As it is well known, the ionosphere, the region of the atmosphere between 50 and 1000 km height approximately containing the most part of its free electrons, has an important effect on Global Navigation Satellite System (GNSS), like the Global Positioning System (GPS) signals. It mainly consists on an advance (i.e. shortening) the carrier phase and a reduction of the group velocity (i.e. pseudorange increase). The effect ranges from few to hundreds of meters, and it is proportional to the line-of-sight integrated electron density of free electrons, and inversely proportional to the squared frequency. As consequence, the top-rank receivers (like the ones deployed worldwide within the International GNSS Service, IGS) provide dual-frequency GNSS measurements, in order to cancel out or, (more interesting for this work) to isolate the ionospheric delay variation with more than 99.9% of precision. In this way the GNSS system becomes a global ionospheric sensor (or

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