Fog or mist is essentially supersaturated air in which some of the water has precipitated out to form small droplets of water, usually < 0.1 mm in diameter (19). Fog is mainly formed from two processes—radiation and advection (6)—and rarely extends more than 100 m above the ground. For this reason, fog is usually ignored in satellite-to-ground link de­sign for frequencies below about 100 GHz. Clouds, however, are much more complicated.

Several models are available for the prediction of cloud at­tenuation (19-23), but there remain two major difficulties in turning these into link design tools: (1) an accurate cumula­tive distribution of the vertical and horizontal extent, on a global basis, of those clouds that contribute to slant-path at­tenuation; and (2) separating this attenuation contribution from that due to rain formed in some of the cloud processes. Many measurements of rain attenuation implicitly include at­tenuation due to clouds. Understanding, and accurately pre­dicting, the contribution of clouds to path attenuation is a key element in the modeling of the proposed V-band (50/40 GHz) satellite services. Of equal importance will be the develop­ment of modeling procedures that can predict the combined effects of all the attenuating phenomena on an earth-space link (24,25).

Updated: 27.05.2014 — 16:40