Assessment of Integrity Monitor Impacts on GBAS Performance under Ionospheric Scintillation

Kurzfassung

Ionospheric scintillation introduces phase transitions that corrupt carrier-smoothed pseudoranges in Ground-Based Augmentation System (GBAS). These phase transitions—discrete full-cycle carrier phase changes occurring during continuous tracking—are not addressed in existing GBAS error models, creating unmodeled integrity risks for operations in equatorial regions. This study evaluates the effectiveness of GBAS integrity monitors in detecting phase transition-affected measurements, characterizing residual errors remaining after monitoring to determine how these residual errors should be addressed in protection level calculations. The analysis combines empirical data from the NavAer network in Brazil with physics-based phase screen simulations, focusing on Divergence-Free (DFree) L1 smoothing, which exhibits the largest phase transition error amplification and represents a primary smoothing mode for future GBAS architectures. NavAer network data analysis reveals that without monitoring, smoothed pseudorange errors substantially exceed standard GBAS error model bounds during scintillation. Sequential monitoring using the signal power test and carrier phase step test progressively reduces these errors through measurement exclusion. However, sample retention during scintillation decreases with tighter threshold application, reflecting the fundamental trade-off between integrity protection and continuity. Phase screen simulations systematically characterize postmonitoring error models across {S4, ?0, C/N0} parameter space. After combined monitor application, residual sigma overbounds persist at levels comparable to nominal GBAS error parameters. The magnitude of residual errors depends on threshold selection: tighter thresholds yield better error reduction but cause severe continuity loss, whereas looser thresholds maintain measurement availability but permit larger residual errors. The results demonstrate that monitor-based mitigation significantly reduces but does not completely eliminate phase transition-induced errors. Post-monitoring error models derived from simulation provide quantitative foundations for addressing these residual errors in GBAS protection level calculations for equatorial operations, with potential approaches including dedicated scintillation-specific parameters or inflation of existing error model terms.