Through the use of opposite polarisations two different signals can be transmitted on the same frequency without interference. Under adverse weather conditions this may not hold. At satellite bands of 12 GHz or higher rain and other forms of atmospheric water can have drop sizes that are significant in relation to the wavelength of the satellite transmissions. Signal depolarisation can occur because the raindrops are non-spherical and this causes differential attenuation and scattering of incident microwave signals.
The term used to describe the effect is Cross Polar Discrimination (XPD). It defines the dB level of a signal that starts out as one polarisation but finishes up as the opposite polarisation due to atmospheric effects.
The graph below provides an illustration of the problem.
Figure 26: Cumulative frequency distribution diagram for a series of XPD measurements performed in the USA [Ref 9].
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