The ITU Radiocommunication Assembly, considering that free-space propagation is a fundamental reference for radio-engineering, recommends that the methods in Annex 1 be used for the calculation of attenuation in free space.
ITU-R P.525-2 FRENCH-1994 Referenced Document
ITU-R P.368 Ground-Wave Propagation Curves for Frequencies between 10 kHz and 30 MHz*, 2007-01-01 Update