Space-earth Propagation

In the early years of satellite communication the cost per watt of radio frequency power from a satellite transmitter was very high. Technical developments have reduced that cost, but economy in the use of satellite power remains a factor that dominates system design. Thus, the power of the carrier wave that is intended to reach an earth station receiver is determined rather precisely, to provide an adequ­ate carrier to noise ratio for a sufficiently high proportion of the time, but no more.

The free space transmission loss from a geostationary satellite to an earth station is very much bigger than the corresponding loss for any terrestrial line of sight path. Figure 51.7 shows that a typical value for this loss is 204dB at l0GHz; this falls to 196dB at 4GHz and rises to 214dB at 30GHz. This loss is the principal factor determining what the satellite power level will be. However, a margin of power must be provided in addition to ensure that channel performance targets are reached. This margin provides in part for under achievement of equipment performance and operating targets and in part for propagation loss, due to absorption in the tropos­phere, which is additional to the free space value.

The principal cause of tropospheric loss at frequencies in use or in prospect of use for satellite communication is absorption in rain. Molecular resonance in water vapour begins to be significant as the frequency rises above 20GHz. The extent of the loss due to rain varies greatly, with frequency and climate but also with the angle of elevation of the satellite as seen from the earth station, and Figure 51.8 can give no more than a very general indication of how these losses vary with frequency under given weather conditions.

The margin of satellite carrier power that must be allowed, to protect link performance against the effects of rain, depends signi­ficantly on the percentage of the time during which deterioration of link performance can be accepted.

In reading Figure 51.8, it may be noted that a rainfall rate of lOmm/hour can be expected in many temperate climates for 0.1% of the time and that rates of 65mm/hour are found for 0.1 % of the time in tropical high rainfall areas.

Significant depolarisation of signals arises in heavy rain, and it may be necessary to use adaptive compensation for this at earth stations in rainy climates which depend on a large measure of polarisation discrimination.

Дата добавления: 2015-07-20