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 th e input for tomographic reconstruction. Except for a constant, the ini tial phase difference value, the data can be considered to be the line integral of electron density along the straight line from the satelli te to a ground based receiver (''slant electron content''). In ionosph eric physics a projection onto the vertical is used: (vertical) electr on content (TEC). Many investigations have been based on this quantity alone. This work discusses some of the properties of the measured dat a (instrumental and ''ionospheric'' limitations in section II), and th e 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 di scussed by means of model calculations (section IV). The models are ma thematically defined two-dimensional electron density distributions th at are used to calculate the latitude dependence of both slant and ver tical TEC. Among others, the model calculations are useful in showing that strongly different electron density distributions can lead to sim ilar electron contents. Tomographic reconstruction would probably not be able to distinguish between the different distributions, which lead s to the recommendation to incorporate additional data in the reconstr uction process. It is well known, and reflected in the model electron contents, that without additional data tomographic reconstruction is n ot accurate in height determination. Layer height variations with lati tude, however, can have a strong influence on slant and vertical elect ron content. (C) 1994 John Wiley and Sons, Inc.