es, the annual and diurnal variations are generally smaller. present, relevant data for such frequencies are very limited. Work is The prediction of transmission loss, for various frequencies, path in progress to produce prediction models for various categories of lengths, antenna heights, etc. is an important practical problem. An path; e.g. over dense buildings, sparse buildings, woodland, bare example of the kind of data required is given in Figure 9.8. ground, etc. Satellite imagery may assist in this, supplementing data At frequencies above 10GHz, the heavy rain occurring for small on terrain height obtained from detailed maps. Preliminary results percentages of the time causes an additional loss due to absorption, at 1.8GHz suggest that the path losses can be some 20dB or more but the accompanying scatter from the rain may partly offset the greater than at 900MHz when trees and buildings are along the path. effect of absorption. Tropospheric effects in space communications 9/15 Other problems related to fine structure are space and frequency diversity. On a v.h.f. scatter link with antennae spaced normal to the direction of propagation, the correlation coefficient may well fall to say 0.5 for spacings of 5-302 in conditions giving fairly rapid fading. Again, however, varying meteorological factors play a dominant role. In frequency diversity, a separation of say 3 or 4MHz may ensure useful diversity operation in many cases, but occasion- ally much larger separations are required. The irregular structure of the troposphere is also a cause of gain degradation. This is the decrease in actual antenna gain below the ideal free-space value. f = 100 MHz Several aspects of the irregular refractive-index structure contribute to this effect and its magnitude depends somewhat on the time interval over which the gain measurement is made. Generally, the f = 500 MHz decrease is only significant for gains exceeding about 50dB. 9.9 Tropospheric effects in space communications In space communication, with an Earth station a