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In certain observations of a downcoming radio wave, made for the purpose of measuring horizontal drifts of ionospheric irregularities, time shifts between fading curves obtained at three spaced receiving points are used to calculate the velocity and direction of drift of the amplitude pattern over the ground. Two simplifying assumptions about the pattern are frequently made, namely (i) the detail remains constant as it moves, and (ii) it is statistically isometric, i.e. the separation of two points for which the amplitudes have a given fixed correlation is independent of direction.
Phillips et al. (Mon,) studied this question.