Study of functional Ionospheric Models

132-141The ionosphere has played an important role for propagation of electromagnetic waves. They contribute to a major portion of the propagation delay, especially for the low frequency navigation signals, as they pass through earth’s atmosphere. The amount of delay of these signals depends on ionospheric total electron content, which has a spatio-temporal variation. While the dual frequency users can remove the delay errors in signals, these delays pose a major threat mostly for single frequency users, who has to resort to the ionospheric models for the removal these delay errors in the signals, and hence for improving the positioning accuracy. This work has reviewed various Ionospheric Models, both theoretical and Empirical, which have been designed for a better determination of variation of TEC values. These models, still useful for both the navigation and other scientific purposes are important for understanding the characteristics of the ionosphere and gaining insight about its temporal and spatial variations