Estimating the absolute total electron content, spatial gradients and time derivative from the GNSS data

Global navigation satellite systems have enabled to study the ionosphere in different regions of the world. The total electron content (TEC) of the Earth ionosphere can be determined from code and phase dual-frequency pseudorange measurements performed by receivers of GNSS signals. This technique is widely described in the literature (B. Hofmann-Wellenhof, H. Lichtenegger, J. Collins. New York: Springer-Verlag Wien, 389 p. 1998). To obtain the absolute TEC values, phase measurements are usually used, because they are weakly noised, and the ambiguity of the initial phase definition is eliminated with code ones. Thus, there occurs a systematic error termed differential code biases (DCBs). To determine absolute TEC accounting for DCBs from the data of a single GPS/GLONASS station as well as spatial gradients and time derivative, we have developed an algorithm. The algorithm includes estimating DCBs by using a simple model of measurements: equation where IV is the absolute vertical TEC value; Δ□(Δl) is the latitude (longitude) difference between the ionospheric point coordinate □ (l) and that of the station □0(l0); Δt is the difference between the measurement time t and the time t0, for which the calculation is performed; G□=∂IV/∂□, Gl=∂IV/∂l, Gq_□=∂²IV/∂□², Gq_l=∂²IV/∂l² are linear and quadratic spatial TEC gradients; Gt=∂IV/∂t and Gq_t=∂²IV/∂t² are the first and second time derivatives.