4-12] GROUND RETURN 219 



Frequency spectrums were obtained for 15-second samples of recorded 

 data (corresponding to 3750 ft along the sea surface), and also frequency, 

 B-scope records of the spectrum as a function of position of the illuminated 

 patch on the sea surface (250 ft long). These will be referred to as the 

 A display and the B display, respectively. 



For low sea states, the average spectrum had a Gaussian shape, and width 

 between half-power points of 2 to 3 knots (60-100 cps at X band). The 

 corresponding B display was generally smooth on upwind and downwind 

 edges for all ranges. Fig. 4-41 (a) shows a sample of the A and B displays 

 for a low sea condition (wave height 2 ft, wind 9 knots). The 3-db band- 

 width of 82 cps in this sample corresponds to a velocity spread of 2.55 knots. 



As the wind increased and white caps became evident, the A display 

 broadened asymmetrically to 5 knots or more. The B display then was 

 broadened on the downwind edge to an extent which varied irregularly with 

 range, but the upwind edge remained smooth. Fig. 4-41 (b) shows a sample 

 of the A and B displays for a medium sea (wave height 5 ft, wind 16 knots). 

 Here the 3-db bandwidth is 172 cps, corresponding to a velocity spread of 

 5.35 knots. These characteristics suggest that the irregular downwind 

 broadening was due to spray filaments or patches associated with the white 

 caps, blown off the wave crests and moving downwind more rapidly than 

 the crests. 



4-12 GROUND RETURN 



The applications of airborne radar over land cover an even broader field 

 than operations over sea. As in the case of sea clutter, reflections from a 

 land surface form a clutter background which tends to obscure the desired 

 echo, e.g. from a target aircraft flying at low altitude. At small depression 

 angles, ground return generally is considerably larger than sea return. 

 Hence the problem of detecting ground targets obscured by ground clutter 

 is correspondingly more difficult. 



In another type of radar application — ground mapping — the most 

 important characteristic is the contrast obtainable between objects and 

 their immediate surroundings as determined by the nonuniformity of the 

 return. This characteristic governs the type of ground map which may be 

 obtained by radar techniques, as was discussed in Paragraph 1-4. 



The ground return which competes with or obscures the target echo is 

 confined to the return from ground elements at the same apparent range as 

 the target. Such returns can be received either on the main beam or the 

 sidelobes of the antenna pattern. A special form of sidelobe clutter — the 

 altitude line — will be discussed in the next paragraph. 



In a pulsed radar the returns which arrive at times precisely separated by 

 the interpulse period appear at the same apparent range. This gives rise to 



