electrical fluctuations in a receiving antenna caused by the heat radiation of the earth in the radio spectrum, which, like atmospheric noise, worsen the quality of radio reception (seeFLUCTUATIONS, ELECTRICAL).
A quantitative characterization of terrestrial noise is given by the noise temperature, which is equal to the product of the physical temperature of the absorbing layer of the earth (≈300°K) and the coefficient of absorption of this layer. The latter depends on the orientation of the antenna with respect to the earth. Terrestrial noise is comparatively great at small altitudes of the main lobe of the radiation pattern of a high-directional antenna placed on the earth and in those cases where an antenna mounted on a low-flying object is directed at the earth. If the main lobe is directed at the zenith, terrestrial noise is usually negligible.