Molecular absorption experienced by radiowaves propagating through the Earth’s atmosphere at centimetric and millimetric wavelengths is primarily due to oxygen (O2) and water vapour. Nitrogen (N2) - the most abundant gas (78% by volume of air) - has no microwave lines due to lack of permanent dipole moment but exhibits weak non-resonant absorption due to collision-induced dipole moments. This absorption becomes significant above 100 GHz in very dry conditions. Trace gases (e.g. CO, CO2, N2O and O3) exhibit resonant absorption that can be significant in very dry conditions, especially for frequencies higher than about 70 GHz [Waters, 1976]. Some applications like atmospheric remote sensing and navigation require knowledge of the contribution from these minor constituents.