10 



ELEMENTS OF AIRBORNE RADAR SYSTEMS DESIGN PROBLEM 



In the second method, Fig. 1-5B, fixed antennas are mounted on each side 

 of the aircraft. The motion of the aircraft with respect to the ground 

 provides the scanning means. Thus the picture obtained by this radar is 

 a continuous map of two strips on either side of the aircraft flight path. 

 In each case the detail is very diflFerent than that obtainable from photo- 

 graphs of the same terrain under conditions of good visibility. Never- 

 theless, a considerable amount of potentially useful tactical information 

 can be obtained from such pictures. The distinction between land and 

 water areas is particularly striking, and prominent targets — large ships , 

 airfields, and cities — can also be clearly distinguished. 



The basic capabilities of radar provide several attractive features in 

 the performance of the mapping function. The range to the target is 

 directly measurable. Smoke, haze, darkness, clouds, and rain do not pro- 

 hibit taking useful radar pictures (depending on the radar parameters 

 chosen). A camouflaged target that might be exceedingly difficult to 

 distinguish by visual means is often readily unmasked by a radar picture. 

 Finally, a radar picture does not necessarily have the same problems of 

 perspective that tend to distort a visual picture. 



The change of target characteristics with frequency can be employed to 

 provide increased contrast. The basic principle is illustrated in Fig. 1-6, 

 which shows hypothetical backscattering curves for the sea and a target. 

 If the mapping is performed at two frequencies,/i and/2, and if the returns 

 at these frequencies are transformed into green and blue, respectively, 

 on a visual display, then the target will appear green and the sea blue. 

 This color transposition utilizes the human eye's ability to discern color 

 differences (see Paragraph 12-7), thereby improving the contrast in cases 

 where a relationship similar to Fig. 1-6 exists. 



By the use of the doppler (velocity 

 discrimination) eff"ect, a mapping 

 system may also be provided with 

 the capability for distinguishing 

 moving targets that have a compo- 

 nent of velocity along the sight-line 

 of the radar. This is known as woy- 

 ing target indication (MTI). 



Another type of radar mapping 

 does not involve the generation and 

 transmission of microwave energy 

 by the radar. Rather, it utilizes the 

 fact that all bodies — as a conse- 

 quence of their temperature and em- 

 issivity characteristics — emit energy in the microwave spectrum. By using 

 highly directional antenna and a receiver that is sensitive to these radia- 



TRANSMITTED FREQUENCY-f 



Fig. 1-6 Utilizing the Change of Target 

 Characteristics with Frequency to En- 

 hance Mapping. 



