Availability of GAST D GBAS Considering Continuity of Airborne Monitors

Standards are being developed for using a ground based augmentation system (GBAS) to provide guidance for CAT III approach and landing operations, known as GBAS Approach Service Type D (GAST D). To evolve from CAT I (GAST C) to GAST D an additional method for mitigating potential errors due to ionospheric anomalies must be implemented in the aircraft. This paper considers the continuity risk from monitors and protection levels affected by this additional ionospheric anomaly mitigation. Three such processes are analyzed: 1) dual smoothing ionospheric gradient monitoring algorithm (DSIGMA) monitor, 2) fault-free vertical protection level (VPLH0) and 3) reference receiver fault monitor (RRFM). A geometry constraint needed to independently ensure acceptable continuity risk for each process is first identified. The constraint needed for RRFM has already been included in specifications. However, only geometry constraints needed for adequate integrity protection with DSIGMA have been specified. Thus, the analysis assesses whether continuity constraints for DSIGMA and VPLH0 must also be implemented. The analysis determines the continuity risk from enforcing all or only some of these constraints. Results show that the VPLH0 continuity risk predominates and a corresponding constraint is needed. However, given only the constraint for VPLH0 the continuity risk from DSIGMA could arguably be acceptable. Moreover, even if all constraints are enforced availability of adequate satellite geometry is no less than that limited by the separate geometry constraints needed for proper integrity protection with DSIGMA.