Position-Domain Geometry Screening to Maximize LAAS Availability in the Presence of Ionosphere Anomalies

Abstract

All fault modes in the Local Area Augmentation System should be mitigated within the specified integrity risk allocation to guarantee the safety of a landing aircraft. However, monitoring within the LAAS Ground Facility is insufficient to completely protect users from unacceptable errors due to ionosphere spatial gradient anomalies. A methodology has been developed to inflate the broadcast ópr_gnd and óvig so that subset satellite geometries (i.e., subsets of the set of approved GPS satellites for which the LGF broadcasts valid corrections) for which unacceptable errors are possible are made unavailable to users. The required sigma inflation factors are computed offline and are input into each LGF site during site installation. These offline simulations and the resulting inflation factors are updated periodically to insure that they remain sufficient to mitigate residual ionosphere anomaly risk. This paper describes the updated ionosphere threat space and the geometry screening algorithm required to be implemented to support the FAA/Honeywell LAAS Provably Safe Prototype (PSP) at Memphis airport. It also demonstrates by simulation results that the required availability of integrity for CAT I approaches is achievable with the proposed method.