Peculiarities of the HF radio wave propagation over round-the-world paths

Peculiarities of the HF propagation over round-the-world paths are discussed. An estimation of the F2 ionospheric region critical frequency variations using analysis of round- the-world radio sounding data has been made. Experimental data obtained by the Russian chirp-sounders network have been used. Using experimental data and numerical simulation, the quantitative dependency between the minimum magnitudes of the F2-region critical frequencies over round- the-world propagation paths and round-the-world maximum usable frequencies has been obtained.