Transionospheric Propagation of FM Signals

Communication links to earth satellites are not usually affected adversely by propagating through the ionosphere. However, the nonlinear variation of the refractive index with frequency causes a dispersion which can result in significant signal distortion under certain circumstances. Such a case occurs, for example, when a wide-band frequency-modulated (FM) signal propagating at the lower carrier frequencies traverses a low-elevation path through a disturbed ionosphere. Expansions are now available for the spectrum of the output phase of an angle-modulated signal which has been passed through a generalized linear filter. The ionosphere is modeled as such a filter in this study and the spectrum of the demodulated signal is determined. From this, formulas are obtained for the output signal-to-distortion and signal-to-crosstalk ratios. The spectral results are presented graphically for FM with a uniform baseband, and two cases of current interest are examined numerically