Tham khảo tài liệu 'geoscience and remote sensing, new achievements part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 448 Geoscience and Remote Sensing New Achievements Eo zo E 0 eJkzo 23 r p2 zo2 0 5 zo 1 1 - T 24 2 z p 25 r - zo V v 1 2 zo we obtain E zo E zo E zo J J f D N D Idz dD 26 0 0 where I J J p e JP p d p d ty 27 0 0 and where F is the radius of the first Fresnel zone. This integral expresses the contributions of all rain drops in the infinite plane perpendicular to the direction of radiowave propagation. In fact the contribution is considered from ashlar of the infinitesimal length in the direction of propagation z axis in our picture E z0 E z0 1 J Lz1 C ko 28 where C J f D N D dD 0 29 Finally 2l r J T C L T lim Az 0 1 J 4 AzC L e k ko 30 where T is the ratio of the attenuated field strength to the non attenuated one transmission f is the complex scattering function The Role of DSD and Radio Wave Scattering in Rain Attenuation 449 Transmission T was computed as a product of a number of L Az ashlars of infinitesimal lengths arranging in the series of the certain length L. The formula for specific rain attenuation a in the dB km unit is then given as 10 logio T a 2 10 3 J Im f D N D dD dB km 31 0 The next parametrical approximation for the drop size distribution - DSD symbol N after Marshall-Palmer is used very frequently for so called average rain - - D N D R 8000 - e R 0 21 mm 1 m 3 32 where R is rain rate or rain intensity in mm h units and D is equivolumetric rain drop diameter see Marshall and Palmer 1948. The same formula 31 was derived through similar way by Van de Hulst 1957 . The mostly used and very simple approximation for specific rain attenuation is this one a a - R b dB km 33 where a k alternatively and b a alternatively are constants depending on frequency polarisation and temperature. An example is shown in next table ITU-R report f GHz 10 12 15 20 25 30 To interpolate a 0 0094 0 0177 0 0350 0 0722 0 1191 0 1789 Logarithmically b 1 273 1 211 1 143 1 083 1 044 1 007 linearly 10. Variability of DSD The majority of existing models estimating the .
