The angular accuracy in. radar is largely determined by the horizontal 

 radiated beam width. The antenna beam width, i.e., half -power point, is inversely 

 proportional to the horizontal dimensions of the antenna . It is this dimension 

 and the transmitted pulse width which determine the horizontal target resolution 

 of the system. Horizontal beam widths of 1-3° are most common for marine 

 radar equipment. Vertical beam widths of 15-40° are quite common . The larger 

 vertical widths are required where antenna stabilization is not used to prevent 

 echoes from becoming lost because of rolling and pitching of the vessel . 



Radar systems display both a range to the point of interest and a map 

 of the surrounding area. The map is produced by coordinating the rotation of the 

 sweep on the cathode -ray tube with the rotation of the antenna beam . The map type 

 of display on the PPl is the equivalent of a large number of trilateration fixes if 

 the prominent reflection points of the area are known. In this type of operation, 

 a knowledge of the surroundings is used to fix the position of the radar. 



The use of a meaningful PPI display requires high range resolution. 

 The over -all pulse length is important because it determines the minimum 

 spacing between discrete reflection points which is necessary to produce separated 

 echoes on the screen. For PPl display purposes , radar pulses are made as 

 short as the rise and fall time will permit. Thus, range resolution as well as 

 angular resolution is important in the display of radar information on a radar 

 PPI. This resolution is dependent not only on the pulse length and beam width 

 but also on the minimum spot size on the PPI screen. 



The limit of resolution attainable in radar systems depends on both the 

 pulse length and beam width of the system. Fast -rise -time pulses require that 

 the transmitter and receiver bandwidth, which is inversely proportional to the 

 pulse rise time, be wide. 



Experimental radars with pulse lengths of a small fraction of a micro- 

 second, giving range resolutions of a few feet, have been built. In general, 

 their useful range is reduced to line of sight or less . Similarly, antennas with 

 beam widths of 0.1° which require an aperture of about 600 wave lengths have 

 been built . 



The navigational use of radar generally requires a comparison of the 

 PPI display and an appropriate chart, whether supplemented or not by standard 

 radar views or photographs . A superposition or chart overlay method is prob 

 ably most effective in matching a chart to the screen. Generally the range rings 

 of the PPI display are matched to those of the chart. Some matching of chart 

 features to those of the PPI is also required. 



136 



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