Ionospheric Effects

Radio waves traveling between satellites and earth stations must pass through the ionosphere. The ionosphere is the upper region of the earth’s atmosphere, which has been ionized, mainly by solar radiation. The

free electrons in the ionosphere are not uniformly distributed but form in layers. Furthermore, clouds of electrons (known as traveling ionospheric disturbances) may travel through the ionosphere and give rise to fluctuations in the signal that can only be determined on a statistical basis. The effects include scintillation, absorption, variation in the direction of arrival, propagation delay, dispersion, frequency change, and polarization rotation (CCIR Report 263-5, 1982). All these effects decrease as frequency increases, most in inverse proportion to the frequency squared, and only the polarization rotation and scintillation effects are of major concern for satellite communications. Polarization rotation is described in Sec. 5.5.

Ionospheric scintillations are variations in the amplitude, phase, polarization, or angle of arrival of radio waves. They are caused by irregularities in the ionosphere which change with time. The main effect of scintillations is fading of the signal. The fades can be quite severe, and they may last up to several minutes. As with fading caused by atmospheric scintillations, it may be necessary to include a fade margin in the link power-budget calculations to allow for ionospheric scintillation.