Mitigation of anomalous ionosphere threat to enhance utility of LAAS Differentially Corrected Positioning Service (DCPS)

Local area augmentation system (LAAS) can be used for both Category I (CAT I) precision approach and differentially corrected positioning service (DCPS) navigation applications. Through its support of DCPS, the LAAS ground facility (LGF) is required to help meet the integrity requirements of terminal-area navigation and other operations that could use the LAAS VHF Data Broadcast in addition to precision approach. The current LAAS standards indicate that DCPS integrity risk shall not exceed 10-7 per hour [1,2]. This paper demonstrates that the requirement in current LAAS standards is hard to achieve under anomalous ionosphere and proposes potential requirements changes to improve DCPS availability. Horizontal position error (HPE) is calculated from current ionosphere threat model and is applied to individual satellites in all the subset geometries. Limited subset geometries and screening Horizontal Alert Limit (HAL) are considered as requirements changes of current LAAS MOPS for DCPS. Limited subset geometries with drill down to N-2 satellites (or SVpsilas) mitigate maximum unbounded HPE caused from 6 kilometers to 110 meters. The screening HAL of 550 meters allows maximum HPE of 300 meters.