Assessment of Ionospheric delay on GPS Signals, Using a Global Electron Density Model

Ionosphere is the upper part of earth’s atmosphere, extended from about 60 km to 1000 km and higher altitudes. Free electrons and ions in this region affect the propagation of radio waves. Amount of these particles is proportional to the solar radiation intensity and to the gas density in the atmosphere. For GPS radio signals, ionosphere is a dispersive medium. Meaning that signals traveling through the ionosphere are affected proportional to their frequencies. This effect is called the ionospheric refraction. By increase in solar radiation, ionosphere has turned into the major error source in GPS measurement, causing errors from about 1 m to 100 m or more in range observations. Therefore the effect of ionosphere on GPS radio signals should be studied in detail. As it will be shown, studying this effect is restricted to determination of Total Electron Content, (TEC). In this study, TEC is obtained using the International Reference Ionosphere, (IRI). Results are compared with the TEC derived from double frequency GPS measurements.