We present the results of application high-orbital (GPS/GLONASS) and low-orbital (Parus/TRANSIT) satellite navigation systems to the study of different-scale irregularities in the ionospheric total electron content (TEC), such as troughs, crests of equatorial anomaly, traveling ionospheric disturbances and so on. We also present the results of the comparison of the global ionospheric maps (GIM) of vertical TEC, which are now widely used in ionospheric research, with the results of low- and high-orbital radio tomographic ionospheric imaging and with the data of UV spectral imaging form GUVI instrument (Global Ultraviolet Imager). The data from low-orbital radiotomography systems in Russia (Moscow-Svalbard) and Alaska (Arctic Village-Cordova) were involved in the comparison as well as the data of the IGS (International GNSS Service) network. The comparisons cover the time interval from 2003 to 2008, which includes both geomagnetically quiet and disturbed periods. We also demonstrate the possibilities of GPS/GLONASS TEC studies in connection with solar flares and artificial ionospheric heating. (5 pages)