The refractive index of the atmosphere as a factor in tropospheric propagation far beyond the horizon

In order to plan a tropospheric beyond-the-horizon radio link, it is at present necessary to make transmission tests to establish the median path loss and to determine the magnitude and duration of the path loss variations. Recent measurements indicate, however, that the relation existing between transmission loss and the refractive index of the atmosphere may enable estimates to be made, from climatic data alone, of the transmission characteristics of any particular path. Extreme values of transmission loss are believed to be chiefly due to exceptional variations with height of the refractive index, while the monthly median values of path loss have been found to be a function of the average surface values of the refractive index of the atmosphere on the transmission path. Curves are given showing the relation found on various paths between radio transmission loss and the refractive index of the atmosphere.