8-32] ACCURACY REQUIREMENTS ON ANGLE TRACK STABILIZATION 463 



alternative to returning to the search mode would be to replace the average 

 level of the stabilization loop command, ordinarily the radar error signal, 

 with a fixed d-c voltage to eliminate the noise. Unfortunately, this requires 

 extra mechanization which may not be worth the additional space and 

 complexity. However, during the fading period, it is necessary to provide 

 sufficient antenna space stabilization, with the stabilization control loop, 

 to keep the positional deviation incident to aircraft disturbance motion an 

 order of magnitude lower than the maximum allowable deviation. There- 

 fore, for the illustrative case depicted by Fig. 8-40, the peak positional error 

 incident to aircraft motion should be less than 0.125°.^^ Although this is not 

 as much attenuation as is required to reduce the rate signal errors, it may 

 not be so in all cases. Usually, it is necessary to check the amount of 

 stabilization needed for each type of error to determine which will require 

 the greatest amount of attenuation. 



Stabilization Accuracy Required to Prevent System Instability. 



As mentioned in Paragraphs 8-25 and 8-32, antenna stabilization is needed, 

 especially in a system with autopilot control, to prevent system instability 

 incident to coupling between the aircraft space commands from the antenna 

 gyros and the resulting antenna motion from the commands. Unlike the 

 human pilot, the autopilot responds to all signals. Ordinarily, it does not 

 learn to reject the signals generated by the coupling between the aircraft 

 and antenna motions. Since this is a form of positive feedback (an undesired 

 or parasitic loop) it is necessary to keep the magnitude of this loop gain less 

 than one-half at all frequencies to ensure that instability will not occur.^* 



Fig. 8-41 Coupling Between the Air Frame and the Antenna. 

 G2 = amplifier, actuator, antenna; Gs = rate gyro; Gc = fire-control computer; 

 Ga = autopilot and aircraft; Gcl ~ GzGcGa\ Pc = computed projectile angle with 

 respect to aircraft; Vr = rate command; Atl = tracking line angle; yffz, = 

 tracking line rate; Aa = aircraft space angle; Otl = antenna angle with respect to 



aircraft. 



^^This is an arbitrary figure. It is one-twentieth of the antenna beamwidth to make it 

 insignificant with respect to the fading error. 



'•^Actually, the degree of stability of any feedback loop depends on the magnitude and phase 

 of the loop characteristic at all frequencies, but if the gain is less than one-half, a stable loop 

 is assured regardless of the phase variation with frequency. 



