THE RADAR RECEIVER 761 



advantage of expansion of the range information by a factor of approxi- 

 mately three times for the same size screen. The A and J types are em- 

 ployed extensively in fire-control radar equipments. 



In the range versus azimuth class of radar presentations the B scan his- 

 torically preceded the other forms shown. Its application was found 

 originally in airborne radar systems for interception purposes. It was com- 

 monly employed in conjunction with an auxiliary C-type indicator for 

 target elevation determination. The B type of display suffers from a dis- 

 tortion due to the reproduction of polar coordinate information directly 

 on a rectangular coordinate field. This particular form of distortion is not 

 of major importance where only a few isolated aircraft targets are to be dis- 

 played, and in the case of guiding an aircraft to intercept the target the 

 relative expansion of the azimuth scale at short ranges may be a slight 

 advantage. When the B type of presentation is employed for navigation 

 and observation of ground features this inherent field distortion becomes 

 very objectionable when map comparisons of the radar image are required. 

 The B type of display has also been employed extensively on narrow 

 sector rapid scanning naval fire-control radar systems. 



The plan position indicator (PPI) type of display was developed to over- 

 come the objectionable distortion of the B-type display and to afford a 

 method of presenting a 360° azimuthal pattern when rotating antenna struc- 

 tures were employed. This form of display essentially replaced the B-type 

 display for aircraft search radar systems and has been since universally 

 employed for ground and naval vessel search systems. Here the linear range 

 trace on the screen is directed outward from the center of the tube, its 

 radial position being synchronized with the instantaneous bearing of the 

 scanning antenna. The map presentation is exact for ground and naval 

 vessel radar locations and for low-flying aircraft radar systems the dis- 

 tortion is negligible, since the slant radar range to the target at low altitudes 

 is essentially comparable to the range as measured on the ground plane. As 

 the altitude of a radar equipped aircraft is increased, the map distortion of 

 the simple form of PPI display also becomes quite objectionable and several 

 modified forms of this display can be employed to improve the presentation. 

 One of these involves delaying the time of start of the linear range sweep by 

 a time interval corresponding to the propagation time of the radar pulse to 

 the ground and return. In this manner a simple but desirable improve- 

 ment in display is realized. As the military requirements during the later 

 period of the past war became more exacting with the emphasis on high- 

 altitude radar bombing, the remaining distortion of the delayed PPI presen- 

 tation was found undesirable, and the necessity for accurate map display 

 directly beneath the aircraft resulted in the development of the ground plan 

 indicator (GPI). In this type of display the range trace is deflected as a 



