778 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



An = dA[<f>ARG + h] (14-28) 



Arj = -4>Ah (14-29) 



Ar, = 4>aRg. (14-30) 



Examination of the last set of equations indicates that the roll stabilization 

 of the antenna in the side-looking system will reduce the displacement to 

 the single term BaH along the / axis. As already noted, the maximum 

 displacement that can be tolerated without undue distortion is difficult to 

 define analytically; let us specify arbitrarily that the maximum along the 

 / axis shall not exceed 1 beamwidth. Referring to the beam of Fig. 14-19, 

 at an altitude of 10,000 ft and a slant range of 35,000 ft this represents a 

 distance of approximately 150 ft. Thus the maximum allowable pitch angle 

 will be less than 1°. Studies of airplane motions in turbulent air indicate 

 that displacements of this magnitude will probably occur with sufficient 

 frequency to make stabilization in pitch desirable. Stabilization would be 

 necessary in both axes for forward-looking systems also. 



The range of a radar ground mapping system is affected by several factors 

 in addition to the customary signal-to-noise ratio considerations which will 

 be discussed in Paragraph 14-19. The maximum range for a given radar 

 altitude will be limited by the angular coverage in the vertical dimension of 

 the fan beam. In seeking to extend the range of coverage by mapping from 

 higher altitudes, the increase in packet dimensions and its effect on resolu- 

 tion must be considered (see Fig. 14-19). Another problem in connection 

 with mapping at long ranges and low grazing angles is the shadowing effect 

 produced by projections above the terrain such as mountain ridges. 



The problem of attaining all-weather capability has been alluded to 

 previously in connection with the effect of wavelength on the beamwidth. 

 Selection of wavelength must, in the final analysis, represent a compromise 

 based upon the considerations of atmospheric and weather attenuation, 

 resolution requirements, and the state of the art. 



14-18 MAJOR CHARACTERISTICS AND COMPONENTS 



Type of Transmission. In general, present-day radar ground 

 mapping systems are ordinary pulse radars employing a low-duty cycle 

 pulse, the duty cycle being of the order of 10~^. Side-looking systems can em- 

 ploy coherent-pulse doppler techniques to obtain a narrow beam by rejecting 

 all return except from points at or near the perpendicular to the airplane 

 ground track (see Sec. 6-6). The transmission periodicity must be com- 

 patible with the maximum and minimum mapping ranges. For example, 

 a mapping radar might have a maximum slant range of 30 n.mi. and a 

 minimum slant range of 10 n.mi. Based on the maximum range only, the 

 maximum pulse repetition frequency would be approximately 2700 pps. 



