in this section). The distance to the object is determined by the time 

 between the emission of an energy pulse and its return to the transmitter. 

 The direction of the object is determined by varying the orientation of a 

 narrow beam antenna. Images are constructed by scanning the scene and 

 displaying the results on the PPI scope. The sweep of the PPI is syn- 

 chronized with the antenna rotation so that a return received by the 

 antenna is recorded on the PPI at the appropriate azimuth. The object 

 is imaged as an illuminated spot on the PPI at a distance from the center 

 of the PPI, proportional to the distance of the object from the radar. 

 In the CERC system, the antenna rotates at constant speed around a ver- 

 tical axis. Thus, the site of the antenna appears near the center of 

 the PPI scope. 



1. Radar Resolution . 



The resolution of the radar along a radial line is determined by the 

 pulse length. For an object to be detected, a pulse of energy must go 

 out to the object and return to the transmitter; i.e., to detect an 

 object at a distance r, the radiation must cover a distance of 2r. 

 Thus, the distance is obtained as 



(*) 



ct (1) 



where c is the speed of electromagnetic radiation, and t the time 

 interval between the emission of an energy pulse and the return of the 

 same pulse. The pulse, however, has finite duration, t. Equation (1) 

 should then be written 



r + 6r = (±) c(t + t) (2) 



(I) 



where Sr is the uncertainty in measuring r, or the resolution of the 

 radar system in a radial direction The nominal resolution is shown to be 



6r = f- . (3) 



Resolution in a radial direction is improved by using small values of t. 

 Reducing the pulse length also reduces the power of the returned signal, 

 and extremely short pulses may also be associated with a reduction in the 

 reliability of electronic components. A pulse length of 50 nanoseconds 

 (50 x 10" 9 seconds) used in the CERC radar system for imaging waves is 

 the shortest pulse width available in commercial marine radars. This 

 type of radar is used in the automatic wave direction system because of 

 its reliability and lower cost. Some special purpose and research radars 

 use shorter pulses which may eventually be used in marine radars. The 

 CERC system has a nominal resolution of 



6r = ( -r-J 3 x 10 8 meters per second x 50 x 10" 9 seconds 



6r = 7.5 meters. 



25 



