Estimation of ionospheric total electron content (TEC) from GNSS observations

One of the largest errors in Global Positioning System (GPS)/Global Navigation Satellite System (GNSS) positioning is attributable to the ionosphere. The ionosphere is a part of atmosphere that extends from about 60 km altitude to more than 2000 km. The ionosphere contains a partially ionized medium, as result of the X and UV rays of solar radiation and the incidence of charged particles. Total electron content (TEC) is a versatile parameter for ionospheric studies and easily derived from the relatively low-cost dual frequency GNSS receivers. Taking advantage of the dispersive nature of ionosphere, delay due to ionosphere is derived by differencing range observations made from received signal from two frequencies. This chapter explains in detail the process of TEC derivation from raw GNSS observations, and few of its applications.