776 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



length are shown in Fig. 14-19 as a function of slant range to the ground and 

 radar altitude. 



Another important area with regard to system resolution concerns the 

 receiver and display. Here, one of the basic considerations is their dynamic 

 range, i.e. the range of signal amplitudes which they can accommodate 

 without distortion. Since a radar map is a brightness pattern, variations 

 of intensity within the map contribute to the resolution of certain features. 

 The range of signal amplitudes encountered in a given mapping operation 

 may be quite large. Recent data taken from a number of different types of 

 terrains indicate that the range will be of the order of 30 db or more." If the 

 receiver or display system cannot accommodate such a range of signal 

 amplitudes without distortion, loss of much detail within the map will 

 occur. If the gain or intensity is set at the noise threshold, strong targets 

 will "bloom," thus obscuring nearby weaker targets. If the gain or intensity 

 is set too high, the weaker targets will not be mapped. In connection with 

 the display, the minimum spot size of the CRT will be an important factor 

 also in determining the overall system resolution. 



Another consideration in connection with system resolution is the manner 

 in which the antenna is scanned. When the beam is scanned relative to the 

 aircraft, as in the conventional forward-looking system, the return from a 

 given point will tend to fluctuate from scan to scan, except at very long 

 ranges. This is a result of the change in aspect angle between scans. 

 Fluctuations of the return from a target on successive scans will reduce the 

 detail with which it is painted. In the side-looking system where scanning is 

 accomplished by the motion of the airplane all return from a given point is 

 within a single beamwidth and fluctuations in the return are greatly 

 reduced. This produces a more integrated map with greater detail. 'Not all 

 the factors involved in this integrating effect are yet fully understood, but 

 the performance of comparable scanning and side-looking systems has 

 demonstrated the superior resolving capabilities of the latter type of scan. 



System accuracy is equally important in many respects as system 

 resolution. Distorted or "smeared" maps make it difficult to obtain a true 

 measure of ground distances or to resolve details within the map. Types of 

 errors that can occur are: 



1. Display system errors 3. Drift errors 



2. Altitude errors 4. Stabilization errors 



Display errors can result from a number of factors — nonlinear sweeps, 

 irregularities in the start of the sweep trace, imperfect stabilization of the 

 display. Most errors of this type can be eliminated or minimized by good 

 system design. Errors in the measurement of the altitude of the mapping 



"C. R. Grant and B. S. Yaplee, "Back Scattering from Water and Land at Centimeter and 

 Millimeter Wavelengths," Proc. IRE 45 (July 1957). 



