nt can be on tropospheric propagation. While absorption in atmospheric gases determined geometrically allowing for refraction effects. For this is not a serious practical problem below 40GHz, the attenuation in purpose we can assume that the refractive index, n, decreases rain and wet snow can impair the performance of links at frequen- linearly by about 40 parts in 106 (i.e. 40 N units) in the first cies of about 10GHz and above. Moreover scattering from precipi- kilometre. This is the equivalent to increasing the actual radius of tation may prove to be a significant factor in causing interference the Earth by a factor of 4/3 and drawing the ray paths as straight between space and terrestrial systems sharing the same frequency. lines. The horizon distance d, from an antenna at height h above an The importance of interference-free sites for Earth stations in satel- Earth of effective radius a is given by Equation 9.1. lite links has also stimulated work on the shielding effect of hills and mountains. In addition, the use of large antennae of high gain in d = (2ah)12 (9.1) space systems requires a knowledge of refraction effects (especially at low angles of elevation), phase variations over the wavefront, and For two antennae 100m above ground the total range is about the associated effects of scintillation fading and gain degradation. 82km, 15% above the geometric value. Particularly at higher microwave frequencies, thermal noise radi- Beyond the free-space range, diffraction around the Earth's sur- ated by absorbing regions of the troposphere (rain, clouds, etc.) may face and its major irregularities in terrain is the dominant mode, with be significant in space communication. Much of the current re- field strengths decreasing with increasing frequency and being search is therefore being directed towards a better understanding of typically of the order of 40dB below the free space value at 100km the spatial structure of precipitation. at v.h.f. for practical antenna heights. As the distance increases, the Survey 