SBAS and GBAS Integrity for Non-Aviation Users: Moving Away from "Specific Risk"

SBAS and GBAS enhance standalone GNSS navigation to meet the safety and availability requirements of civil aviation. SBAS and GBAS are also freely available to other users, such as automobiles, buses, and trains on land as well as ships near shore. However, integrity as implemented there are significant differences between the aviation interpretation of navigation integrity and the one that would be natural to most users. This paper explains the differences between “specific risk” as defined by aviation and “average risk,” which is used in most other fields and which is the foundation of Probabilistic Risk Assessment (PRA). Maximum errors for the FAA WAAS version of SBAS are presented and compared to the protection levels determined from WAAS to support aviation approach operations to illustrate the degree of conservatism that is built into the “specific risk” interpretation of integrity. Based on this information, several means are proposed to remove this conservatism from SBAS and GBAS for non-aviation users who do not need it. The resulting benefits, in terms of smaller error bounds and/or improved availability, can be substantial.