Performance of Ionospheric Scintillation Index S4c 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 system (GNSS) receivers, and the S 4 c c 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 4 c c indices derived from GNSS dual-frequency observations at 1-, 5-, 15-, and 30-s sampling rates during scintillation events over Brazil. These indices are systematically compared and analyzed with the amplitude index ( S 4 ), the rate of TEC index (ROTI), and global-scale observations of the limb and disk (GOLD) nighttime image. Results show that all S 4 c c indices with different computation strategies exhibit strong consistency with these scintillation means, and all S 4 c c indices variants effectively respond to S 4 variations during scintillation events. However, detection thresholds for identifying scintillation vary across sampling rates. Notably, S 4 c c index values derived from 5-, 15-, 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.