772 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



As in the design of any equipment involving compromises, the designer 

 of airborne weather radar must weigh carefully the relative importance of 

 all the many factors concerned in the light of his own specific application, 

 in order to arrive at the optimum design. Figs. 14-16a and 16b give some 

 idea of the relative size, weight, and power consumption of two typical 

 commercial weather radars ' — one a 3.2-cm system, the other a 5.7-cm 

 system. 



14-15 INTRODUCTION TO ACTIVE AIRBORNE GROUND 

 MAPPING SYSTEMS 



The mapping of large areas and the capability of providing data which 

 permit rapid and accurate identification, location, and interpretation of all 

 important features within the area surveyed are among the primary 

 requirements of ground mapping systems. Airborne radar ground mapping 

 systems have the potential to satisfy many of these requirements. They 

 combine the advantage of radar, with its ability to operate during the night 

 or day under adverse weather conditions, with the mobility and range of an 

 airborne platform. 



The following discussion will outline the basic principles of active 

 airborne ground mapping systems and examine system considerations as 

 well as the major system characteristics and components. The basic radar 

 range equation will be considered as it applies to active ground mapping 

 systems, and factors which determine the limits of resolution will be 

 discussed. Finally, the future possibilities of this type of mapping system 

 will be considered briefly. 



14-16 BASIC PRINCIPLES 



The basic principle of an active radar ground mapping system is the same 

 as that of any active radar; it transmits energy and detects the part of it 

 scattered back from a target. However, instead of the usual point target, 

 the target in this case is the ground, which can be considered as an extended 

 array of scatterers. The radar map is obtained by scanning or "painting" 

 the ground and displaying the return on a cathode ray tube or photographic 

 film. Since the scattering characteristics of the ground will vary from point 

 to point, the map will be in the form of a varying brightness pattern. 

 Variations of intensity in this brightness pattern can be interpreted in terms 

 of the topographical and man-made features of the terrain. For example, 

 the energy back-scattered from a smooth surface such as calm water will be 

 much less than that from a rough surface such as the ground. As a result 

 land-water boundaries are usually among the most easily identifiable 

 features on a radar map. The degree of correspondence between the 

 brightness pattern and the features of the terrain depends to a considerable 



