Ionospheric tomography: its limitations and reconstruction methods

Recent modeling and experimental efforts carried out by several research groups indicate that tomographic reconstruction is a promising experimental tool for obtaining images of ionospheric structures. The technique makes use of navigation satellites and a network of ground-based receiving stations which are used to measure the total electron content (TEC) on a number of paths through the ionospheric region of interest. In practice, the available TEC data are inevitably sparse and incomplete and this imposes certain limitations on the reconstructed images. The nature of these limitations is described in this paper. In addition, a unified discussion of some of the various reconstruction algorithms is presented. The difference between the reconstruction methods arises from different choices for the basis functions which are used to 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 priori information about the ionosphere. An example illustrating the application of the method to image the equatorial ionosphere with extremely sparse data is described. The technique is shown to produce an excellent reconstruction for this case which was not well reconstructed using techniques that do not incorporate the a priori information. Finally, the paper is concluded with a short review of experimental work that has been reported in the literature to date.