WAAS and the Ionosphere – A Historical Perspective: Threat Model Evolution

<title>Abstract</title> The Wide Area Augmentation System (WAAS) renders the United States’ Global Positioning System (GPS) safe and reliable for aircraft navigation over North America. This paper is the third in a sequence of companion papers providing a comprehensive review of how WAAS, over the first 20 years of its operation, has mitigated threats posed by the ionosphere to the accuracy and integrity of position estimates derived from measurements of GPS signals. The initial paper (<xref>Sparks et al., 2022</xref>) reviews how WAAS has protected the user from threats generated by large-scale ionospheric storms. The second paper (<xref>Sparks et al., 2026</xref>) provides an overview of the methodology WAAS has applied to protect the user from the potentially harmful impact of ionospheric disturbances that are more modest in magnitude. This paper traces the evolution of the undersampled ionospheric irregularity threat model used by WAAS to augment the integrity confidence bounds that confine the user’s positioning error.