 (1990a) Radiometeorological Data, CCIR Report 563, conductivity is low. A comprehensive set of these curves is to be XVIIth Plenary Assembly, V, Dusseldorf. found in CCIR (1990d). CCIR (1990b) Cross-Polarisation due to the atmosphere, CCIR Report 722, XVIIth Plenary Assembly, V, Dusseldorf. 6.6.7 The ground conductivity CCIR (1990c) Propagation data required for space telecommunica- tions systems, CCIR Report 564, XVIIth Plenary Assembly, V, It is necessary to add a few words about the way a conductivity Dusseldorf. figure is ascribed to the Earth for the present purposes. In the case CCIR (1990d) Ground-wave Propagation Curves for Frequencies where the ground is uniform and composed of similar material at all between 10kHz and 30MHz, CCIR Recommendation 368, depths, and composed of material having a conductivity which XVIIth the Plenary Assembly, V, Dusseldorf. remains constant independent of frequency, the meaning of the term is fairly clear. However, the geological structure of the real Earth CCIR (1990e) Electrical Characteristics of the Surface of the Earth, has considerable variations in the constituent materials near the CCIR Report 229, XVIIth Plenary Assembly, V, Dusseldorf. surface. To determine the depths to which the characteristics of the CCIR (1990f) Attenuation by Atmospheric Gases CCIR Report 719, Earth's surface are significant, the 'skin depth' of the current flow XVIIth Plenary Assembly, V, Dusseldorf. must be calculated. This skin depth varies between 70m for low frequencies and poor ground conductivity to 20m for high frequen- Chu, T.S. (1980) Analysis and prediction of cross-polarisation on cies and more conductive ground. For the sea, with its greater Earth-space links. In Proc. URSI (Commission F), International conductivity, the skin depth at medium frequencies is less than 1m. Symposium on Effects of the lower atmosphere on radio propa- The effective conductivity, for land, is best taken as a mean value gation at frequencies above 1GHz, Lennoxville, Canada. averaged over the values obta