GAST D (GBAS Approach Service type D) in the Low Latitude Region and use for CAT I Service

Susumu Saito

Abstract:	GNSS ground-based augmentation system (GBAS) is designed to support precision approach and landing of aircraft. Though GBAS approach service type (GAST) C is operational in the mid-latitude region in the world, ionospheric issues prevent GAST C from full availability. In this study, possible use of GAST D for CAT I operation is examined. The GAST D protection levels in all the analyzed cases were below the vertical alert limit of 10 m. However, the airborne satellite geometry screening degraded the CAT I availability for the cases with the E_ig parameter, which is the maximum range error assured by the ground subsystem, greater than 2.75m. Two potential approaches to relax the threshold value in the airborne satellite geometry screening (limitS_vert^2) are considered. One is to set a larger max E_v value, which is the maximum allowed vertical error, for CAT I operation in the airborne receiver in addition to that for CAT II/III operation, and the other is to deflate the broadcast E_ig value. While the first approach is straight forward but requires airborne receiver standard changes, the second approach requires changes in the broadcast parameter only. When the deflated E_ig is broadcast in the second approach, CAT II/III operations must be prohibited. Nevertheless, deflated E_ig for CAT I could be beneficial, because the period of CAT II/III conditions are much less than the CAT I condition. Deflated and non-deflated E_ig values could be used depending on the weather condition at the airport, if certain mechanism to notify aircraft on the supported approach operations.
Published in:	Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021) September 20 - 24, 2021 Union Station Hotel St. Louis, Missouri
Pages:	1133 - 1141
Cite this article:	Saito, Susumu, "GAST D (GBAS Approach Service type D) in the Low Latitude Region and use for CAT I Service," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 1133-1141. https://doi.org/10.33012/2021.18049
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