angle is small and the first-order Bragg scattering is due to ocean waves 

 with lengths on the order of one-half the radar wavelength. For X-band 

 radar, which has a X r of about 3 centimeters, the Bragg scattering is 

 from the capillary and short gravity ocean waves. 



3. Role of Wind in Scattering . 



At times no waves appear on the radar images, although waves are 

 known to be present because they can be observed in the surf zone due 

 to radar return from facets and spray as specular reflection. A case 

 where no waves are seen outside the surf zone is shown in Figure 25. 

 The figure clearly shows a wide surf zone which was caused by swells 

 from two directions. Outside the surf zone, no waves are visible in 

 Figure 25, because since the winds were calm or low, little capillary 

 wave development was present to provide back-scatter radar return. 

 Figure 26 shows the same image taken about 2 hours after Figure 25 when 

 the winds had freshened. The offshore swell is now clearly visible be- 

 cause of the radar return from the well-developed capillary waves modu- 

 lated by the swell. In general, field tests have indicated that at least 

 a 5-mile-per-hour wind is necessary for the radar to image waves outside 

 the surf zone. 



4. Scattering from Wave Crests . 



When an X-band imaging radar is used at a shore location, a return 

 is seen from the surf zone due to specular reflection and scatter from 

 small droplets and spray facets perpendicular to the radar beam. A 

 return from outside the surf zone is caused by Bragg scattering, where 

 a strong return is obtained from along the ocean wave crests. Several 

 factors contribute to this phenomenon. 



The slope of the sea surface is one factor. The strength of the 

 radar return, measured in terms of radar cross section, a, is a func- 

 tion of the local grazing angle. The radar cross section, a, is a 

 measure of the ratio of the power density scattered toward the receiver 

 to the power density incident on a target. Figure 27 shows a cross sec- 

 tion per unit area o° plotted versus grazing angle (a° is in decibels; 

 i.e., a°(dB) = 10 log a ). In the figure, the graph for an X-band 

 horizontally polarized signal shows an increase in return with increased 

 grazing angle. Thus, a better return would be expected from the upper 

 part of the forward face of gravity waves, where the local grazing angle 

 between the water surface and a radial line from the radar antenna is 

 largest. 



A second factor is that the antenna is mounted at such a low eleva- 

 tion that most wave troughs are shadowed by the previous crests but high 

 enough to ensure that a high wave would not shadow any following wave 

 crests. The optimum antenna elevation would be between 10 to 20 meters 

 (33 to 66 feet). With the antenna below 10 meters, the grazing angle is 



28 



