Nearshore currents might be measured by tracking float -mounted 

 corner reflectors on a sequence of radar images. 



3. Methods for Analysis of Radar Photos . 



Because the CERC radar system was designed primarily for obtaining 

 wave direction statistics, a method of analysis is needed to permit rapid 

 and accurate interpretation of the direction of wave propagation from a 

 large set of radar photos. An additional complication is that a single 

 radar picture often shows several wave trains, each of which has a di- 

 rection and a wavelength to be determined. An effective and convenient 

 method of analysis is to scan the film with a device similar to that 

 shown in Figure 22. The operator can control a 16-millimeter projec- 

 tor while viewing the images, in cinema mode or individually, on a rear 

 projection screen. A rule with a crossbar assembly that fits over the 

 screen can be lined perpendicular to the direction of wave propagation 

 in an area of the image to read the relative angle from a protractor. 

 A true direction can be derived by referencing a known azimuth available 

 from charts. The shore direction or some landmarks shown in the radar 

 photo can be used to define a true azimuth. Any 180° ambiguities can be 

 resolved by observing two successive frames to note the direction the 

 wave crests are propagating. The distance between wave crests can be 

 measured with the rule. Figure 23 shows a radar image with annotated 

 directions and wavelengths. The measurement of the wave direction rela- 

 tive to the shore is shown for two wave trains, where 9i and 02 are 

 the angles between the wave train propagation direction and the shore- 

 line. The measurement of the wavelength for each wave train Xi and X2 

 is also shown. 



4. Measurement Errors . 



A measurement of the propagation directions for the principal wave 

 trains in the CERC system can be obtained with an accuracy on the order 

 of 8°. Uncertainties in the direction measurement are due to (a) radar 

 angular resolution of about 1°, (b) resolution on the protractor of 0.5°, 

 (c) errors in lining up the ruler perpendicular to the wave crests and in 

 determining a reference angle (estimated to total 5°), and (d) motion of 

 the waves during the time the radar sweeps the sea surface (an error of 

 1° to 2°). 



Errors in measuring wavelength due to resolution of the rule would 

 be least significant in measurements at shorter ranges, such as the 1.39- 

 kilometer (0.75 nautical mile) range. Errors of from 5 to 10 percent in 

 measuring wavelength are encountered at this range. 



IV. THEORY OF RADAR IMAGING 



Objects are detected with radar by illuminating the scene with short 

 bursts of electromagnetic energy of a discrete frequency. A part of the 

 radiated energy may be returned to the transmitting antenna by specular 

 reflection (mirrorlike reflection) or by back-scattering (discussed later 



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