V/UHF space radars: spatial phase decorrelation of transionospheric signals in the equatorial region

The spatial decorrelation of V/UHF signals by equatorial ionospheric turbulence is studied using 150 and 400 MHz signals transmitted from low-earth-orbiting beacon satellites. The signals are monitored on a linear array of spaced antennas located on Ascension Island, and processed coherently to determine the cross-correlations of the phases of the received signals. Analyzing signals from the low-inclination satellite C/NOFS has provided an opportunity to investigate the correlation variations in and out of scintillating structures. As a necessary step, the geometrical component of the phase difference between antennas has been accurately removed by adjusting the satellite orbital information using the measured phases. In order to unambiguously measure the spatial phase correlation without any temporal effects, the phase cross-correlation was calculated as the cross-correlation function of time-synchronized signals. As expected, the VHF signals were more affected by scintillation than the UHF signals. When the signal propagated through patches of strong scintillation, the VHF signal became completely uncorrelated over an ionospheric distance of 130 m, while over the same distance the UHF phase correlation decreased to 0.55. The time-synchronized technique limited the spatial variations assessed to east-west distances of similar to 300 m. To extend this range, a novel 'phase reconstruction' technique was developed to link arrays of samples together. In the absence of scintillation the measured decorrelation distance is similar to 10 km at both frequencies, but with increasing scintillation, the decorrelation distance falls to similar to 100 m at VHF and 300 m at UHF. A clear relation between the decorrelation distance of the measured phase and S-4 is observed and a simple empirical model has been derived.