The ITU Radiocommunication Assembly considering a)that, in the planning of a digital communication network, it is necessary to define the overall system performance and availability achieved for a given percentage of the time;b)that designers of radio systems in the VHF (metric), UHF (decimetric) and SHF (centimetric) bands require to know, from the viewpoint of sustained satisfactory operation, the tropospheric propagation characteristics and the resulting transmission loss that is not exceeded for a large percentage of the time for each particular frequency band, over the distance corresponding to the service range, which may extend from about 100 km to more than 500 km.