8-32] ACCURACY REQUIREMENTS ON ANGLE TRACK STABILIZATION 461 



Stabilization Accuracy Required to Reduce Position Errors Due 

 to Aircraft Motion. The radar antenna must not deviate from its 

 desired position within a specified angle incident to aircraft motion even if 

 the radar signal should not be present for a short time because of temporary- 

 loss of target return. In discussing the accuracy required to accomplish 

 this, it will be necessary to discuss the position accuracy required and the 

 temporary loss of target return which is often referred to as signal fading. 



The position accuracy needed again depends on the type of fire control 

 being considered. In some missile systems where a proportional navigation 

 course is flown, only antenna rates of the missile seeker are needed to 

 compute the proper course; angular information is not used. Therefore, 

 the antenna angle need not be maintained very accurately. It is only 

 necessary to keep the antenna positioned toward the target within the 

 antenna beamwidth, which may vary from 1° to 15° depending on the 

 antenna size and radiating frequency. Actually, the stabilization accuracy 

 needed to maintain antenna position is an order of magnitude less than that 

 needed to provide the necessary attenuation of antenna motion in the rate 

 signal; and therefore it is usually not used as a criterion for stabilization 

 loop design for this type of application. 



In many fire-control systems, however, the antenna position is used to 

 compute the course of the aircraft. From a system analysis, analytical or 

 simulated, the random and bias errors in position may be specified. These 

 values may range between 2 mils and 10 mils as described in Paragraph 2-27 

 for a typical lead collision course. The total random error should be less 

 than 2.62 mils (0.15°) rms and the bias error should be less that 9.15 rms 

 (0.525°). 



Tracking loop errors contribute to the total position error, but as will be 

 shown in Chapter 9, where the tracking loop errors are considered, the 

 position error component due to platform motion does not require the 

 attenuation needed to reduce the corresponding rate errors to the specified 

 values. Usually, the allowed error in antenna position may be greater than 

 that allowed in the rate signal because the rate signal, and its errors, are 

 multiplied by a large multiplying constant, the error signal to angular rate 

 sensitivity factor discussed in Chapter 2. Therefore, the rate error specifi- 

 cation is usually more severe, and the required stabilization accuracy may 

 be determined from it. 



At long ranges the target may be detected and tracking may begin, but 

 the radar return signal may become weak shortly thereafter and even fade 

 away completely as the target changes its aspect angle with respect to the 

 interceptor.''^ After two or three seconds, if the target signal does not 

 reappear, the antenna search mode is reactivated. However, during the 



*2The change in magnitude of the reflected radar signal is discussed in Paragraph 4-3. 



