than those seen by the pressure gage. The precise position of the pres- 

 sure gage was not known. The error could be accounted for if the wave- 

 length measurements were made on the radar image at a location shoreward 

 of the actual pressure gage position. This hypothesis is supported by 

 a scatter plot (Fig. 37) of the wave period measured by the pressure 

 gage and the wave period obtained using the wavelength as measured by 

 CERC radar, the depth from charts, and Airy theory. This is the same 

 data set used in Figure 36, but for period no systematic error is seen. 



An extensive comparison study of wave direction as determined by the 

 CERC radar, aerial photos, SAR imagery, and a pressure gage array is pre- 

 sented in Mattie, Evans, and Hsiao (in preparation, 1979). 



VI. OTHER RADAR DEVICES AND DIRECTION MEASURING SYSTEMS 

 1. Radar Devices . 



Radar is used in many different ways to obtain information on ocean 

 waves and winds. Radar altimeters used on aircraft and satellites can 

 provide information about wave height (Rufenach and Alpers, 1978). These 

 satellite altimeters measure significant wave height to an accuracy of 

 1 meter (3.28 feet) or 25 percent of the actual height over a range of 

 1 to 20 meters. Images of an ocean surface area can be obtained from 

 aircraft and satellites by the SAR (Elachi, 1978). In the SAR system, 

 the return along the flight path is combined in the data processing so 

 that the angular resolution is greatly improved, and an antenna is simu- 

 lated that is much longer than the one actually carried on the platform. 

 With this increased angular resolution, the satellite SAR's have a reso- 

 lution cell 25 by 25 meters (82 by 82 feet). Spatial wave and current 

 information can also be obtained with high frequency (HF) radar. A 

 typical radar of this type has a wavelength of 5 to 100 meters (16.4 to 

 328 feet), and usually requires an antenna array on the order of one to 

 five wavelengths long to form a fairly narrow azimuthal beam. Barrick 

 (1977) has developed a phase-difference technique to obtain narrow beams 

 with small HF antennas. Most HF units are doppler radars which measure 

 the speed of the radar scatters from the doppler shift of the radar sig- 

 nal. When HF radar is scattered from the sea surface, the first-order 

 doppler return can give a measure of surface currents. Lipa (1978) dis- 

 cusses the theory and verification from one experiment for a method to 

 obtain wave directional spectra from the information contained in the 

 higher order doppler return. The resolution provided by this system is 

 unlikely to be fine enough to meet the needs of coastal engineers for 

 nearshore measurements. 



A variety of other specialized radars is available for making speci- 

 fic measurements in the ocean environment; e.g., a radar scatterometer 

 provides estimates of local wind velocities by measurement of the radar 

 cross section, because at some radar wavelengths, the cross section is a 

 function of windspeed. An inexpensive radar with a resolution fine enough 

 to obtain images of sea surface wave fields for ranges to include coastal 

 wave phenomena is needed. The CERC radar will meet this need for many 

 operational conditions. 



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