The Earth’s ionosphere as part of the upper atmosphere is the most important atmospheric layer for radio wave propagation in space between the satellites and the Earth. It has, for example, a large impact on the accuracy of positioning and navigation, but also on up-to-date GNSS applications such as autonomous driving and precision farming. Under usual circumstances, the ionosphere is the largest error source in point positioning with single frequency GNSS receivers. As an example, the Figure below shows a global map of the Vertical Total Electron Content (VTEC) on March 17, 2015, the St. Patrick’s day, at 13:00 UT. These large VTEC variations are mostly caused by a strong geomagnetic storm (Erdogan et al., 2020).
The ionospheric influence on radio waves is twofold: the signal travel times are changed and the signal paths are bent. The latter can be neglected for most applications. The ionospheric delay depends directly on the integral of the electron density along the signal path between the transmitter and the receiver, the so-called slant total electron content (STEC). In other words, if the STEC along the ray path is known from ionosphere models, the delay for a single-frequency GNSS measurement can be computed and to some extent corrected. High-precision ionosphere models are required, for instance, if precise navigation applications have to be performed with single-frequency receivers.