In panel a), the green line represents the effective altitude reconstructed by the comparison of the cross-correlation and Fresnel algorithms for the velocity reconstruction. The color of the scatter plots represents the relative percentage error between the two velocity methods. The comparison is done with the velocities measured using the 140 MHz data, as reported in the plot title. S4 values, measured at the same frequency, are also shown as a red line. Panel b) shows the velocity obtained by the cross-correlation method, measured at the effective altitude. The color of the scatter plots represents the relative percentage error between the two velocity methods.