1.61 km 



Figure 31. Schematic showing the illumination of wave 

 crests by radar at 4.6 meters elevation. 



Several other complicating factors make exact calculations of maximum 

 range difficult in every situation. These include the decrease of radar 

 cross section with incidence angle (see Fig. 26) , the unknown phase of 

 the radar scatters with respect to the long waves, variation in tempera- 

 ture and humidity in the air, and the variation in height of the long 

 waves. Thus, the choice of the optimum antenna elevation must be made 

 empirically with consideration of the wave height to be expected, the 

 features to be investigated, and the sites available. 



V. VALIDATION 



Wave data from imaging radar should be compared with wave data from 

 other devices. At least two types of comparative data may be used: 



(a) Other types of wave imagery, including aerial photog- 

 raphy and aircraft or satellite radar such as synthetic aper- 

 ture radar (SAR) and side- looking radar (SLAR) . 



(b) Data from conventional wave gages or arrays of conven- 

 tional wave gages. 



All imaging processes present a nearly instantaneous, two-dimensional 

 view of the water surface. For aerial photography, the duration of an 

 observation is a few milliseconds; for scanning radar (used in these 

 experiments), successive parts of the image are formed at later times, 

 and the duration of data collection is about 1 to 2 seconds. In all 

 imaging processes, ripples are required on the water surface to reveal 

 the presence of longer waves. 



With aerial photography, the principal process for obtaining infor- 

 mation about the water surface is specular (mirrorlike) reflection; for 

 surface-based radar, the principal process is Bragg scattering. 



34 



