480 AUTOMATIC TRACKING CIRCUITS 



Maximum Random Tracking Errors. The random errors in the 

 tracking loop should be less than 0.15° rms total or 0.105° rms per channel 

 in the position measurements, and 0.1 5 /sec rms or 0.105° /sec rms in the 

 rate measurements (see Table 2-4). 



The random errors are produced not only from radar noise but also from 

 inaccuracies in the measuring devices and from random platform space 

 motion. These errors and the bias errors are described in more detail in 

 Paragraph 9-6. 



9-5 ANGLE TRACKING SYSTEM ORGANIZATION 



For an adequate solution of the fire-control problem, measurements must 

 be maintained within limits discussed in Paragraph 9-4. As implied, there 

 are many factors affecting the accuracy of the system: {a) target signal 

 fading, {b) interceptor-controlled maneuvers, (c) interceptor gust disturb- 

 ances, {d) target noise inputs, {e) interceptor induced vibrations (due to 

 buffeting, structural resonances, etc.), and (/) radar receiver noise. 



To minimize the effects of these disturbances, some form of feedback 

 control must be^sed. There are a variety of basic control systems that can 

 be devised. A few of these are indicated in Fig. 9-2. The figure shows 

 representative block diagrams of three types of systems, and it has corre- 

 sponding diagrams which define the angular variables that are being 

 measured or commanded in space. 



The first part of the figure, (a), represents a system where the projectile 

 launching mechanism is on a gimbal system, separate from that of the 

 antenna in the aircraft, and it moves the projectiles into proper launching 

 position according to commands computed from the tracking antenna 

 position and rate information. A tail turret is often mechanized in this 

 manner. In this case the projectile launching mechanism is a cannon and, 

 as shown in the figure, it is commanded through a position feedback loop 

 to reduce the effect of actuator and amplifier variations. The turret and 

 the antenna are subjected to the same aircraft motion disturbances, and 

 in addition there is usually a structural coupling between the tracking 

 antenna and the turret which can actually produce system instability as 

 well as induce disturbances into the antenna tracking loop. 



The second part of Fig. 9-2, (b), illustrates a system in which the aircraft 

 is controlled directly from the tracking radar error signal. In this way the 

 antenna is moved directly by the aircraft and also by a position command, 

 the lead angle with respect to the aircraft, computed from measured 

 antenna space rates. However, it is the aircraft which is continuously 

 directed by the radar tracking error signal toward the future position of the 

 target. The antenna lead angle is similar to a calculated bias signal in the 

 loop which prevents the aircraft from flying directly toward the present 

 target position as the tracking error is reduced. 



