uncertain; however, it is not essential to a determination of wave direc- 

 tion or wavelength. The direction of wave propagation can be determined 

 from the orientation of the lines of scatterers. 



The distance at which waves will give a return detectable on the 

 radar depends on several factors, including the antenna elevation, the 

 steepness of the long gravity waves, and the atmospheric conditions. 

 With commercial radars such as that used in the CERC system, the maximum 

 range for wave imaging is restricted by the parameters of the radar unit 

 to 5.28 kilometers (3 miles). The pulse width of 50 nanoseconds needed 

 for wave resolution is available only for ranges of 5.28 kilometers or 

 less. A minor change to the radar would allow use of this shorter pulse 

 for the longer ranges. However, the radar power would then become the 

 limiting factor for the ranges longer than 5.28 kilometers. Unless the 

 transmitted power could be increased, the return may be too weak to give 

 wave images. 



If radar radiation is assumed to travel in straight lines, then the 

 range would also be expected to be restricted by the curvature of the 

 Earth as shown in Figure 30. If d = 5.28 kilometers, then a minimum 

 radar height, h, is found by 



(r + h) 2 = r 2 + d 2 



r 2 + 2 hr + h 2 = r 2 + d 2 (8) 



h(2r + h) = d 2 

 but 



r >> h 



h * f- , (9) 



2r 



r = 4,000 miles (6,439 kilometers) 



h = ( 9 \ 5,280 feet 

 \8,000y 



h - 6 feet (1.8 meters) . 



Since radar waves follow a straight line only in a medium of constant 

 index of refraction, this method is not quite correct. In the atmosphere, 

 the gradients of humidity and density cause the radar beam to bend. Typi- 

 cally in a standard atmosphere, radar radiation starting out parallel to 

 the Earth will travel along an arc with a radius approximately equal to 

 4/3 the radius of the Earth. The actual radius varies with the vertical 

 gradient of temperature and humidity in the atmosphere . The radius is 

 least when temperature increases and humidity decreases rapidly with 



32 



let 



then 



