The X-mode and O-mode traces of the above ISIS-2 digital ionogram were used as the input to the true-height inversion program. This program assumes vertical propagation in a horizontally-stratified ionosphere and uses the X-mode input data to calculate the electron density profile from the satellite altitude down to the altitude of the F region peak density and the O-mode vertical-reflection trace expected to be produced by the sounder in the presence of this topside electron density profile. This calculated O-mode trace is presented in Figure 2 together with the scaled O and X-mode traces. The good agreement between the scaled and calculated O-mode traces in the frequency regions where a scaled O-mode trace was available provides a degree of confidence in (1) the original trace identification as a X-mode vertical reflection trace, (2) the scaling of this trace and (3) the electron density profile (Figure 3) derived from the true-height inversion program (since this profile is used to calculate the O-mode trace). Figure 2 can be used as a guide for identifying the traces on the original ionogram, e.g., in Figure 1. The third ionospheric reflection trace of greatest delay in Figure 1, and most intense between 3.5 and 5.0 MHz, is an oblique reflection trace. The abrupt change in reflections at 5 MHz is due to a transition from long to short sounding dipoles during the frequency sweep.
An analysis program based on the inversion program developed by Jackson  is available from the NSSDC anonymous ftp archive ( Description of the Program). This analysis program requires IDL for display, and FORTRAN for the true-height inversion and satellite world-map programs.
Jackson, J. E., The reduction of topside ionograms to electron-density profiles, Proc. IEEE, 57, 960-976, 1969.
Jackson, J. E., E. R. Schmerling, and J. H. Whitteker, Mini-review on topside sounding, IEEE Trans. Antennas Propagat., AP-28, 284-288, 1980.