12-3] 



TYPES OF DISPLAYS 



617 



as shown in Fig. 12-6. The radial direction of the sweep corresponds to the 

 true bearing of the antenna. The video is presented as an intensity- 

 modulated signal similar to the B or C scope. The distance from the center 

 of the screen to the intensity-modulated signal indicates the range of the 

 target. The relative bearing of the target is given as the angle between 

 forward heading (direction is such that forward position is at the top of 

 the display) and the position of the target as shown in Fig. 12.6. 



The antenna for a PPI is driven in the horizontal plane at a speed of 

 1 to 30 rpm. Since the direction of radiation changes as the antenna is 

 rotated, the sweep on the scope is rotated so as to be always correlated with 

 the direction of radiation. The PRF is very high compared with the 

 frequency of rotation; therefore the antenna is relatively motionless during 

 a radial range sweep. The displacement between sweeps is also relatively 

 short. If, for example, the antenna is rotated at 20 rpm and the PRF is 

 800 cps, then the radial changes in sweep position (and change in antenna 

 position) between transmitted pulses is 0.15°. This radial change is directly 

 proportional to the antenna speed and inversely proportional to the PRF. 



The high azimuth resolution coupled with the wide-angular coverage 

 makes this type of display particularly adaptable to AEW and mapping 

 or navigational radar systems. One of the disadvantages of the polar 

 coordinates plot is the poor resolution at short ranges. This problem is 

 particularly important for ground-mapping radar displays which employ 

 very narrow antenna beamwidths. 

 At only one range can the angular 

 sector corresponding to the azimuth 

 beamwidth of the antenna and the 

 spot size be equal. Below this range, 

 the spot size determines the azimuth 

 resolution rather than the radar. 

 Beyond this range, the radar angular 

 resolution is the determining factor. 

 These conditions are illustrated in 

 Fig. 12-7. The degree of poor focus 

 is determined by a number of vari- 

 ables: the spot size, the azimuth 

 beamwidth, the maximum range dis- 

 played, the physical length of the 

 sweep, the angle of sector scanned, 

 and the angle of display sector. 



One method of improving the aximuth resolution at short ranges is by 

 mechanizing an open center PPI. This type of display, as shown in Fig. 

 12-6, overcomes the minimum spot size limitation by changing the zero 

 range origin out beyond the point where the angular resolution of the 



Fig. 12-7 



Resolution Limitation of PPI 

 Display. 



