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Introduction The order of ionospheric delay of microwave is generally larger than tropospheric delay due to neutral atmosphere, and it depends on number of electron and frequency of microwave. We can remove the first order delay due ionosphere with dual frequency observation in GPS (GNSS) because of dependency of the delay on frequency (called as dispersion). We can also estimate STEC and VTEC with dual frequency observation in GPS (GNSS), and it is used for monitoring of space weather and research on ionosphere. Figure 1: The illustration above is conceptual figure which explains how VTEC (vertical total electron content) is estimated in ground-based GPS processing. The figure assume very thin layer of ionosphere at altitude of 350 km. VTEC is computed by mapping the STEC (slant TEC) for each satellite into surface of the earth, and then we can have VTEC information around the GPS network. Figure 2: The map above shows snapshot of VTEC at 12:20 UTC, DOY 236, 2005 based on GEONET data in Japan. Wave of VTEC is clearly seen over the Japanese islands. This is called Traveling Ionosphere Disturbance (TIC), one of the disturbance in ionosphere. There are seven "Japanese islands" in the map. Microwave from seven satellites passed through thin layer of ionosphere, and STEC is mapped into the surface as suggested in the plot in Fig. 1.