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

摘要

The Federal Aviation Administration's (FAA) Wide ArearnAugmentation System (WAAS) for civil aircraftrnnavigation is focused primarily on the ConterminousrnUnited States (CONUS). The ionospheric correctionrnalgorithms for WAAS have been characterizedrnextensively for this mid-latitude region of the ionospherernwhere benign conditions usually exist. Researchers arernfacing a more formidable challenge in addressing thernionospheric impact on navigation using Satellite-BasedrnAugmentation Systems (SBAS) in other parts of the worldrnsuch as the South American region. At equatorialrnlatitudes, geophysical conditions lead to the so-calledrnAppleton-Hartree (equatorial) anomaly phenomenon,rnwhich results in significantly larger ionospheric rangerndelays and range delay spatial gradients than is observedrnin the CONUS region.rnIn this paper, we use data from the South American regionrnto perform a preliminary quantitative assessment of thernperformance of WAAS correction algorithms in thisrnregion. For this study, we accessed a world-wide networkrnof 230 dual-frequency GPS receivers. The networkrnincludes: 1) the Continuously Operating Reference Sitesrn(CORS) in the United States; 2) stations in and near SouthrnAmerica as part of the Brazilian Network of ContinuousrnMonitoring of GPS (RBMC), operated by the BrazilianrnInstitute of Geography and Statistics (IGBE); and (3) sitesrnincluded in the International GPS Service (IGS) globalrnnetwork. Data sets have been selected to include both arnquiet and geomagnetically disturbed day. To providernground-truth and calibrate GPS receiver and transmitterrninter-frequency biases, we processed the GPS data usingrnGlobal Ionospheric Mapping (GIM) software developedrnat the NASA Jet Propulsion Laboratory to computerncalibrated high resolution observations of ionosphericrntotal electron content (TEC).rnWe assessed the WAAS's planar fit algorithm in thernequatorial region where the spatial gradients and thernabsolute slant TEC are known to be the highest in thernworld. We found that in Brazil the dominant error sourcernfor the WAAS planar fit algorithm is the inherent spatialrnvariability of the equatorial ionosphere with ionosphericrnslant range delay residuals as high as 15 meters and rootmeanrnsquare residuals for the quiet day of 1.9 meters. Thisrncompares to a maximum residual of 2 meters in CONUS,rnand 0.5 meter RMS. We revealed that ionosphericrngradients in Brazil are at the 2 meter over 100 km level.rnContrary to results obtained for CONUS, we discoveredrnthat a major ionospheric storm had a small impact on thernplanar fit residuals in Brazil.