Radio noise from different sources introduce a certain unwanted background RF level at the input stage of any receiver that the wanted signals have to overcome for successful reception. Recommendation ITU-R P.372 defines the term radio noise as well as its different sources and states average values for each source separately. Below 1 GHz, noise from one or more of the following sources may be dominant, depending on the frequency: Galactic noise; Atmospheric noise due to lightning; Man-made noise (MMN).
ITU-R REPORT SM.2155-2009 Referenced Document
ITU-R P.311 Acquisition@ Presentation and Analysis of Data in Studies of Tropospheric Progagation