768 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



C-band weather radars are usually designed with at least 3 db more peak 

 transmitted power than their X-band equivalents in order to match more 

 nearly the X-band range under light attenuation conditions, while further 

 enhancing their superiority under very heavy rainfall conditions. 



Antenna Size. Because the antenna size enters into the range 

 equation twice, affecting both transmitting antenna gain and receiving 

 antenna cross-sectional area, it becomes one of the most effective para- 

 meters for increasing range. From Equation 14-18 we see that power into 

 the receiver is proportional to the square of antenna gain G, which is in turn 

 proportional to the square of the antenna diameter, thus making overall 

 system gain vary with the fourth power of the diameter (d*). Since maxi- 

 mum range is proportional to the fourth root of power, it is apparent that 

 system range varies directly with dish diameter (again assuming non-beam- 

 filling targets). 



Pulse Length. As seen in Equation 14-21, back-scattered energy 

 returned to the antenna from weather targets is directly proportional to 

 pulse length. Doubling the pulse length should increase the power return 

 by 3 db, assuming that the target is greater than rc/2 in depth. A practical 

 limit of about 5 Msec is placed on this factor by the minimum range resolu- 

 tion that can be tolerated. 



Minimum Usable Receiver Power (Often referred to as Minimum 

 discernible signal). This is principally a function of the noise figure of the 

 receiver. Because of the requirements for a simple, rugged receiver it is not 

 generally feasible to obtain a noise figure for the airborne equipment 

 comparable with its large ground-based counterpart. A great deal of work 

 has been done and considerably more needs to be done to reduce this figure. 

 IF strips can now be designed having noise figures sufficiently low to make 

 no sizable contribution to the overall noise figure. Crystal mixer character- 

 istics and waveguide losses are being improved to the point where a 9-db 

 receiver noise figure is feasible on a production basis utilizing conventional 

 techniques. Furthermore, the development of small solid-state devices, 

 capable of amplification at microwave frequencies, is making possible still 

 greater reductions in noise figure. 



Another factor over which the designer has cognizance is receiver 

 bandwidth. Reducing receiver bandwidth, so long as it is wide enough to 

 pass the main components of the pulse spectrum, improves the signal-to- 

 noise ratio. 



One last factor to be considered for its effect on radar performance is the 

 ability of the scope indicator to discriminate minimum discernible targets 

 from noise incident to the integration effect provided by tube persistence. 

 Noise, being random, does not integrate to the same extent as do repetitious 



