452 REGULATORY CIRCUITS 



smaller. In practice, however, the actuators often have the same size for 

 production economy. Finally, it should be emphasized that the control loop 

 and actuators should not be designed to have a performance much greater 

 than that required, not only because increased size and weight would be 

 involved but also because physical limitations inherent in the gimbals and 

 the antenna structures of a given size place an upper practical limit on 

 linear design, ^^ and as this limit of performance is approached, the cost and 

 complexity of control equipment increases rapidly. Specifically, noise is 

 always present in the search control loops, although not to the extent that 

 it is in other control loops associated with fire-control systems, and its 

 detrimental effects become more of a problem as an attempt is made to 

 increase loop performance. In addition, structural resonant frequencies in 

 the antenna make it nearly impossible to construct a stable loop with a 

 crossover frequency near the resonant frequencies. Thus, there is a practical 

 upper limit for the control loop bandwidth which is governed by the 

 antenna characteristics. Since the performance of the loop is primarily 

 a function of the bandwidth, the performance itself is limited. 



8-30 STABILIZATION DURING TRACK 



As was discussed in Paragraph 8-25, the tracking antenna must be 

 stabilized in space to prevent: 



{a) system errors caused by the course computer operating on informa- 

 tion from coupling between the interceptor and antenna motions. 



{b) the antenna radar beam from drifting away from the target during 

 brief periods of radar signal fading 



(c) instability incident to coupling between the interceptor and antenna 

 motions. 



A portion of the required space stabilization is provided by the automatic 

 tracking loops discussed in Chapter 9, except during periods when the radar 

 signal fades. However, during normal operation, the typical tracking loop 

 cannot provide effective isolation above frequencies equal to about one-half 

 the track loop bandwidth, or about 3.0 rad/sec for a typical system. On 

 the other hand, the interceptor may have appreciable motion at higher 

 frequencies as indicated in Table 8-3. To provide the necessary space 

 stabilization, a special automatic control loop is designed to move the 

 antenna relative to the aircraft in a direction opposite to that of the aircraft 

 motion in space. 



To provide space stabilization, the control loop must obtain antenna 

 space motion information. This is obtained and converted into useful 

 electrical signals with gyroscopes mounted on the antenna or on the aircraft, 



''^It is possible to devise nonlinear control loops which will provide increased performance 

 in special cases. 



