This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 5 M

Year: May 2013

Rain Attenuation:

1. Water droplets scatter and absorb impinging radiation, causing attenuation.
2. Rain effects become most se at wavelength approaching the water drop size, which is dependent on the type of rainfall.
3. In heavy rains raindrop size may be a cm and serve absorption may occur at frequency as low as 10 GHZ. Thus rainfall effects can become extremely severe at satellite frequency X band and above.
4. If a satellite link is to be maintained during a rainfall, it is necessary to have enough extra power called power margin to be transmitted to overcome the maximum attenuation induced by the rain hence it is necessary to assess expected rain loss while evaluating link parameters.
5. To evaluate the additional rain loss we first obtain expected rainfall in millimeter/hour for the region of the communication link, and then we use the curves as shown in fig 4.3 to read off db loss per path length at operating frequency.
6. The rainfall attenuation used is of form

$Db loss/length=Ar^b$

$r=rainfall$

$A and b are frequency dependent coefficient$