azimuth from the location of observation. Further, scintillation indices S4 (blue) and \(\sigma_{\varphi}\) (orange) for Signal 2 are given \begin{table}
\begin{tabular}{c l c c c c} \hline \hline \multirow{2}{*}{**S. No**} & \multirow{2}{*}{**GNSS Constellation**} & \multirow{2}{*}{\
\begin{tabular}{c} **Maximum Depletion** \\ **Depth \(\Delta\)(sTEC) in TECU** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Apparent Duration** \\ **(t\_TEC) in Minutes** \\ \end{tabular} } & \multicolumn{2}{c}{**Ionospheric Delays in Meters**} \\ \cline{5-6}  & & & & **Signal 1** & **Signal 2** & **Signal 3** \\ \hline
1 & GPS (G) & 45 & 48 & 7.29 & 12.05 & 13.09 \\
2 & GLONASS (R) & 25 & 45 & 3.92 & 6.47 & - \\
3 & Galileo (E) & 22 & 40 & 3.56 & 5.87 & 6.40 \\
4 & BeiDou (C) & 32 & 28 & 5.28 & 8 & 8.83 \\
5 & GAGAN (S) & NA & NA & NA & NA & NA \\
6 & IRNSS/ NavIC (I) & NA & NA & NA & NA & NA \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The maximum sTEC depletion depth \(\Delta\)(sTEC), apparent duration t\({}_{\text{TEC}}\), and their corresponding ionospheric delay for all the GNSS constellations and their available frequencies. Figure 13: The sTEC (black), elevation angle (pink) and their respective Signal 1 scintillation indices S4 (blue) and \(\sigma_{\varphi}\) (orange) of the BeiDou PRNs 142, 148, 149, 152, 153, and 156, showing the depleted TEC between 15:00 UT and 18:00 UT on the 23 April 2023 geomagnetic storm day. in Figure 15a for GPS, GLONASS, and Galileo and in Figure 15b for BeiDou constellations.