Correlation Structure of the Equatorial Ionosphere

With the successful implementation of the Wide Area Augmentation System (WAAS) for the North America, there is interest in implementing similar systems in other parts of the world. The ionosphere over certain parts of the globe, notably equatorial regions and Brazil in particular, can have a far greater impact on the signals received from the satellites. The Ionospheric corrections form an integral component of the WAAS solution. The main requirement is to come up with a bound on the user vertical error, the first step of which is to perform a correlation analysis of the ionosphere. The WAAS MOPS and ICAO SARPS specify a thin-shell approximation at a height of 350 km to model the ionosphere. This model is used to convert slant range delays into equivalent vertical observations. The correlation analysis of the Vertical Total Electron Count (VTEC) measurements is performed using processed data for Jan 11 2000 and Feb 19-21 2002 from a set of 12 Reference Stations in Brazil. The data indicates that even co-located IPP’s may have significantly different equivalent vertical ionospheric delay values, differences greater than 4 m at L1 on a quiet day. Thus, the thin shell model is much less accurate for the equatorial ionosphere than it is for mid-latitudes. We will discuss the correlation methodologies used in this analysis, which will bring out differences for the equatorial structure from the mid-latitude regions.