ambient waves. Multiple photos will permit the resolution of any 180° 

 ambiguity about wave direction and will minimize the chance that the 

 random selection of observation times will give misleading data. The 

 system can be operated for prolonged periods with a constant range if 

 desired. In the photographic mode the camera shutter is opened when the 

 radar antenna sweep crosses from land to sea and remains open until the 

 entire sea has been scanned. The shutter then closes and the film is 

 advanced one frame. The duration of a 360° radar sweep is 1.82 seconds. 

 Additional variation of parameters can be programed into the system if 

 experience shows this to be desirable. Power to the radar antenna is 

 shut off when the antenna is not facing the sea. 



The unit has a clock which provides a time signal in hundredths of a 

 second to the light emitting diodes (LED) on the PPI that document each 

 photo. With normal operation, one roll of 16-millimeter film will last 

 about 1 week. The developed film is analyzed in the CERC laboratory with 

 the aid of a viewing device described in Section III. 



III. RADAR RESULTS 



Radar images of waves presented in this report were collected using 

 two different radars. A short sequence of images was collected with a 

 Decca radar during an experiment in January 1976 at Nauset Beach on Cape 

 Cod, Massachusetts, to test the concept of using radar to provide wave 

 direction information. The Decca radar was part of a vessel tracking 

 system on loan from the U.S. Coast Guard Research and Development Center 

 in Groton, Connecticut. All other radar images were obtained with the 

 CERC radar system. For several months the CERC system regularly collected 

 wave images in an automated mode at Channel Islands Harbor, California, in 

 support of a sediment transport study. An additional set of images was 

 collected at San Diego, California, with the CERC radar during 22 Febru- 

 ary to 31 March 1977 in support of the West Coast Experiment organized 

 through the Jet Propulsion Laboratory (JPL) , Pasadena, California. The 

 Decca radar images clearly showed single and multiple wave trains, and 

 led to the construction of the present CERC radar system. 



This section shows the type of images available from the two radars 

 and highlights some special features. 



1. Images of Single and Multiple Wave Trains . 



Figure 5 shows a PPI photo of a single wave train imaged by the Decca 

 radar during the Cape Cod test. The wave train is from the east with a 

 wavelength of 90 meters (295 feet) near the outer limit of the image. 

 One disadvantage of the Decca radar is that when the gain is turned high 

 enough to see the waves near the edge of the scope at 1.39 kilometers 

 (0.75 nautical mile) from the radar, the surf area is saturated and the 

 waves in the surf zone cannot be distinguished due to the bright return. 

 The Raytheon radar in the CERC system handles this problem much more 

 satisfactorily. Figure 6 is a photo of the PPI scope taken by the CERC 

 radar in California at the 1.39-kilometer range, and shows one primary 



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