338 of the CCIR gives the following values for a frequency of 4GHz, for the worst month of the year, for average rolling terrain in northwest Europe: 0.01% of time: Figure 9.6 Tropospheric propagation modes and path length 20km; 11dB or more height-variation index, n below free space path length 40km; 23dB or more gases becomes increasingly important. This factor may determine below free space system design and the extent to which co-channel sharing is possible; for example, between terrestrial and space communication Over very flat, moist ground the values will be greater and, for a services. There are strong absorption lines due to oxygen at 60 and given path length, will tend to increase with frequency. But at about 119GHz, with values of attenuation, at sea level, of the order of 15 10GHz and above, the effect of precipitation will generally domi- and 2dB km respectively. At 183GHz, a water vapour line has an nate system reliability. attenuation of about 35dB km ¹. Between these lines there are The magnitude of attenuation in rain can be estimated theoreti- 'windows' of relatively low attenuation; e.g. around 35, 90 and cally and the reliability of microwave links can then be forecast 140GHz. These are the preferred bands for future exploitation of the from a knowledge of rainfall statistics. But the divergence between millimetric spectrum, such as short-range communication systems theory and experiment is often considerable. This is partly due to the or radar. Further details are given in Report 719 of the CCIR. variation which can occur in the drop-size distribution for a given rainfall rate. In addition, many difficulties remain in estimating the 9.8.1 Ground-wave terrestrial propagation intensity and spatial characteristics of rainfall for a link. This is an important practical problem in relation to the possible use of route When the most distant maximum in Figure 9.5 occurs at a distance diversity to minimise the effects of absorption fading. Experimental small compared with the optical range, it is oft