Introduction This Report presents a method of calculating attenuation@ noise temperature@ and link performance@ based on ITU-R Recommendations concerning the characteristics of radio wave propagation. The method can be extended to include equipment characteristics. The method includes consideration of some effects that become important for systems with noise temperatures below about 30 K when operated in frequency bands below about 15 GHz. These factors are cosmic background noise@ galactic noise@ and the effect of atmospheric attenuation on their values. For many other systems with higher noise temperatures@ these factors may be neglected. The method will be explained in the context of providing data for the selection of preferred frequency bands on the basis of link performance.