Closed-Form Study of Undetected Range Errors Induced by Ionospheric Anomalies for GAST-D GBAS

In ground-based augmentation system (GBAS) approach service type D (GAST-D), various ionospheric monitors are implemented in both aircraft and ground facilities to detect ionospheric anomalies. Additionally, the largest undetected differential range errors induced by ionospheric anomalies must be examined because these errors are used in geometry screening to identify potentially unsafe satellite geometries. Based on the ionospheric front threat model, a closed-form expression of the largest undetected ionospheric range error has been established for GBAS approach service type C (GAST-C), where only ground ionospheric monitoring is involved. This paper presents a closed-form expression for GAST-D, and both the ionospheric front model and plasma bubble threat model are taken into consideration. Based on exhaustive simulations among all possible ionospheric threat conditions, the expression is determined as a linear function of the relative speed and gradient magnitude of the ionospheric anomaly. Compared with the linear expression of ionospheric errors for GAST-C, the expression for GAST-D demonstrates that the use of additional ionospheric monitors and a smaller time constant for the code-carrier smoothing filter can effectively reduce the largest undetected ionospheric range error.