Performance of ionospheric scintillation index S_4c derived from geodetic GNSS observations at low latitudes: strategies and validation

In recent years, increasing attention has been paid to ionospheric scintillation monitoring leveraging geodetic global navigation satellite systems (GNSS) receivers, and the S_4c index has emerged as an effective parameter in characterizing scintillation phenomena. Although high sampling rates (e.g., 1 Hz) are standard in current practice to capture rapid signal fluctuations, a systematic evaluation of how different computation strategies influence S_4c remains limited. This study investigates the performance of various S_4c indices derived from GNSS dual-frequency observations at 1 s, 5 s, 15 s, and 30 s sampling rates during scintillation events over Brazil. These indices are systematically compared and analyzed with the amplitude index (S₄), rate of TEC index (ROTI), and GOLD nighttime image. Results show that all S_4c indices with different computation strategies exhibit strong consistency with these scintillation means, and all S_4c indices variants effectively respond to S₄ variations during scintillation events especially. However, detection thresholds for identifying scintillation vary across sampling rates. Notably, S_4c index values derived from 5 s, 15 s, and 30 s data are numerically similar but lower than those based on 1 s measurements. These findings suggest that GNSS observations with 1 Hz sampling rates remain the optimal choice for accurate scintillation monitoring.