Effect of the Earth's oblateness on the estimation of global vertical total electron content maps

Global Navigation Satellite Systems like the Global Positioning System and the Russian counterpart GLONASS permit the estimation of ionosphere parameters in the form of vertical total electron content (vTEC) values. Although the GNSS reference systems are rotation‐symmetric ellipsoids, simple approximations of gravitational equipotential surfaces, the modeling of global vTEC maps is carried out in spherical systems, since this simplifies mathematical expressions. Due to the Earth's oblateness this approximation is in slight disagreement to the geometric situation of the observations. Thus it is expected that estimated vTEC values will change when the WGS84 ellipsoid is taken as a reference for all computations. We will discuss both, the direct effect (i.e due to a shift of the ionospheric pierce point) and the indirect effect, caused by a change of the mapping function, on global vTEC values using a single layer model of the ionosphere. We show that the estimated global TEC maps change slightly by up to ±0.4 TECU when refined mathematical models are considered and we discuss how differential code biases are affected by this change.