SCATTERING AND ATTENUATION OF CENTIMETRE AND MILLIMETRE RADIO SIGNALS BY TROPICAL RAINFALL.

Abstract

The theoretical investigation of impairment to millimetre and microwave signals by tropical rainfall is reported. Various numerical techniques such as the point matching, perturbation, coupled azimuthal potentials, least squares fitting, and so on, have been used to compute hydrometeor scattering parameters. In the present work, spheroidal raindrops scatterin parameters were evaluated for linear polarization, using the least squares fitting method which is more applicable to distorted large drops prevalent in tropical rainfall. Typical results of these scattering functions are presented for the frequency range 1-100 GHz for elevation angles of 0°, 50° and 70° for application in both terrestrial and Earth-space systems. Comparison has been made with the ITU-R results based on point matching technique, which is more applicable to dropsizes in the temperate region. The lognormal drop size distributions previously obtained from experimental data at three locations in Nigeria for drizzle, widespread, shower and thunderstorm rainfall have been used together with the computed scattering parameters to study the various impairments to microwave and millimetre waves in a tropical environment. In particular, the specific attenuation and phase shift were investigated for terrestrial and satellite systems. While results presented show that drizzle and widespread rain types degrade microwave and millimetre waves to some extent, shower and thunderstorm rain types on the other hand, degrade these short wavelength signals significantly. The computed results were compared with that of the ITU-R, based on the Laws and Parsons dropsize distribution which is applicable to temperate regions. The shower and thunderstorm results show the largest deviation. In addition, comparison of the results derived from the tropical drop size distributions of Ajayi-Olsen show good agreement with those for the present study, especially for thunderstorm rain over almost all frequencies. Comparison of the computed specific phase shift shows that at frequencies higher than 30 GHz, the temperate models overestimate phase shift significantly. For ease of practical application, especially over short path, the coefficients and exponents of the power law scaling relationship between attenuation, phase shift and rain rate were calculated for each rain type over the frequency range 1-100 GHz.