 Km) Frequency (GHz) Figure 51.7 Transmission loss and delay as a function of Figure 51.8 An approximate indication of tropospheric satellite altitude absorption on space-earth paths In reading Figure 51.8, it may be noted that a rainfall rate of 10mm/hour can be expected in many temperate climates for 0.1% 51.4.2 Space-earth Propagation of the time and that rates of 65mm/hour are found for 0.1% of the time in tropical high rainfall areas. In the early years of satellite communication the cost per watt of Significant depolarisation of signals arises in heavy rain, and it radio frequency power from a satellite transmitter was very high. may be necessary to use adaptive compensation for this at earth Technical developments have reduced that cost, but economy in the stations in rainy climates which depend on a large measure of use of satellite power remains a factor that dominates system design. polarisation discrimination. Thus, the power of the carrier wave that is intended to reach an earth station receiver is determined rather precisely, to provide an adequ- 51.4.3 The transponders ate carrier to noise ratio for a sufficiently high proportion of the time, but no more. The first INTELSAT IV satellite was launched in January 1971 but The free space transmission loss from a geostationary satellite to the design plan of the transponders for these satellites was highly an earth station is very much bigger than the corresponding loss for innovative (Jilg, 1972) and it has remained a basic model for most any terrestrial line of sight path. Figure 51.7 shows that a typical satellites that have followed. The satellite used the 6GHz band for value for this loss is 204dB at 10GHz; this falls to 196dB at 4GHz up-links and the 4GHz band for down-links. Figure 51.9 shows the and rises to 214dB at 30GHz. This loss is the principal factor basic elements. determining what the satellite power level will be. However, a Signals in the band 5.932GHz to 6.418GHz from the receive margin of power must be provided in addition to ensure 