IONOSPHERIC TOMOGRAPHY - ITS LIMITATIONS AND RECONSTRUCTION METHODS

Recent modeling and experimental efforts carried out by several resear ch groups indicate that tomographic reconstruction is a promising expe rimental tool for obtaining images of ionospheric structures. The tech nique makes use of navigation satellites and a network of ground-based receiving stations which are used to measure the total electron conte nt (TEC) on a number of paths through the ionospheric region of intere st. In practice, the available TEC data are inevitably sparse and inco mplete and this imposes certain limitations on the reconstructed image s. The nature of these limitations is described in this paper. In addi tion, a unified discussion of some of the various reconstruction algor ithms is presented. The difference between the reconstruction methods arises from different choices for the basis functions which are used t o represent the unknown electron density structure. Three such choices for the basis functions are discussed. One of these uses the concept of expansion in terms of model ionospheres which can incorporate a pri ori information about the ionosphere. An example illustrating the appl ication of the method to image the equatorial ionosphere with extremel y sparse data is described. The technique is shown to produce an excel lent reconstruction for this case which was not well reconstructed usi ng techniques thal do not incorporate the a priori information. Finall y, the paper is concluded with a short review of experimental work tha t has been reported in the literature to date.