WAAS Performance Improvement in Mexico

Ephrem Paredes, Nitin Pandya, Timothy Schempp

Abstract:	The Wide Area Augmentation System (WAAS) broadcasts vertical signal delay estimates and their confidence bounds on an ionospheric grid. Aviation service availability at a given user location can be quantified in terms of the user's computed Vertical Protection Limit (VPL) relative to a Vertical Alert Level (VAL), specified for a given level of aviation service. The magnitudes of the VPLs of WAAS are primarily driven by the magnitudes of the Grid Ionospheric Vertical Errors (GIVEs). GIVEs are error bounds for the WAAS estimates of vertical ionospheric delays for a set of Ionospheric Grid Points (IGPs) from which the user estimates their receiver-to-satellite slant ionospheric delay corrections. The GIVEs must be large enough to protect against two categories of ionospheric gradient threats: sampled and undersampled. The sampled gradients are those that are measured by the available Ionospheric Pierce Points (IPPs). The GIVE algorithm protects against sampled ionospheric gradient threats using the Irregularity Detector at each IGP, the system-wide Extreme Storm Detector, and a dynamic ionospheric uncertainty inflation factor in the computation of the GIVE. Undersampled gradients are between or beyond the available station IPPs. The GIVE algorithm protects against undersampled (or unsampled) ionospheric gradients using a threat model developed from a comprehensive analysis of the data collected during the last solar cycle. The recent expansion of WAAS includes the addition of five new stations in Mexico, a region characterized by a generally higher level of ionospheric activity. Optimizing performance in Mexico affects the construction of the GIVE algorithm. This paper summarizes the GIVE algorithm focusing on a trade study between various components of the GIVE, specifically: the Pierce Point Filter Line (PPFL), the undersampled ionospheric gradient threat model, and the Irregularity Detector trip threshold value, that protect against the sampled and undersampled gradient threats. The trade study results show an improvement in WAAS performance, particularly in Mexico.
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	881 - 893
Cite this article:	Paredes, Ephrem, Pandya, Nitin, Schempp, Timothy, "WAAS Performance Improvement in Mexico," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 881-893.
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