Fade statistics and propagation events at C band for two overwater, line‐of‐sight propagation paths over a 1‐year period

Abstract

We examine signal fading statistics over a 1‐year period corresponding to two overwater, line‐of‐sight, propagation links in the mid‐Atlantic coast of the United States. These links are composed of a transmitter on a tower at Parramore Island, Virginia, operating at 4.7 GHz, sending simultaneous CW signals to two receiver systems located on a lighthouse and a lookout tower on Assateague Beach, Virginia, at distances of 44 and 39 km, respectively. The receiving sites are separated by approximately 5 km. Cumulative fade distributions corresponding to yearly, monthly, and diurnal time scales were derived. The annual distributions showed fades in excess of 49 and 54 dB during approximately 7.7 hours of the year, for the lighthouse and lookout tower links, respectively. Fade duration statistics corresponding to sustained attenuation events were also derived. These events, which were arbitrarily defined as having fades relative to free space powers in excess of 20 dB for durations of 2 hours or more, are believed to be generally due to extreme subrefractive conditions. Analysis of synoptic weather conditions and nearby rawinsonde data during four sustained deep fading periods showed atmospheric conditions consistent with extreme subrefraction, where the refractivity height profiles had positive lapse rates. A measured refractivity profile, having a positive lapse rate, was injected into a propagation model based on an approximation to the scalar Helmholtz wave equation, and fade levels in the vicinity of the antenna locations were calculated. Although the refractivity profile was measured during an earlier period in the vicinity of the links, it was obtained over the water and showed an extreme positive lapse rate in refractivity, indicating that such profiles do in fact occur. Furthermore, the calculated fades were noted to have the same range of values as measured during the sustained deep fade events, showing consistency with the hypothesis that severe subrefraction may be the cause of these events.