Improved models for long-term prediction of tropospheric scintillation on slant paths

The prediction models for tropospheric scintillation on earth-satellite pat hs from Karasawa, Yamada, and Allnutt and ITU-R are compared with measureme nt results from satellite links in Europe, the United States, and Japan at frequencies from 7 to 30 GHz and elevation angles of 3 to 33 degrees. The e xisting prediction models relate the long-term average scintillation intens ity to the wet term of refractivity at ground level. The comparison shows t hat the seasonal variation of scintillation intensity is well predicted by this relation, but for the annual average some additional meteorological in formation is needed. A much better agreement with measurement results is fo und when a parameter representing the average water content of heavy clouds is incorporated. This confirms the assumption that scintillation is, at le ast partly, associated with turbulence inside clouds. The asymmetry between the distributions of signal fade and enhancement can also be explained by turbulence inside clouds. The asymmetry depends on the intensity of the sci ntillation, which is consistent with the theory assuming a thin layer of cl oudy turbulence. A new model based on this theory predicts the distribution s of signal fade and enhancement significantly better.