396 REGULATORY CIRCUITS 



interpreted by the radar as amplitude modulation of the mean signal level, 

 is independent of range if a good automatic gain control (AGC) system is 

 used. 



Angle noise, with an rms value of (Xang-, is defined as the variation in the 

 apparent angle of arrival of the echo from a target relative to the line-of- 

 sight to the center of reflectivity of the target. Angle noise is a function of 

 the spacing of surface elements producing echoes, and the relative am- 

 plitude and phase of these echoes. Since angle noise is a function of the 

 linear dimensions of the target, a variation inversely proportional with 

 range results as long as the target subtended angle is small compared with 

 the beamwidth of the antenna. At times, incident to angle noise, the 

 direction indicated by the apparent angle of arrival of the target echo may 

 fall outside the target extremes. 



Bright spot wander noise, with an rms value of o-6s,„, is defined as the 

 variations in the center of reflectivity of the target relative to a selected 

 physical reference point on the target. The summation of angle noise plus 

 bright spot wander noise is the variation in the apparent angle of arrival 

 of the echo from a target relative to the selected physical reference point on 

 the target. Bright spot wander noise is a function of the relative spacing of 

 target reflecting elements and the amplitude of echoes from these elements. 

 Like angle noise, bright spot wander noise (in angular units) varies inversely 

 with range. However, the peak excursions of the center of reflectivity of the 

 target cannot extend beyond the target limits. 



Internal Noise Inputs. In addition to the primary function of 

 location and tracking of targets in space, radar outputs to computers utilize 

 rates of change of the basic position information. Tracking smoothness 

 and accuracy depend upon the manner in which the external inputs are 

 processed by the radar system. Internal radar noise components may be 

 categorized as follows. 



Receiver noise, with an rms value of o-rec, is defined as the variations in the 

 radar tracking arising from thermal noise generated in the receiver and any 

 spurious hum pickup. Receiver noise is inversely proportional to the signal- 

 to-noise ratio in the receiver, and since the signal power varies inversely 

 as the fourth power of the range to the target (excluding propagation 

 anomalies), this effect is directly proportional to the fourth power of range. 



Servo noise, with an rms value of aser, is defined as the variations in the 

 radar tracking axis caused by backlash and compliance in the gears, shafts, 

 and structures of the antenna. The magnitude of servo noise is essentially 

 independent of the target and is thus independent of the range. 



Tracking Noise Definitions. An optimum radar system design can 

 result only from proper consideration of the nature of all the external and 

 internal noise sources. One principal objective of tracking system design 



