GNSS-Based Radio Tomographic Studies of the Ionosphere at Different Latitudes

We present the results of ionospheric imaging by the radio tomographic (RT) methods based on the Navigation Satellite Systems (GNSS). GNSS include the first-generation low orbiting (LO) systems (Tsikada, Transit, etc.) and second-generation high orbiting (HO) systems (GPS and GLONASS, which have been put in operation, and Galileo, BeiDou, and QZSS systems, which are currently under development in Europe, China, and Japan).