An Assessment of the Current WAAS Ionospheric Correction Algorithm in the South American Region

Attila Komjathy, Lawrence Sparks, Anthony J. Mannucci and Xiaoqing Pi

Abstract:	The Federal Aviation Administration’s (FAA) Wide Area Augmentation System (WAAS) for civil aircraft navigation is focused primarily on the Conterminous United States (CONUS). The ionospheric correction algorithms for WAAS have been characterized extensively for this mid-latitude region of the ionosphere where benign conditions usually exist. Researchers are facing a more formidable challenge in addressing the ionospheric impact on navigation using Satellite-Based Augmentation Systems (SBAS) in other parts of the world such as the South American region. At equatorial latitudes, geophysical conditions lead to the so-called Appleton-Hartree (equatorial) anomaly phenomenon, which results in significantly larger ionospheric range delays and range delay spatial gradients than is observed in the CONUS region. In this paper, we use data from the South American region to perform a preliminary quantitative assessment of the performance of WAAS correction algorithms in this region. For this study, we accessed a world-wide network of 230 dual-frequency GPS receivers. The network includes: 1) the Continuously Operating Reference Sites (CORS) in the United States; 2) stations in and near South America as part of the Brazilian Network of Continuous Monitoring of GPS (RBMC), operated by the Brazilian Institute of Geography and Statistics (IGBE); and (3) sites included in the International GPS Service (IGS) global network. Data sets have been selected to include both a quiet and geomagnetically disturbed day. To provide ground-truth and calibrate GPS receiver and transmitter inter-frequency biases, we processed the GPS data using Global Ionospheric Mapping (GIM) software developed at the NASA Jet Propulsion Laboratory to compute calibrated high resolution observations of ionospheric total electron content (TEC). We assessed the WAAS’s planar fit algorithm in the equatorial region where the spatial gradients and the absolute slant TEC are known to be the highest in the world. We found that in Brazil the dominant error source for the WAAS planar fit algorithm is the inherent spatial variability of the equatorial ionosphere with ionospheric slant range delay residuals as high as 15 meters and root- mean square residuals for the quiet day of 1.9 meters. This compares to a maximum residual of 2 meters in CONUS, and 0.5 meter RMS. We revealed that ionospheric gradients in Brazil are at the 2 meter over 100 km level. Contrary to results obtained for CONUS, we discovered that a major ionospheric storm had a small impact on the planar fit residuals in Brazil.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	1286 - 1296
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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