92 THE DEVELOPMENT OF WEAPONS SYSTEM REQUIREMENTS 



of the order of that indicated in Fig. 2-25 should be adequate for the tactical 

 problem. 



One other problem related to vertical beamwidth is of great importance 

 to the AEW problem, namely ground and sea return. Fig. 2-25 shows that 

 the fan beam intersects the surface of the water — or land, as the case may- 

 be — at all ranges greater than 10 n.mi. Thus reflections from the ground 

 will compete with target signal reflections at all ranges greater than 10 n.mi. 

 This fact requires that means be provided in the AEW system to distinguish 

 between returns from the surface of the earth and target returns. 



Interactions between the radar, the target, and the ground constitute 

 a very complicated problem. The polarization of the radar transmission, 

 surface characteristics, and AEW and target altitudes all interrelate to 

 produce nulls and reinforcements which influence the system capability. 

 These factors are discussed in some detail in Chapter 4. 



In an actual design study, the quantitative aspects of this problem should 

 be carefully studied and set forth at this point of the systems requirements 

 development. The interrelations of the tactical geometry and propagation 

 and scattering characteristics must be ascertained to define the magnitude 

 of the problem implied by the requirements for distinguishing between 

 ground and target returns. 



2-19 FACTORS AFFECTING HEIGHT-FINDING RADAR 

 REQUIREMENTS 



The height-finding radar for the example problem is positioned in the 

 aircraft nose. It is directed to point in a given azimuth direction at a target 

 located by the fan-beam radar. It is then nodded up and down to determine 

 the elevation of the target with respect to a horizontal reference in the AEW 

 aircraft. The nodding action causes the target return to vary as a function 

 of the space (or angle) modulation characteristic (elevation) of the height- 

 finding beam. The particular type of space modulation characteristic that 

 is used depends upon the accuracy requirements of the height finder. 



The requirements of the height-finding radar are dictated primarily by 

 the following tactical considerations: 



1. The characteristics of the expected threat including possible varia- 

 tions from the specified values. These characteristics include speed, 

 altitude, and number of aircraft. 



2. Height-finding requirements during threat evaluation. 



3. Height-finding requirements during vectoring. 



4. Height-finding requirements dictated by the need to supply early 

 information to the ground-to-air missile system. 



To meet these requirements within the limitations of the hypothesized 



