Schematic representation of bubble detection by quasi co-located DGS and a GNSS receiver. Both detectors are placed at the bottom vertex of the triangle. $T_i^{\mathrm{DGS}}$ and $T_f^{\mathrm{DGS}}$ correspond to the starting and final times at which the EPB is observed by the DGS, whereas $T_i^{\mathrm{GNSS}}$ is the starting time at which the EPB is detected by the GNSS receiver. The DGS spans an observation range of α ≃ 35° with respect to the vertical direction, and the altitude of the ionospheric layer, H_ion, is taken at 350 km.