Rain is the major variable in the design of 12 GHz (and higher) satellite link budgets. The statistical variation of rainfall and the fact that rain rate prediction models are still being refined to achieve better agreement with observed data mean that rain attenuation predictions should only be considered as giving a general guide to system performance. Rain attenuation prediction like weather prediction is not an exact science.
Attenuation due to rain depends on frequency and the rainfall intensity, or rain rate. Rain rate data is presented in units of mm/hr but in practice it is measured with a tipping bucket rain gauge over intervals of typically 5 minutes. Rain rate data has been collected by the Bureau of Meteorology for a selection of sites around Australia. Rainfall rate is highly variable, both in terms of location and time (includes diurnal, seasonal and yearly variations).
Rain attenuation, because of the statistical variation in rainfall rates, is given as a probabilistic value. This is often expressed as a rainfall rate exceeded for a certain percentage of the time (usually 1% of the worst month). These rainfall attenuation rates can be presented as contours on a map of Australia (see figure 23).
Fig 23: Chart showing the rainfall attenuation rates for 1 % of the time for the worst month [Ref 10].
One method of predicting rain attenuation at a particular location is as follows:
It is notable that Flavin's work has shown that for elevation angles above 30o there is little dependence of attenuation on elevation angle.
Figure 24: Graph showing the strong dependence of specific attenuation (dB/km) with rain rate and frequency [Ref 2].
It is worthwhile noting that rain attenuation is strongly dependent on frequency. Rain attenuation at 4 GHz is minimal even for very heavy rain but at 12 GHz it can be a significant factor.
To overcome the effects of severe localised rain fades at important reception (or transmission) sites it is possible to interconnect two earth stations separated by a distance greater than the likely size of a rain cell. This arrangement is called space diversity reception(transmission). If a rain cell is likely to affect reception (or transmission) from one earth station the signals are switched to the second earth station which statistically speaking is very unlikely to be suffering the same rain fade conditions. This sort of arrangement is of greatest value for transmission (uplinking) because fading at the uplink site will affect all receiving sites. Rain fades on the downlink however only affect the local area around the rain cell.
Next Section
Return to Index