18.3 meters (60 feet) or shoreward of the tower with wave direction cal- 

 culated at NUC by refraction theory (McClenan, 1975). The measurements 

 were made manually with a protractor and straight edge. The true bearing 

 of 275° from the NUC tower to the radar site was used as a reference. To 

 resolve the 180° ambiguity, several radar images were viewed which showed 

 that the waves were moving shoreward. For the U-2 photos, it was assumed 

 that the waves were propagating toward the shore. 



In both cases, two wave trains were detected on the radar images, and 

 the directions for these wave trains agreed well with measurements from 

 the U-2 photos. However, in both cases, another very short wave train 

 appeared in the U-2 photo but was not detected by the radar. Perhaps 

 this was due to the misadjustment of the pulse width resulting in a 

 radar resolution that was too poor to image the short wave train. With 

 the radar optimally adjusted, it is expected to produce images of waves 

 with lengths on the order of 20 meters. Wave trains with lengths of 35 

 meters (115 feet) have been measured by a radar with the same design 

 resolution as the CERC radar; these results are discussed later in com- 

 paring pressure gage data with radar measurements at Cape Cod. 



Wave images were also collected by JPL during the West Coast Experi- 

 ment using a SAR mounted in the NASA Convair 990 aircraft. On 28 March, 

 a SAR image of the waves was taken in the Mission Beach area at the same 

 time CERC radar images were obtained (see Fig. 33) . Figure 34 is a two- 

 dimensional Fourier transform of the marked area of the SAR image. On 

 the transform, the predominant wave train shows as a bright radial band. 

 A close examination also shows a faint second band oriented 28° in a 

 counterclockwise direction from the predominant band, which corresponds 

 to a second wave train. These light lines or bands in the transform are 

 normal to wave crests. Wave direction and period for the SAR data were 

 taken from direction plots derived from the Fourier transform and sup- 

 plied by JPL. The CERC radar results for 1800 on 28 March are based on 

 an average of three images taken within ±30 minutes of 1800 (photo of 

 the radar scope for this observation is shown in Fig. 35). The 285° 

 wave train appears on all three images. Each of the 261° and 242° trains 

 appears on only two of the three averaged images. The Table shows that 

 when the direction of the prominent wave trains of the SAR image are 

 refracted to the NUC tower (location of the CERC radar measurements) , 

 there is good agreement in the wave direction measured by the two instru- 

 ments for these wave trains. However, a third train from 242° with a 

 period of 15 seconds appears on the CERC radar and not on the SAR. A 

 possible explanation is that three wave trains were present-the prominent 

 train from 285°, a second from 263°, and a third swell train from 242°. 

 Such a swell, even if the wave height were low, could show up on the 

 CERC radar because of shadowing of wave troughs with the low antenna 

 elevation. It might not have appeared with the SAR because of the small 

 amount of modulation of the radar-scattering capillary waves resulting 

 from such a wave train's small wave steepness. 



The Table shows that for 14 and 29 March the wave periods for the 

 long-wave trains measured from the CERC radar image do not agree well 



38 



