Validation of Ionospheric Anomaly Monitor for DFMC GBAS under Disturbed Ionospheric Conditions

The ionospheric anomaly monitor for the Dual Frequency Multi-Constellation (DFMC) Ground Based Augmentation System (GBAS) was tested by using the real Global Navigation Satellite System (GNSS) data obtained under the ionospheric disturbances at Ishigaki (24.4N, 124.2E, corresponding to 19.7N in the magnetic latitude), Japan. We used data obtained on 1 November 2021 when multiple plasma bubbles were observed to test the ionospheric anomaly monitor. We tested ionospheric delay difference (dI) estimation methods based on the single-frequency smoothed (SFS) pseudoranges, divergence-free smoothed (DFS) pseudoranges, and carrier-phase (CP). Two smoothing time constants of 100 and 600 seconds were compared. When the ionospheric gradient of 230 mm/km was observed for the GPS PRN27 satellite, the dI estimation by DFS with the smoothing time constant of 600 seconds outperformed the SFS with 100 seconds. SFS with a longer time constant (600 seconds) showed dI variation but with the opposite sign as those of CP and DFS. The smoothing time constant of 100 seconds was not enough for both DFS and SFS to distinguish the ionospheric anomaly from multipath. For the analyzed case, DFS with 600 seconds showed good performance. Recursive leveling of dI estimated by CP showed good convergence with dI as estimated with DFS. Because the conditions were not the same as the expected operational environment, more ionospheric disturbance data at high satellite elevation angles as well as in the airborne environment are necessary to be analyzed to fully characterize the performances of the different methods.