Spatial decorrelation of VHF and UHF trans-ionospheric signals measured at Ascension Island

The effect of spatial decorrelation of signals of space-based synthetic-aperture radars by equatorial ionospheric turbulence, is studied by monitoring 150 and 400 MHz signals from LEO beacon satellites on an array of spaced antennas located on Ascension Island, and analysing the cross-correlations of the phases of the received signals. The special property of this approach is that it does not convolve the spatial fluctuations with the movement of the satellite. The geometrical component of the phase difference between antennas is accurately removed by adjusting the satellite position using the measured phases. As expected, the phase of VHF signals is much more affected by scintillation than the UHF phase. In the example given, the spatial correlation over 130 m falls to zero at VHF and 0.55 at UHF. Correlations over longer distances have been determined using a novel ´phase reconstruction´ technique. In the absence of scintillation the decorrelation distance is ~10 km for both frequencies; with increasing scintillation, it decreases to around 100 m at VHF and 300 m at UHF.