760 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



factors as they affect radar signals. Even where weather is not related to a 

 system's prime function, the designer would do well to have some appre- 

 ciation of radar meteorology, if only to safeguard his design from its 

 detrimental effects. 



This section, therefore, although primarily concerned with the airborne 

 weather radar system, will include some discussion of the nature of the 

 meteorological disturbances which are its targets and will describe the 

 utilization of special circuitry designed to display most efficiently the 

 information available in these targets. 



14-10 METEOROLOGICAL EFFECTS AT MICROWAVE 

 FREQUENCIES 



Meteorological Scatterers. One of the effects of precipitation 

 particles suspended in the atmosphere (see Paragraph 4-16) is back- 

 scattering of the radar energy.* The degree of backscattering is usually 

 given in terms of the back-scattering coefficient n. By definition n is the area 

 of an isotropic scatterer which returns an amount of energy in the direction 

 of the receiving antenna equivalent to that returned by a unit volume of the 

 specific weather target. When n is multiplied by the volume of precipitation 

 particles illuminated by the radar beam, the resultant gives the total 

 back-scattered energy. When wavelength (X) is long compared with the 

 scatterer diameter D (droplets are assumed spherical), the Rayleigh 

 scattering approximation gives n in terms of 



„ = 'J^. (14-14) 



The expression SD^ is the summation of the sixth power of the diameters 

 of the scatterers being illuminated and \KY is a factor related to the 

 complex refractive index of the particles. A number of important considera- 

 tions can be drawn from this expression. 



1. Target returns increase as a fourth power of frequency. 



2. Target returns vary as the sixth power of drop diameter for spherical 

 droplets; therefore it is to be expected that rain would provide a much 

 better target than clouds or fog, particularly at frequencies where X » D. 

 Since the determination of drop diameters becomes extremely cumbersome 

 (see Paragraph 4-16) it is desirable to convert the expression to an equiva- 

 lent one in terms of R, rainfall rate in mm hr~^, a parameter much more 

 readily measured:^ 



2:D« = kR^-K ' (14-15) 



^For an excellent study of meteorological echoes the reader is referred to D. E. Kerr, (Ed.), 

 Propagation of Short Radio Waves, McGraw-Hill Book Company, Inc., New York, 1951. 



5D. Atlas and J. S. Marshall, Weather Effects on Radar, Air Force Surveys in Geophysics 

 No. 23, Air Force Cambridge Research Center, December 1952. 



