Since the first development of the TOMION(TOmographic Model of the IONosphere),in1995,gAGE/UPC has been improving the performance and reliability of this technique. The TOMION kernel relies on the computation of the ionospheric electrondensity by means of using a 4D model of the ionosphere. At the beginning the technique was mainly focused on ionospheric determination from a single or few stations using a 2-layer tomographic approach. Further developments lead to the use of TOMION as a global ionospheric scanner,allowing the use of TOMION to produce Global Ionospheric Maps(GIMs)for the International GNSS Service(IGS) jointly with CODE(University of Bern),EMR (Energymines and Resources,NRCAN),ESA (European Space Agency) and JPL(Jet Propulsion Laboratory) since 1st June 1998. Inparallel to this activity, the TOMION capabilities were increased, and the use of more than 2-layers was a natural extension of the model. However, other approaches were introduced, and as a result of that effort the Improved Abel technique, for electrondensity profile retrieving, was developed. This technique was based on the separability hypothesis, which overtook the limitations of the spherical hypothesis for electrondensity retrieval.

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