Ground GNSS Ionosphere Sounding

Ionospheric delay will bring errors for GNSS navigation and positioning when the electromagnetic wave signal goes through the earth\textquoterights ionosphere from satellites to receivers. The amount of ionospheric delay of GNSS varies from a few meters to decades of meters, but could reach more than decades of meters during severe ionosphere storms. In contrast, the GNSS ionospheric delay may provide some useful information on the ionosphere, e.g. the total electron content (TEC). In this chapter, the theory and methods of ground-based GNSS ionospheric sounding are introduced, including vertical TEC, differential code biases, 2-D and 3-D ionospheric mapping. In addition, some applications are presented and discussed, e.g., GNSS TEC climatology, solar flare and storms response and co-seismic ionospheric behaviors.