Limiting frequencies of radio wave reflection from the inhomogeneous ionosphere for vertical- and oblique-incidence sounding

We compare the maximum observable frequencies (MOF) measured at two subpolar oblique-incidence ionospheric sounding (OS) paths with classical maximum usable frequencies (MUF) calculated from vertical-incidence ionospheric sounding (VS) data. We show that the accuracy of scaling VS data to OS data for subpolar paths is lower than at midlatitudes, particularly in the winter season at high solar activity. The results can be explained by the influence of large-scale irregularities (LSI) of the ionosphere electron density N. We propose a procedure for processing of VS data, which improves the agreement between measurement and calculation results.