9-3] EXTERNAL INPUTS: UNDESIRED AND DESIRED 475 



9-3 EXTERNAL INPUTS: UNDESIRED AND DESIRED 



Before an automatic control system can be designed, some information 

 about the system inputs and the allowable tracking accuracy must be 

 known. This knowledge may be used to determine what the control loop 

 must do, and it dictates what characteristics are necessary to accomplish 

 the desired function. The track loop inputs are discussed in this paragraph 

 and the allowable errors are described in the next paragraph. 



The undesirable inputs to the tracking loop may, in general, be classified 

 as noise or as disturbances. Ultimately it is desired to eliminate the effects 

 of the undesirable inputs from the useful output signals. One form of 

 disturbance, aircraft motion, was described in Paragraph 8-24. Another 

 form of undesirable input, noise, is described in Paragraph 4-4. If radar 

 is used as a detector, there will be considerable noise in the system, and the 

 system output noise increases with increasing bandwidth. Therefore it is 

 necessary to keep the system bandwidth as low as possible without creating 

 significant time delays in the measured information about the target's 

 motion in space. 



The desirable inputs are those which the angle tracking system must 

 detect and follow. Essentially, the desirable inputs are the time functions 

 which describe the space motion of the sight line between the radar antenna 

 and the target. The expected sight line motion may be determined from 

 a study of the various attack courses that the interceptor is to fly. 



Possible target maneuvers should also be considered in the general study 

 of course dynamics, although for preliminary considerations the target is 

 often assumed to have a constant velocity. 



Course Dynamics. Course dynamics, with respect to the angle track 

 system, concern the angular motion of the sight line in space corresponding 

 to all tactically feasible attacks. The maximum angular rates comax and 

 angular acceleration co^ax of the total line-of-sight motion resulting from 

 these attack courses may be calculated from knowledge of the equations 

 describing the course. Typical sight-line rates and accelerations with time 

 during a lead collision course have been shown in Fig. 2-45. 



Another possible mode of attack for a high-speed fighter is the lead- 

 pursuit course wherein the aircraft is flown in a trajectory such that the 

 correct lead angle for firing is maintained throughout the course, rather 

 than at a single point as for the lead collision course. Typical data for 

 lead-pursuit courses against maneuvering targets are shown in Fig. 9-1. 

 The maximum values of angular rates and angular accelerations of the lead 

 pursuit course sight lines are summarized in Table 9-1 along with the 

 lead-collision data. Higher derivatives have been found to be relatively 

 small and unimportant in establishing system specifications, and they are 

 not included in the table. However, cjmax and w^ax are the principal charac- 



