where a is the weighting factor that determines the relative importance of the two scale sizes and and have the same functional form as that given for the single scale size. In addition to the spatial correlations described above, it is often necessary to determine the temporal correlations of the wave field. It is straightforward to transform the spatial correlation to the temporal correlation for the case where the irregularities are drifting with a constant velocity and not changing in form. Let V be the horizontal velocity of the irregularities in the plane containing the x axis and the propagation direction. Now if and are the distances to the field point and the source from the irregularities and i is the angle of incidence, then the diffraction pattern at the observation point moves along the x axis with the velocity given by: using the above equation, the time constant for which the correlation falls to can be obtained from the corresponding spatial correlation scale size D. A computer program has been developed to compute the rms phase and amplitude
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