Directional resolution is determined by the ratio of the effective 

 aperture of the antenna (a measure of the effective length presented by 

 the antenna to the incident wave) to the wavelength of the radar waves. 

 The CERC system has a nominal horizontal angular resolution of about 

 0.9° or 0.0157 radians. Equipment now available (after the CERC system 

 was purchased) will permit an improvement to 0.6°. 



The resolution normal to the radius is given by 



6c = r69 



= 0.0157r . (4) 



Thus, the nominal resolution along a radial line and normal to it is 

 equal at a range of 477 meters (1,565 feet). The angular resolution 

 is inferior to the radial resolution at greater distances. 



The angular resolution in a vertical direction (23° for the CERC 

 system) is not critical, and little effort has been expended in opti- 

 mizing this. 



The radar in the CERC wave imaging system transmits 3,600 pulses per 

 second with a pulse length of 50 nanoseconds. Since the antenna rotates 

 at 33 revolutions per minute, about 18 pulses are emitted for each 1° 

 rotation of the antenna. Near the center of the scope, at least, the 

 smallest element of the phosphor will sustain an angle of more than 1°. 

 Thus, each spot will display an average of many radial scans. The theo- 

 retical optimum radial resolution is also degraded due to distortions of 

 the pulse on scattering and slight misadjustments to radar circuitry. 

 When all factors are considered, the manufacturer claims a resolution of 

 10 to 20 meters in the radial direction and 0.9° in an azimuthal direction. 

 Therefore, the shortest wave likely to be detected with the system in deep 

 water can be expected to have a period of 3.6 seconds. The shortest detect 

 able periods in shallow water will be even longer. 



Radars which use pulse lengths much longer than 50 nanoseconds and 

 angular resolutions of less than 0.9° are unlikely to be satisfactory 

 for imaging the wave field. 



2. Scattering Mechanisms . 



Radar is scattered from the ocean by two mechanisms. The first is 

 specular reflection, where the microwave radiation is reflected by a 

 facet or a surface that is perpendicular to the radar beam. Radar al- 

 timeters and other radars that look straight down at the sea obtain re- 

 turn signals via this mechanism. A return can be seen when viewing the 

 surf zone with a radar at grazing angles; facets caused by the breaking 

 waves present surfaces that are perpendicular to the radar beam. Waves 



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