Surface refractivity gradient data for radio system design

Reliable information about the cumulative distribution of surface refractivity gradient is often required in the design of radio systems. Strong negative gradients, or super-refraction, may lead to interference between terrestrial stations, both terrestrial links and satellite earth stations. Predicted positive or sub-refractive gradients are taken into account in determining the minimum antenna heights, to ensure terrestrial links achieve their required availability. This paper reviews sources of refractivity gradient data, and considers how refractivity gradient at a point may relate to an effective value over a terrestrial path.