operation, the more significant the atten- uation due to rain. For links working at 12-15 GHz the attenuation may reach 10-12 dB/km, which is significantly high. Transmission planning or microwave link planning engineers should remember that the rainfall rate varies with time and location. Thus, the accuracy of the calculations depend upon how the rainfall was recorded and how stable the cumulative distribution remains over a given period. In general, it has been seen that one year of data has 0.1% of error, while two years of data contains 0.01% of error; if the data are collected for a period of 10-15 years, the error rate reduces to 0.001%. In the absence of any data, it is recommended that the ITU maps be used instead. PRE-PLANNING IN TRANSMISSION NETWORK The attenuation (A) due to rain is given by: A = a R8 (3.13) where R is the rainfall rate and a and B are constants. Both the parameters a and B are defined for spherical drops and they are independent of polarization. The value of these constants depends upon the drop size distribution, canting angle, terminal velocity and the properties of water. During network design these values are not easily available, SO transmission planning engineers are recommended to use the ITU recommendation 838 as it gives the values for these parameters for both horizontal and vertical polarisations. Attenuation also increases with distance and frequency (see Figure 3.14). Thus, in sum- mary, attenuation increases with distance, frequency and rain rate. 38 GHz V 38 GHz H 23 GHz V 23 GHz H 10 GHz V 10 GHz H Distanc e (Km) Figure 3.14 Attenuation due to rain How does rain cause attenuation? Energy is lost from the microwave signal when the raindrops scatter or absorb it. Scattering occurs when a wave impinges on an obsta- cle without being absorbed, SO the effect of the scattering is to change the direction of travel. If the particle is small compared with the incident wavelength, the process is de- scribed by the Rayleigh scattering phenomenon (<10 GHz); when the particle is of