A considerable effort has been directed toward the elucidation of parameters that are important to the design of earth-space systems in order to counter the scintillation problem. Communications systems may counter the effects of substantial fading by using space diversity. If the paths from a single satellite are sufficiently well separated (depending upon the details of the inhomogeneity wave number spectrum), then fading in the two links is uncorrelated and a net performance gain may be achieved through use of diversity. Separations of the order of a kilometer are involved, but these useful minimum separations are certainly larger than ship dimensions and will only function most efficiently for ground installations. As a result, shipboard and airborne terminals must be designed to provide compensation through pursuit of other countermeasures. One might logically suspect that radio links sufficiently separated in frequency, polarization, or transmission time would be independent and could be effective in combating scintillation. Unfortunately, this generalization is not true, even for orthogonal polarizations. Furthermore, separations of up to 100 MHz may be required to obtain an adequate diversity improvement in the frequency domain. However, time diversity is a demonstrated procedure for overcoming scintillation at UHF for disadvantaged mobile platforms.
Fluctuation Frequency (Hz) Fluctuation Frequency (Hz)
Figure 4-24: Scintillation data at 138 MHz obtained at Poker Flat, Alaska. The abscissa is fluctuation frequency (Hz) for both types of power spectra, (a) Phase spectrum, (b) Intensity spectrum. The so-called S4 index is the integral of the intensity spectrum. Most of the contribution to this integral is at the low frequency Fresnel frequency cutoff. This is a geometrical effect that is not encountered with the phase spectrum, which has no low frequency cutoff. From Secan .
Was this article helpful?