Top panel: the difference dTEC = TEC(TaD) – TEC(GNSS) obtained by varying the correction factor k with a step 0.01. In this sample, where TaD is applied to a profile from Athens Digisonde, dTEC becomes zero at k = 3.25, denoted as k0. Bottom panel: the same profile case shown in the top panel, shown here with default value k = 2.5 (no adjustment – blue curve) and k0 = 3.25 (with adjustment – red curve).