| A global empirical background TEC model is built by using the CODE vertical TEC data for full 13 years, 1999-2011. It represents the state of the ionosphere for a particular epoch of solar activity and can be used for planning radio systems operations. The solar activity is described by both F10.7 and its linear rate of change. In this way we distinguish the different ionospheric behaviour at rising and declining part of the solar cycle at one and the same F10.7. The model offers TEC maps which depend on geographic coordinates (5ox5o in latitude and longitude), the relationship between geographic-modip latitudes, and UT at given solar activity and day of the year. At each latitude the diurnal variability of the TEC model is described by 2D (longitude-time) sine functions with zonal wavenumbers up to 4 and 4 harmonics of the diurnal period, while the seasonal variability is presented by sine functions including 4 harmonics of the yearly period. Not only a detailed statistical assessment of the model is performed, but also an error model is developed. |