Radio Duct

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Radio Duct

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A rather shallow, almost horizontal layer in the atmosphere through which vertical temperature and moisture gradients are such as to produce an index of refraction lapse rate of greater than -48 N-units per 1000 feet. Strong temperature, or moisture inversions, or both are necessary for the formation of radio ducts. The resulting superstandard propagation is such as to cause the curvature of rays traveling through it to be greater than that of the earth. Radio energy which originates within the duct and leaves the antenna at angles near the horizontal may thus be trapped within the layer. See anomalous propagation, skip effect. </dd>
The effect is similar to that of a mirage (it is sometimes called radio mirage), and radar targets may be detected at phenomenally long ranges if both target and radar are in the duct. The greater the elevation angle between radar and target, the less the possibility of serious distortion due to transmission through ducts. Ducts may be surface based or elevated, with thickness ranging from a few tens of feet up to a maximum of 1000 feet. Elevated ducts are generally associated with subsidence or frontal inversions and are seldom found above 15,000 to 20,000 feet. [[/a>|/a> ]]

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use