Three years of C band signal measurements for overwater, line‐of‐sight links in the mid‐Atlantic coast: 2. Meteorological aspects of sustained deep fades

Abstract

The morphology of deep and long‐lived fades for line‐of‐sight, overwater propagation links in the mid‐Atlantic coast are examined. Such events, known as sustained deep fades (SDF), are analyzed employing weather maps, in situ measurements from radiosondes, an instrumented helicopter, and sensors on coastal platforms. These events occurred exclusively from November through July over the 3 years in which fade statistics were amassed; no SDF events were observed during the months August, September, or October. The SDF events biased the 3‐year fade statistics described in a companion paper. The results have demonstrated that synoptic weather conditions created by a sustained high‐pressure system over the subtropical Atlantic and occasionally by a sustained low‐pressure system whose center lies west of the coastal region result in a steady flow of warm, humid air at higher altitudes. When coupled with colder water conditions, such a flow results in a surface temperature inversion, an increase of water vapor pressure with altitude, and a positive lapse rate of the radio refractivity. This condition leads to severe subrefraction resulting in fades ranging from 20 to 60 dB for durations in excess of 2 hours and lasting as long as 2 days. Propagation modeling using a refractivity profile derived from sensors on board a helicopter during an SDF event predicted the same range of fades as were measured during that period.

K factor statistics for subrefraction in the mid‐Atlantic coast of the United States

“…Application to higher-altitude antennas (i.e., above 45.5 m) would implicitly be assuming that the refractivity profiles contributing to the derived distributions continued to have subrefractive gradients at higher altitudes. In fact, refractivity profiles measured during subrefractive events suggest that the subrefractive layer has a typical height of 100-300 m, above which the refractive profile is standard [Goldhirsh et al, 1994b;Babin, 1995].…”

“…An examination of weather maps during severe subrefractive events have, however, demonstrated that this phenomenon does extend over significant portions of the east coast of the United States [Goldhirsh et al, 1994b].…”

“…Three years of signal level statistics for two line-ofsight links in the mid-Atlantic coast have been described previously by Goldhirsh et al [1994aGoldhirsh et al [ , 1994b. The signal level statistics were heavily biased by extreme subrefraction defined such that the refractivity N increases with altitude.…”

“…This phenomenon is attributed to the meteorological condition where warm moist air from subtropical regions is advected over colder water, resulting in a temperature inversion. The temperature inversion and the presence of moisture at altitude result in the condition where the vapor pressure near the Earth's surface increases with height above the surface, giving rise to a positive lapse rate of reftactivity with altitude [Goldhirsh et al, 1994b]. Although the examined fade statistics and associated The bending effects of propagating rays may be partially accounted for by replacing the Earth's radius "a" by an effective radius "Ka," where K (hereinafter called the K factor) is a constant that may be smaller or greater than 1.…”

Signal level statistics and case studies for an over‐the‐horizon mid‐Atlantic coastal link operating at C‐band

“…With the availability of advected subtropical moisture at the higher altitudes, a local increase of water vapor pressure usually occurs. This combination of temperature and water vapor pressure inversions results in a positive lapse rate of the refractivity and the condition of extreme subrefraction [Goldhirsh et al, 1994b]. This type of phenomenon affects not only communications but also radar detection capabilities.…”