Propagation measurements and modeling at C band for over-the-water, line-of-sight propagation links in the mid-atlantic coast

Fade, radar, and propagation modeling results are described for two nearby over-the-water, line-of-sight, propagation links operating at 4.7 GHz in the Mid-Atlantic coast of the United States. During a 5 day “intense experimental period” in August 1989 a focused examination of the propagation mechanisms relating to time series fading was conducted. Cumulative fade distributions associated with both propagation paths are presented for the individual days and the overall S-day period. During this period, helicopter measurements of the refractivity structures along the propagation paths, boat measurements of the refractivity structure near the ocean surface at the center of the paths, and radar measurements of the clear atmosphere and ocean backscatter were executed. The refractivity measurements were interfaced with a complex parabolic equation propagation model which incorporated the two-dimensional structure of the refractive index to yield normalized power density structures as a function of range and height for the geometry of the propagation links. These results were compared with the time series fade measurements and provided clear indications of the mechanisms for fading and nonfading events. Azimuthal and vertical radar scans employing a high-resolution radar at the NASA Wallops Flight Facility showed excellent correlation between strong backscatter returns from the ocean surface well beyond the standard horizon and ducting events as evidenced by signal fading along the propagation links.