4-13] 



ALTITUDE RETURN 



223 



it gives rise to the "altitude circle," while on an A display it is referred to as 

 the "altitude line". In many cases this return is prominent because of the 

 marked increase of cr° which occurs for depression angles near 90° (see 

 Figs. 4-37, 4-38, 4-39, and 4-43). 



To a radar altimeter the altitude return is the desired signal, while 

 to target detection and tracking radars it is a source of interference or 

 "clutter." Since the antennas of these two classes of radars have widely 

 different beam patterns, the illumination of the ground as a function of 

 angle may vary widely between different applications. A full discussion of 

 the problem, therefore, is beyond the scope of the present treatment, so that 

 only some of the principal factors will be discussed. 



The expressions (Equation 4-60) were given for the area of a resolution 

 element on the surface. For small depression angles this area is proportional 

 to range, while for large depression angles it is proportional to range 

 squared. The distinction between these two in the case of the altitude line 

 is actually a function of altitude. For example, if both the antenna beam 

 and the pulse shapes are rectangular, and if cr'' is a slowly varying function 

 of angle near vertical incidence (as in the case of Fig. 4-42, for example), 

 then the illuminated area is beamwidth limited if the leading edge of the 

 transmitted pulse passes the outer 

 edge of the antenna beam before the 

 trailing edge of the pulse reaches the 

 ground. The received power of the 

 altitude line then will vary as the 

 inverse square of altitude in accord- 

 ance with Equation 4-60b. Because 

 of the inverse square relationship (as 

 contrasted with an inverse fourth 

 power relationship for a point target) 

 the altitude line return can be very 

 strong. This is particularly true for 

 altitude line return from a flat calm 

 sea which tends to act as a perfect 

 reflector (see Figs. 4-37 and 4-38.) 

 However, if the altitude or beam- 

 width is great enough that the trail- 

 ing edge of the pulse reaches the 

 ground before the leading edge 

 passes out of the antenna beam, then 

 the return is pulse-length limited, 

 and the received power of the alti- 

 tude line will vary as the inverse 

 cube of altitude in accordance with Equation 4-60a (see Fig. 4-44). 



20 



16 



0.01 0.02 0.03 0.04 0.05 0.06 0.07 

 L/2/i 



I \ \ I \ \ 1 I 



0.04 0.08 0.12 0.16 0.20 0.24 0.28 



r(Msec)A(1000ft) 



Fig. 4-44 Angular Extent of Altitude 

 Line vs. Pulse Length. 



