GNSS (Global Navigation Satellite System) signals pass through various layers of the atmosphere until reaching the receiver on the Earth. The ionosphere, one of these layers, is about 70 km to 1000 km above the Earth surface and constantly changes with the time of day, seasons, geographical location and solar explosions. The GNSS signals affected by the variable structure of the ionosphere are proportional to the total electron content (TEC). Determination of the TEC change is important for modelling of the ionosphere. In this study, totally 26 stations, including 14 TUSAGA-ACTIVE (CORS-TR) stations in Turkey and 12 IGS stations, were selected and evaluated. Bernese v5.2 GNSS software was used for evaluation. TEC values were calculated at intervals of two hours, one day per each month, from 2009 to 2015. TEC values, obtained from GNSS measurements using Single Layer Model, were compared with global ionosphere maps (GIMTEC) issued by the Center for Orbit Determination in Europe (CODE), the European Space Agency (ESA), the Jet Propulsion Laboratory (JPL), and TEC (IRI-2012 TEC) values obtained from international ionosphere reference program. The best approach to regional ionosphere model obtained as result of comparison was shown by CODE and ESA. Additionally TEC map was produced for the selected area as utilizing regional and global TEC values.

Key words: GNSS, ionosphere, total electron content (TEC), global ionosphere maps (GIM)