THE RADAR RECEIVER 759 



in space and which are determinable from a single radar observation loca- 

 tion are the range to the target, azimuth angle with respect to a chosen 

 direction axis, and elevation angle as measured from a convenient reference 

 plane. The classification of radar displays shown in Fig. 37 results from the 

 fact that the only available and convenient display device has the property 

 of resolving only two such coordinates simultaneously. The radar display 

 problem is then one of selecting the most important two coordinates for the 

 specific radar application and choosing the presentation means accordingly. 

 For example, if the radar system under consideration is to be employed on a 

 surface naval vessel against similar naval vessel targets, it follows that 

 elevation angle radar information is redundant and, therefore, type-A or B 

 display patterns are quite satisfactory and are in fact the typical presenta- 

 tions which have been universally employed for surface target fire-control 

 applications. The basic A-type indicator presents range-only data, but 

 for fire-control purposes a modified form is often employed with lobe switch- 

 ing by which accurate training of the radar antenna is possible and bearing 

 information is thus secondarily obtained. 



For airborne radar search and bombing applications the presentation is 

 concerned with targets, one coordinate of which is known by other than radar 

 means. Since all targets of interest are in this case located on the ground 

 plane, the relative location of which is determinable by reference to the al- 

 timeter and a gyroscopic artificial horizon within the aircraft, it is sufficient 

 here to present all information as a 2-dimensional map. The presence of 

 targets and to some extent their composition is observable as an intensity 

 modulation of the field of view. For this type of application the PPI or its 

 more exact successor the GPl form of presentation is extensively employed. 



For military radar applications where fire-control information is desired 

 pertaining to targets which are not confined to a definite plane all three de- 

 terminable coordinates must be known and, therefore, presented to the radar 

 observer. In certain instances this requirement has been fulfilled by the 

 use of multiple displays each presenting the information regarding one or 

 two coordinates and in cases where gun training is accomplished through 

 separate operators for each coordinate axis, range, bearing, and elevation, 

 this method has proven entirely satisfactory. During World War II the 

 fast moving and highly maneuverable aircraft target has required a more 

 direct and, therefore, faster system of gun pointing. In these cases, the 

 operator has been provided with a display which electronically duplicates a 

 sighting telescope and which merely requires the operator to position the 

 gun (and associated radar antenna) until the target image is centered. To 

 introduce a measure of range to the target the size or form of the target 

 "spot" is often varied in accordance with the range data. For defense 

 against low-level aircraft attacks this admittedly crude range information 



