The Effect of Rain and Water Vapor on Sky Noise at Centimeter Wavelengths

This study is directed toward evaluating the variability in the noise level of sensitive ground-based microwave receivers to be used in radar or space communications. 1 The sensitivity of such receivers is limited by the antenna noise received from the sky due to emission of radiation by the earth's atmosphere and by noise sources in space. The sun and moon and the galaxy are serious noise contributors in space; however, since the sun and moon are seldom in the antenna beam and since galaxy noise is small at centimeter wavelengths, sky noise is due mainly to emission by the earth's atmosphere. The atmosphere contains oxygen and water vapor, both of which absorb and therefore emit radio waves, especially in the microwave band. The oxygen content is relatively constant in time; it therefore produces a sensibly constant background noise. However, the water vapor content and the noise it produces change with location and season. Condensed water, in the form of rain, is a strong absorber of microwaves and contributes a significant amount of noise, whose level changes with time. A brief theoretical treatment outlining the amount of noise to be 1331