14-18] MAJOR CHARACTERISTICS AND COMPONENTS 781 



side-looking systems should not be too difficult, but stabilization in pitch 

 for long arrays will present more difficult mechanization problems. In 

 forward-looking systems using linear arrays, stabilization in roll will be 

 more difficult. One approach to the stabilization problem is to stabilize the 

 aircraft itself along the axes where antenna stabilization is most difficult. 



Receivers. In general, receivers used in radar ground mapping 

 systems are of the superheterodyne type because of the very high frequen- 

 cies at which such systems operate and the requirements for sensitivity, 

 fidelity, and selectivity. While many radar systems which operate at high 

 frequencies employ receivers of this general type, there are several consider- 

 ations of particular interest for mapping systems. One of these is the effect 

 of the requirement for good range resolution on receiver design. 



To obtain a given resolution in range, ground mapping systems usually 

 employ a very narrow pulse. Therefore, since the optimum IF amplifier 

 bandwidth is given approximately by the reciprocal of the pulse width, the 

 receiver bandwidth required will be comparatively wide. For example, 

 airborne fire-control pulse radar systems, which usually employ pulse 

 widths of the order of 1 or 2 Msec, have receiver bandwidths of the order of 

 several megacycles. Mapping systems, which may use pulse widths of the 

 order of several tenths of a microsecond, will require much wider band- 

 widths; a pulse width of 0.1 Msec would dictate a bandwidth of the order of 

 10 Mc. 



Another consideration, which has been mentioned in Paragraph 14-17, is 

 the dynamic range of the receiver; as noted, the range of signal amplitudes 

 may vary by 30 db or more. The dynamic range of present linear amplifiers, 

 which is of the order of 20 db, will not encompass such a range of signal 

 amplitudes without limiting. Various approaches to the solution of this 

 problem have been considered. One is the lin-log amplifier. It has linear 

 amplification characteristics at low and moderate signal levels, but a 

 logarithmic characteristic for large input amplitudes. Thus, although large 

 signals are compressed, saturation of the receiver will not occur. Systems 

 using this type of amplifier have obtained good results. 



Display and Data Processing. This part of the system usually 

 includes the cathode ray tube with its power supplies, a camera and optical 

 system, and associated circuits which may be used to obtain computer 

 inputs. 



The CRT represents an important factor in determining the overall 

 resolution of the system. Two characteristics of particular importance in 

 determining its resolving capabilities are spot size and contrast. Spot size 

 depends upon tube type, acceleration voltage, beam current, and the type 

 of focusing used (as was explained in Paragraph 12-6). Minimum spot sizes 

 obtainable at present are of the order of 0.1 mm or less, where the spot size 



