DATA FROM ORBITING NAVIGATION SATELLITES FOR TOMOGRAPHIC RECONSTRUCTION

So far, orbiting navigation satellites are the only source for primary data in ionospheric tomography. Phase difference measurements give the input for tomographic reconstruction. Except for a constant, the initial phase difference value, the data can be considered to be the line integral of electron density along the straight line from the satellite to a ground based receiver (''slant electron content''). In ionospheric physics a projection onto the vertical is used: (vertical) electron content (TEC). Many investigations have been based on this quantity alone. This work discusses some of the properties of the measured data (instrumental and ''ionospheric'' limitations in section II), and the role of TEC evaluation assumptions (''mean ionospheric height'' and initial phase constant, in section III). Some of the problems inherent in ionospheric tomography using orbiting navigation satellites are discussed by means of model calculations (section IV). The models are mathematically defined two-dimensional electron density distributions that are used to calculate the latitude dependence of both slant and vertical TEC. Among others, the model calculations are useful in showing that strongly different electron density distributions can lead to similar electron contents. Tomographic reconstruction would probably not be able to distinguish between the different distributions, which leads to the recommendation to incorporate additional data in the reconstruction process. It is well known, and reflected in the model electron contents, that without additional data tomographic reconstruction is not accurate in height determination. Layer height variations with latitude, however, can have a strong influence on slant and vertical electron content. (C) 1994 John Wiley and Sons, Inc.