Supposing the TEC is compressed into an Ionospheric thin layer, the International Space Station (ISS)/Chinese Space Station (CSS) emitter to the receiver on Earth connections intersect at the Ionospheric thin layer, which point is called Ionospheric Pierce Point (IPP), VTEC is the TEC in the vertical direction at IPP and STEC is the slant TEC from ISS/CSS to the Earth. H_b and H_t are the heights from the Earth’s surface to the bottom and top spheres of the ionosphere layer of interest. The green arc between the left-side point IPP and the right-side one is visible from the ground station to the space station.