of ments using satellite transmissions. Special emphasis has been space links difficult and emphasises the value of measured data. given to frequencies between 10 and 30GHz, in view of the effect Some results obtained using the Sun as an extraterrestrial source are of precipitation on attenuation and system noise. Details are given shown in Figure 9.10. in Report 564 of the CCIR. For typical elevation angles of 30° to 45°, the total attenuation exceeded for 0.01% of the time has values of the following order: f=12GHz: 5dB in temperate climates (e.g. northwest Europe) 20dB in tropical climates (e.g. Malaysia) f = 37 GHz 10dB in East Coast USA (Maryland) f=20GHz 10dB (northwest Europe) f = 19 GHz Site-diversity experiments using satellite transmissions show that site spacings of the order of 5-10km can give an useful improvement in reliability. However, the improvement may depend on the site geometry and on topographical effects. At frequencies above say 15GHz, the advantage of site diversity may be quite small if the sites are SO chosen that heavy rain in, for example, frontal systems tends to affect both sites simultaneously. Frequency re-use is envisaged, in space telecommunication sys- items, by means of orthogonal polarisation. But this technique is restricted by depolarisation due to rain and ice clouds and, to a lesser % of time extent, by the system antennas. Experimental data on polarisation distortion, obtained in satellite experiments, are given in CCIR Figure 9.10 Measured probability distribution of Report 564. attenuation A on earth-space path at 19GHz (southeast Further data are still needed, especially from satellite trans- England: elevation angles 5° to 40°. Data from solar missions and at frequencies up to at least 40GHz. In this context, the tracking radiometers) availability of signals from the Olympus satellite at 12, 20 and 30GHz, from 1989 onwards, is important. Measurements at 20, 40 Scatter from rain (and ice crystals at and above the freezing level and 50GHz on the ITALSAT satellite are a