Frequency correlation of line-of-sight signal scintillations

The frequency autocorrelation of amplitude and phase scintillations of radio signals propagated over turbulent line-of-sight paths is calculated theoretically. The single scattering (Born) approximation to the electromagnetic response of the turbulent irregularities is used, limiting the results to small amplitude and phase variations. However, the results are valid for both Fresnel (near zone) and Fraunhofer (far zone) scattering. The calculations are made for an arbitrary model of the turbulent irregularities by using the spectrum method, which postpones the specialization of the analysis to a particular turbulence model until the wave-propagation aspects of the problem are completed. It is shown that the signal variations on adjacent carrier frequencies have high correlation for frequency separations comparable to the carrier frequency for all propagation conditions, indicating that the "medium bandwidth" for line-of-sight paths is very large. This also means that there should be negligible pulse distortion for high-speed data links or high resolution radars operating on line-of-sight paths. These predictions are compared with the few available experimental results.