The mutual coherence function for transionospheric radiowaves

The mathematical properties of the mutual coherence function for radiowaves propagating in a randomly irregular medium have been known since the early development of the theory. Recent satellite experiments that transmit multifrequency, phase-coherent signals have facilitated direct measurement of the mutual coherence function for transionospheric radiowaves. In this paper we report the results of an extensive series of such measurements using the Wideband Satellite multifrequency beacon as a signal source. The measured mutual coherence functions show systematic departures from their simplest theoretical form, particularly under weak-scatter conditions. The results are nonetheless consistent with a power law spectrum for the irregularities but show evidence of an abrupt change in the power law index at high frequencies. This spectral feature has been detected in rocket data, but it is evidently masked by diffraction effects in scintillation phase data. The measurements also verify the relationship between the intensity autocorrelation function and the mutual coherence function that applies under conditions of saturated scintillation.