Figure 35, 



CERC radar image at Mission 

 Beach, 28 March 1977. 



with the periods measured from the U-2 photos. These long waves were 

 part of secondary wave trains which did not show up well on either the 

 aerial photos or the radar images. Thus, the wavelengths were difficult 

 to determine with the manual analysis scheme used. In general, a Fourier 

 transform of the photos should provide a more reliable wave period meas- 

 urement. A reliable wave period measurement from the radar images may be 

 obtained by recording the time for one wavelength to pass a fixed point. 



2. Comparison with One-Dimensional Wave Spectra . 



One-dimensional wave spectra from a pressure gage have also been used 

 in radar imagery evaluation. The Decca marine radar, with specifications 

 similar to the CERC radar, was operated for 2 weeks at Cape Cod. Wave 

 images were obtained with the radar operating at 18 meters (60 feet) 

 above mean sea level (MSL) . Simultaneously, a pressure gage was oper- 

 ated offshore of the radar location. The lengths of the visible wave 

 trains were measured on the radar photos at the same location as the 

 bottom-mounted pressure gage. From the spectrum obtained with the 

 pressure gage, a peak period was determined from which wavelength was 

 calculated using linear theory and depth. A scatter plot (Fig. 36) of 

 radar-measured wavelength versus that from the pressure gage shows a good 

 correlation. The radar-measured wavelengths are systematically shorter 



