462 



REGULATORY CIRCUITS 



signal fading period, it is necessary to prevent the antenna from being 

 moved in space away from the target direction by own-ship's motion. Here 

 the stabihzation loop plays a major role in attenuating antenna space 

 motion caused by aircraft motion. Usually, the attenuation needed is not 

 extremely large, because when the radar error signal (which is also the 

 antenna rate command) fades, it is replaced by the thermal noise of the 

 radar receiver. As is shown in Fig. 8-40, this noise causes the antenna in a 



Antenna should be 

 within the Shaded 

 Area 95% of the 

 Time After the 

 Radar Signal Fades 



Fig. 8-40 Antenna Drift During Radar Signal Fade. 



wh( 



K is per cent modulation/degree tracking error 



r is time between radar pulses 

 coc is track loop bandwidth, rad/sec 



/ is amount of integration in track loop 



/ is time in seconds 

 Ad is antenna drift angle in degrees 



Typical Values 

 50%/deg 

 1 X 10-3 sec 

 6 rad/sec 

 > 10 : 1 

 < / < 5 

 0< 15° 



typical system to drift away from the desired position by as much as 2.0° 

 in 3 seconds. Thus, even if the stabilization loop operated perfectly, the 

 noise commands would probably drive the antenna off target. 



Actually, the positional accuracy of the antenna cannot be maintained in 

 space without a radar signal to provide a space reference. Sufficient 

 accuracy for fire-control computation cannot be obtained, and it is only 

 necessary to prevent the antenna from being displaced from the target sight 

 line by more than the antenna beamwidth, which may be between 1° and 

 15°, to ensure that tracking will be resumed when the target signal strength 

 increases again. Therefore, for the system of Fig. 8-40 it would be well to 

 limit the duration of the fading period to a time less than 3 seconds. If the 

 system is not returned to search at this time, the antenna would probably 

 drift from the target sight line and prevent resumption of tracking. An 



