702 BELL SYSTEM TECHNICAL JOURNAL 



received radar information. Tliis conii)lexity is created out of the compara- 

 tively limited resolution available and from the military need for presenta- 

 tion of detailed information concerning large areas during small time 

 intervals. 



The effect of limited resolution on the choice of presentation means of a 

 radar receiver can be appreciated by considering the following. The micro- 

 wave pulse employed in modern radar systems has a duration in time cor- 

 responding in linear range dimensions of hundreds of feet, while the beam 

 width of commonly employed radar antenna systems likewise includes 

 hundreds of feet of target at useful ranges. Thus, the inherent radio- 

 frequency "resolution" is limiting to an extent that, while it usually enables 

 one to determine the coordinates of the centroid of the target, it will not 

 furnish adequate information as regards the exact size or shape characteris- 

 tics of the target. The radar response of an area of mihtary interest is a 

 function of electrical conductivity and other related characteristics, rather 

 than of the military importance of the target. These factors indicate 

 strongly that the human observer must be required to supply a certain 

 function of interpretation, and that the chosen radar presentation means 

 should be such that this is possible. An effective illustration of this situa- 

 tion is found in the descriptive term "navigation by constellation" which 

 was common among the radar operators in the long-range bombing forces. 

 Here the navigation to and the orientation with respect to the military 

 objective was often possible only through the interpretation of strong 

 radar responses from known landmarks. Offset bombing, where the 

 bombing radar operator carried out his observations on a satisfactory 

 radar target in the vicinity of the final objective and introduced the known 

 offset coordinates in the computed release point so as to strike the military 

 objective, was found to be a successful method of partially overcoming this 

 basic limitation of World War II radar equipments. 



Modern warfare is concerned with rapid movement and extremely large 

 area operations. The display of continuous information regarding these 

 large areas is a basic military requirement of the modern radar system. 

 For specific military applications the radar viewpoint may often be re- 

 stricted to selected limited areas with increased demands on detail and on 

 reproduction of changing information during small time intervals. 



The above considerations have led to a choice of presentation means for 

 the military radar system which is of an optical nature and has the essential 

 characteristics of motion i)ictures. Such a display of complex information, 

 in general, allows the observer to concentrate his attention at any time on 

 any desired region of interest, to orient himself with respect to the broad 

 features of the complete area, and to be cognizant of changes in the scene as 

 they occur. 



