3Troposphere (non-clear air)

• rain effects

  • attenuation
  • depolarisation
  • scattering

• cloud effects

  • attenuation

• system availability considerations

  • high availability system
    • rain significant (e.g. at 0.1% time)
  • low availability system
    • cloud significant (e.g. at 10% time)

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• Third principal propagation category – precipitation.

• Well-known that rain attenuates radiowaves – effect becomes significant >~6 GHz.

• Also, depolarising effects – systems using two polarisation states for frequency reuse rely on discrimination between them; depolarising effect of rain reduces discrimination potentially causing interference.

• Scattering – can give rise to interference path between two stations on Earth’s surface.

• All these effects need to be taken into account for planning systems at >~ 6 GHz; rain can be limiting factor in providing high availability service.

• Cloud – attenuation due to liquid water held in suspension – may seem insignificant compared with rain but need to appreciate time statistics of rain and cloud.

• Never rains for >~5 % time! Cloud present perhaps up to 50% time.

• For system planned for high availability (e.g. 99.9% time), rain will be dominant mechanism; although cloud attenuation occurs, its contribution is insignificant compared with rain.

• For system planned for low availability (e.g. 90% time), e.g. local system using VSATs where user is content to wait for up-time, then rain is irrelevant since it never rains for 10% time. However, cloud is important which, although causing lower attenuation than rain, might account for ~1 dB which is important for link budget of VSAT system.