Interpretation of oblique-incidence ionograms under the assumption of a spherically stratified ionosphere

Oblique sounding ionograms show that an ionogram trace can be of nose shape with typical multipath for which the maximum corresponds to maximum usable frequency. However, ionograms obtained from sounding at short paths are of classical type and do not differ from vertical sounding ionograms in appearance. On the basis of ray tracing in isotropic medium we examined changes in ionogram structure when varying the ground range. It is shown that in a stratified medium, multipath can take place at any nonzero ground range though at short paths, to resolve multipathing in an experiment seems to be impossible. Some possibilities for using the assumption of sphe-rically stratified ionosphere when sounding at long radio paths (of the order of few thousand kilometers) are also under consideration. We compared the distance-frequency characteristics (DFC) produced for horizontally-nonuniform ionosphere defined along the path by IRI-2012 model, as well as for spherical-stratified ionosphere for which the height profile does not depend on horizontal coordinate and agrees with IRI-2012 profile in the middle point of the path. Although the electron density profiles exhibit significant variations along the path, DFCs agree with each other up to fraction of the percent. In this case, the form of the ionogram practically in everything is defined by the vicinity of radio path middle point.