-4-17] 



ATTENUATION BY PROPELLANT GASES 



231 



Calculations of attenuation and back-scattering (radar area) for spherical 

 drops have been made by Haddock"*" on the basis of the drop size distri- 

 butions of Laws and Parsons. "^^ These are reproduced in Figs. 4-48 and 4-49. 

 The total radar area is found by 

 multiplying the value found in Fig. 

 4-49 by the volume of precipitation 

 illuminated by a pulse length. If the 

 entire antenna beam is filled with 

 precipitation, then this volume is 

 R^^QL. The curves in these figures 

 may be extended to longer wave- 

 lengths by assuming a dependence 

 as X-*. 



Snow is a mixture of air and ice. 

 Since the refractive index of ice is 

 much smaller than that of water, the 

 scattering and attenuation due to 

 snow are less than those of a corre- 

 sponding mass of water. However, 

 when a snow flake begins to melt, it 

 becomes coated with a thin film of 

 water. The scattering and absorp- 

 tion then become almost the same 

 as a water particle of the same size 

 and shape and thus increase greatly. 

 This effect has been advanced as the 

 explanation for the radar "bright 

 band" observed at or near the freezing level 



10.0 



1.0 



0.1 



0.01 

 5 

 3 



0.001 



100 5 3 



X(cm) 



Fig. 4-48 The Variation of Attenuation 

 with Wavelength for Various Rainfall 

 Rates. 



4-17 ATTENUATION BY PROPELLANT GASES 



In the transmission of information between a missile and its ground 

 control station, the flame of the propellant gases lies in or near the path 

 between the missile antenna and the ground station antenna. Attenuation, 

 reflection, and refraction of the radio waves by the flame then are an 

 important factor in determining the performance of the radio channel. A 

 discussion of the nature of this problem appears in the Guidance volume of 

 this series. ^^ 



^"F. T. Haddock, Scattering and Attenuation of Microwave Radiation Through Rain, Report 

 of NRL Progress, June 1956. 



*1J. O. Laws and D. A. Parsons, "The Relation of Raindrop Size to Intensity," Trans. Am. 

 Geophys. Union 24, 452-460 (1943). 



42A. S. Locke fEd.), Guidance, pp. 118-124, D. Van Nostrand Co., Inc., Princeton, N. J., 

 1955. 



