14-18] MAJOR CHARACTERISTICS AND COMPONENTS 779 



This value could be increased to about 4000 pps if use is made of the fact 

 that no returns occur in the 0-10-mile range interval; thus, this interpulse 

 space could be used for returns from a 20-30-n.mi. interval. 



Frequency of Transmission. Resolution requirements and weather 

 attenuation effects combine to restrict pulse-type ground mapping radar 

 transmission frequencies to X band and K band. The latter frequency has 

 the advantage of producing a smaller beamwidth for a given antenna size; 

 this fact must be weighed against increased susceptibility to weather effects. 

 Unfortunately, the effect of weather on the tactical utility of a ground- 

 mapping system is most difficult to evaluate. The issue is further compli- 

 cated by the fact that both X-band and K-band operations are adversely 

 affected by heavy cloud cover (it will be recalled that X band is used for 

 weather mapping). For a given application, the designer is well advised to 

 obtain as much experimental information as possible, because experience 

 has shown that a purely analytical approach (using the type of data 

 presented in Chapter 4) can be misleading in many cases. 



In considering doppler techniques for mapping, the choice of frequency 

 becomes more straightforward; the premium on a very narrow beamwidth 

 is reduced. In general, X band is preferable for such applications. 



Polarization. Most present-day radar mapping systems employ 

 linearly polarized beams. Certain types of terrain, particularly rough seas 

 at small depression angles, exhibit some difference between their reflective 

 properties for vertically and for horizontally polarized radiation. This 

 difference has been suggested as the basis of a possible means of identifying 

 different types of terrain, and some experimental investigations have been 

 made. However, the results were inconclusive and it appears that consider- 

 ably more study is necessary to determine the possibilities of this appraoch. 



Beam Configuration. The fan beam is used in most systems designed 

 primarily for mapping. Its characteristics make it particularly suited for 

 the side-look system since the motion of the aircraft provides the scanning. 

 In most systems the beam power pattern is so shaped in the vertical plane 

 that the intensity of the ground return is nearly independent of range. 

 The power gain within this pattern, known as a cosecant-square beam, is 

 expressed usually as 



G{d) cc csc^^Vcos^- (14-31) 



Additional terms are sometimes included to account for the change in 

 atmespheric attenuation due to the varying range to the ground. 



The derivation of Equation 14-31 assumes that the ground is an array of 

 closely packed isotropic scatterers, the incident radiation being scattered 

 equally in all directions. It assumes also that the area to be considered as 



