774 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



map as viewed on the display. In other systems the map may be 

 viewed directly on film from a high-speed recorder. 



14-17 SYSTEM CONSIDERATIONS 



As with all radar systems, the design of a ground mapping radar system 

 involves a series of compromises between requirements and state-of-the-art 

 limitations. Some instances represent a simple choice; others involve a 

 number of system parameters. The choice may represent a trade-ofF 

 between a desired capability and system size, weight, and complexity, or it 

 may represent a fundamental limitation imposed by laws of nature. Some 

 of the most important system considerations for ground mapping radar 

 systems involve: 



1. Resolution 3. Range and operational altitude 



2. Accuracy 4. All-weather capability 



Just as accuracy is the basic measuring stick of navigation systems 

 performance, resolution is the basic measuring stick of performance for 

 radar ground mapping systems. Resolution, a measure of the system's 

 ability to distinguish between closely spaced objects or to delineate the 

 details of a large area, is defined usually in terms of range resolution and 

 transverse or azimuth resolution. While the ultimate resolution attained by 

 the system is a function of many parameters, the single criterion most 

 commonly used to judge it is the pulse packet size as projected on the 

 ground. The system parameters which determine the pulse packet size are 

 antenna beamwidth and pulse length, as measured at the half-power points. 



Beamwidth is a function of the transmitted wavelength and the dimen- 

 sions of the antenna, being expressed approximately by 



e = §^ (14-24) 



where 6 is the beamwidth defined by the half-power points 



X is the transmitted wavelength 



D is the pertinent dimension of the antenna aperture 



K is the constant dependent on the particular aperture. A typical 

 value for this constant is 70 where 6 is expressed in degrees, and 

 X and D are measured in the same units. 



The system designer is confronted at once with a compromise, both in the 

 selection of the transmitted wavelength and the dimensions of the antenna. 

 To narrow the beamwidth, either the wavelength must be decreased or the 

 dimensions of the antenna must be increased. In decreasing the wavelength 



