II. A SYSTEM FOR RECORDING WAVE DIRECTION BY RADAR 



The CERC radar system has been assembled to automatically obtain wave 

 images, and includes a Raytheon 1020/9XR Mariners Pathfinder X-band radar 

 mounted in a van to provide mobility. A rotating antenna is mounted on 

 the roof of the van (Fig. 3), or on a 2-meter (6.56 feet) mast at the end 

 of the trailer for additional elevation. This radar has a pulse width of 

 0.05 microsecond (50 nanoseconds) with a range resolution of 10 to 20 

 meters (32.8 to 65.6 feet). The 2.74-meter (9 feet) slotted-array an- 

 tenna gives a horizontal beam width of 0.9° at 3 decibels and has a ro- 

 tation rate of 33 revolutions per minute. Pulses of electromagnetic 

 energy with a nominal wavelength of 3 centimeters (nominal frequency 

 10 10 hertz) are beamed over the water. A part of this energy is scat- 

 tered back to the antenna by a process explained in Section IV. The 

 back-scattered energy is displayed on a 10-inch-diameter cathode-ray 

 tube (CRT) called a "plan position indicator" (PPI) in the form of light 

 and dark patches which parallel the wave crests. Data are recorded by 

 photographing the CRT using a Bolex 16-millimeter H-16 reflex camera. 

 The PPI and camera are shown in Figure 4. 



A CRT with a fast-decay phosphor is used. The standard CRT uses a 

 medium- to slow-decay phosphor which retains the target for easy viewing. 

 The fast phosphor is used for data collection to obtain sharp images with 

 the time-lapse photography. 



To adjust the radar to obtain wave images, the signal gain is in- 

 creased for the weak sea clutter return to appear on the CRT. The scope 

 intensity is kept low so the CRT is not saturated. Rain clutter and sea 

 clutter controls are turned to a low position. The rain clutter control 

 is used in the "just on" position to limit some of the strong return near 

 the center of the scope which tends to saturate that part of the CRT. 

 This gives better images in the surf zone while not affecting the return 

 from the more distant wave areas. 



The radius of the region displayed may be varied by discrete steps 

 from 0.695 to 44.4 kilometers (0.375 to 24 nautical miles). The opti- 

 mum radius or "range" for wave imaging varies with the ambient wave 

 conditions-shorter ranges are best for shorter waves. 



The CERC system is designed for unattended operation; an automation 

 system turns on the radar at a periodic time interval that can be set 

 from 1 to 9 hours. After a suitable warmup time (about 10 minutes) the 

 radar signal is fed to the antenna as it scans the sea, and a sequence of 

 1 to 9 photos is obtained at each of four ranges, 0.695, 1.39, 2.78, and 

 5.56 kilometers (0.375, 0.75, 1.5, and 3 nautical miles). 



The automation unit controls the opening and closing of the camera 

 shutter and the sector of the sweep which is photographed. The system 

 is then turned off until the next scheduled observation. Photos are 

 taken at several ranges because the optimum range to obtain information 

 on wave direction or length depends on the wavelength and height of the 



