The ionosphere is one of the two major sources of error for WAAS. The other major source of error is the satellite clock bias introduced by intentional and random dithering of the satellite clock for Selective Availability (SA) control. Ionsopheric vertical range delays of over 30 meters can occur in hawaii, and delays of over 20 meters can occur in the CONUS region. The delays at other elevation angles are related to the verttical ionospheric dealsy via the obliquity factor, and at low elevations the ionsopheric delays can be typically three times higher compared to the vertical delays. A network of WRSs (WAAS reference stations) makes measurements of the ionospheric delays in the slant paths to various GPS satellites in view of various WRSs. These measurements are proessed by WMS (WAAS Master Station) to arrive at vertical ionsopheric delays at a set of grid points across the CONUS. The avionics receiver estimates delays to satellites in its view by appropriate interpolation and processing of these grid point delays.
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