Effects of the ionosphere on the radio horizon in HF band on the earth-space paths

A computational study is made of three-dimensional ray trajectories in anisotropic and inhomogeneous ionospheres to estimate the size of the radio window viewed from a satellite with high frequency and radio waves. Model ionospheres are used to calculate the variation of the extent of the iris with changing ionospheric electron density distribution, and the estimated values are compared with observational results obtained by the ISS-b. It is found that the extent of the viewed area from the satellite is enlarged when taking the refractive effect into account. In the case of the ionosphere with constant hmF and NmF, the extent of viewed area increases with increasing F-layer thickness, and for the radio wave with a frequency of 17.5 MHz, the extent of the viewed area is not affected much by the existence of the E-layer with a density of about 4-5 MHz.