Optimizing High-Rate Scintillation Data Downlinks in Radio Occultations

High-rate radio occultation (RO) data obtained from low-Earth orbiting (LEO) satellites play a critical role in global monitoring of ionospheric scintillation, which can disrupt GNSS-based navigation and communication systems [Sokolovskiy et al. 2002]. This study evaluates the effectiveness of current data downlink trigger algorithms, specifically those relying on the S4 amplitude scintillation index, to ensure efficient and targeted data collection. Downlink trigger algorithms are necessary when the continuous downlinking of high-rate data from a satellite is not feasible; in such cases, RO data are only downlinked when an onboard algorithm detects scintillation. Today all the RO capable satellites (e.g., COMIC-2) employ a trigger based only on the S4 index. While the S4-based trigger has proven effective for low-latitude scintillation, which is predominantly characterized by amplitude fluctuations, its suitability for high-latitude regions, where phase scintillation is more prevalent, remains uncertain [Livingston, et al. 1982]. Low-latitude scintillation can arise from plasma density irregularities within the equatorial ionosphere, driven by factors such as post-sunset equatorial spread-F events. These irregularities cause significant amplitude scintillation, which is well captured by the S4 index. In contrast, high-latitude scintillation occurs in regions influenced by geomagnetic activity under open field lines and the auroral oval, where rapid changes in electron density often manifest as phase variations rather than amplitude disturbances. This difference in scintillation characteristics creates a challenge for the current S4-based trigger algorithm, which may fail to detect high-latitude events [Rino 1979]. Using data from COSMIC-2, PlanetIQ, and Spire Global RO datasets [Cook, et al. 2013], this study highlights that the S4-based trigger effectively detects scintillation at low latitudes, with few instances of significant phase scintillation lacking corresponding amplitude fluctuations. However, high-latitude analysis reveals a substantial proportion of events with strong phase scintillation that would have been missed by the S4-based algorithm. These findings emphasize the need to incorporate phase scintillation thresholds into the trigger criteria, particularly for high-latitude regions. The study recommends further refinement of downlink trigger algorithms to account for these latitudinal differences. This includes developing dual-criteria systems that monitor both amplitude and phase scintillation indices, along with larger datasets and improved filtering to enhance operational reliability. By addressing these challenges, the next generation of RO missions can better support ionospheric monitoring and GNSS resilience under diverse geophysical conditions.