A modification to previous ITU-R simplified model for gaseous attenuation

The attenuation due to atmospheric gases is essential to radio system designs above 10 GHz. The specific attenuation can be evaluated accurately by the MPM model (microwave propagation model). However, this model is very complicated. For quick estimation, a simplified model has been recommended in versions 1 and 2 of Recommendation ITU-R PN.676, which is based on curve-fitting to the MPM model. Unfortunately, neither the simplified model nor a possible model derived from it and the zenith attenuation equation in the Recommendation applies to higher altitudes. Therefore a modification is presented. The modification is still based on the MPM model with interest mainly in the frequency range below 300 GHz. Simplification is made first for the terms of the isolated absorption lines and continuum spectra in the MPM model, in which the components of the sum of the line frequency and radio frequency in the terms are neglected. The contribution of the dry air in the band near 60 GHz is fitted to the curve from the MPM model, in which two parts are treated first for below 57 GHz and above 63 GHz, respectively, and then linked by an interpolation polynomial. For both dry air and water vapour, the initial form of the specific attenuation is determined first for the standard atmospheric condition at sea level, and the final form obtained by corrections for the effects of the temperature, pressure and water vapour density.