Attenuation by Atmospheric Gases

A radiowave propagating through the Earth’s atmosphere will experience a reduction in signal level due to the gaseous components present in the transmission path. Signal degradation can be minor or severe, depending on frequency, temperature, pressure, and water vapor concentration. In this chapter the effects of atmospheric gases, primarily oxygen and water vapor at space communications frequencies, are discussed, and methods for calculating the expected attenuation for a radiowave link are presented. Examples are given for atmospheric attenuation at several of the frequencies of interest for space communications systems. Atmospheric gases also affect radio communications by adding atmospheric noise (i.e., radio noise) to the link. This problem is described fully in Chapter 7.