Radar-derived path reduction factors for terrestrial systems

Attenuation due to rain becomes an increasingly important factor as frequency increases above 5 GHz. For terrestrial links operating at high microwave and millimetre wave frequencies, it is considered to be the dominant mechanism affecting system availability. At frequencies above 30 GHz, cloud and fog will also become a significant factor, while in high latitude regions, or high altitude locations in lower latitude regions, attenuation due to melting snowflakes can be considerable. However, the only effect for which ITU-R provide a full statistical prediction model is that of rain. This is contained in Recommendation 530-5, section 2.4. A particular aspect of the model considered is the method used to allow for the inhomogeneity of rainfall. This is achieved through a quantity called the 'effective path length', which is shorter than the actual path length of interest. It is over this 'effective path length' that attenuation, based on point rainfall statistics, is assumed to be uniform. Experimental verification of this procedure is difficult, because it requires several links with different path lengths to be located in close proximity. An alternative approach is through the use of radar data to obtain attenuation statistics for simulated links of various lengths. This approach is adopted.