The scattering function is used as a measure which characterises transmission channels, in particular high frequency channels. Experiences have proved that this function is at the present time sufficient and in some cases necessary to characterise these channels. The scattering function is adjusted for the high latitude channels (DAMSON experience) as well as for the middle latitude or equatorial zones (STUDIO-LETTI). Results presented in this study are based on the measurement of scattering functions obtained by backscattering ionospheric sounding methods. These methods allow to characterise the different channels existing between an emission point and different receiving points located at variable distances from the emission point.