780 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



the effective "echoing" area is the projection normal to the beam of the 

 pulse packet on the ground. ^^ These assumptions raise two problems if one 

 uses the back-scattering cross section o-° defined in Chapter 4. They are: 



(1) a" is a function of the depression angle 6 



(2) The effective echoing area to be used with o-" is the actual area on the 

 ground covered by the pulse packet, as expressed in Equation 4-59. 



The expression obtained for the gain pattern when these factors are 

 considered is 



G(« - "^;i';^P - (14-32) 



Since the dependence of o-" on the depression angle is a function of the type 

 of terrain, exact mechanization of Equation 14-32 for all cases is not 

 possible. However, a o-'* averaged for several types of terrain should be 

 satisfactory. 



Antennas. The antennas used in ground mapping systems include 

 linear arrays which produce fan beams and various parabolic shapes to 

 produce pencil beams. Doppler and correlation techniques which produce 

 the effect of a narrow beam without the large dimensions necessary in 

 conventional antennas are being investigated. These are in the develop- 

 mental stage at present. 



In the forward-looking fan beam system the linear array must be 

 positioned perpendicular to the airplane fuselage to obtain the correct 

 orientation of the beam. Scanning of the beam is accomplished by various 

 mechanical or electronic techniques. The maximum dimensions of the 

 array are limited by mounting problems. The possibility of placing the 

 array in the leading edges of the wings has been suggested as a means of 

 increasing the maximum length of the array that can be accommodated. 

 However, the thinness of the wings of present-day high-speed aircraft seems 

 to preclude this possibility. The side-looking system offers a considerable 

 advantage over the forward-looking system in this respect, since quite long 

 arrays of the order of 10 to 15 feet or more can be mounted along the sides 

 of the airplane's fuselage without undue difficulty. 



Stabilization. As already noted, distortion and smearing within the 

 radar map can occur as a result of aircraft motions about the flight path. 

 The analysis of stabilization errors in Paragraph 14-17 indicates that 

 stabilization is probably required both in roll and pitch. While drift-angle 

 errors can be corrected in the display system, correction for roll and pitch 

 requires stabilization of the antenna in those axes. Roll stabilization in the 



^^Microwave Antenna Theory and Design, Radiation Laboratory Series, Vo. 12, Chap. 14, 

 McGraw-Hill Book Co., Inc.. New York, 1947. 



