Radar performance during propagation fades in the mid-Atlantic region

Periodically through the year, an Atlantic coast vessel traffic control radar, located near the entrance to the Delaware Bay, observes a reduction in detection range from 37 (20 nmi) to about 17 km (9 nmi). Sometimes ships can be seen visually from the radar tower before they can be observed on the radar screen. The reduction in the radar detection range usually lasts several hours and occurs often when fog is present. An investigation of this phenomenon, referred to as a radar deep fade (RDF), was undertaken to find out what causes the fade and to determine the best method of increasing the radar detection range when one occurs. The investigation indicated that these RDFs arose during the presence of a propagation condition known as subrefraction, which is an increase in the atmospheric index of refraction with altitude above the ocean surface at microwave frequencies. This increase in the index of refraction with altitude results in the effective radius of the Earth being much smaller than the usual 4/3 Earth radius that is observed for radar during standard atmospheric conditions. During subrefractive conditions, the effective Earth's radius can be between its true radius and one-half of its true radius. As a result, the radar horizon is shortened and the radar detection range is reduced. These RDFs will occur during propagation conditions known as sustained deep fades (SDFs), which arise when subrefraction lasts for more than two hours and causes one way losses exceeding 20 dB relative to free-space for line-of-sight (LOS) geometries. Subrefraction is caused by the movement of subtropical moist air over the cold ocean surface. The recommended solution for increasing the radar range in the presence of subrefractive conditions is to place the radar on a higher tower than the present 19-m-high tower, one that is perhaps 60-m high. This would increase the radar range by 50% when the Earth's effective radius is half of normal. This increase in radar height also usually increases the radar detection performance when skip zones associated with surface-based ducts occur or when evaporation ducts (EVDs) are present.