8-2] p:xternal, internal noise inputs to radar system 395 



Motions of the aircraft carrying the radar set can modulate the incoming 

 signal and cause loss or degradation of the target signals. Automatic space 

 stabilization systems are often required to cope with this problem. 



Finally, the measurement problem is complicated by externally and 

 internally generated noise. The origins of such noise and the effects of the 

 noise upon range- and angle-tracking accuracies are described in the next 

 paragraph. This discussion is particularly important to the subsequent 

 discussion of AGC in this chapter and the angle and range tracking as 

 discussed in Chapter 9. 



The remainder of this chapter will deal with the basic considerations 

 governing the preliminary design of the AFC, AGC, and space stabilization 

 loops. The problem of electronic power regulation is not discussed in detail 

 since this is largely a matter of good electronic design practice, a topic 

 beyond the scope of this volume. 



8-2 EXTERNAL AND INTERNAL NOISE INPUTS TO THE 

 RADAR SYSTEM 1 



Paragraph 4-7 presented some of the basic measurements of target noise 

 characteristics. This paragraph will define the noise sources in a form more 

 immediately useful to the closed-loop control designer to illustrate the 

 means for utilizing the measured information for design purposes. 



External Noise Inputs. Variations in the external input to the radar 

 system fall into two basic categories, i.e. frequency components associated 

 with motion along the target flight path and other frequency components 

 normally referred to as noise. Noise includes propagation path anomalies 

 and atmospheric noise (sferics) as well as noise caused by the complex 

 nature of the target, random motion, and reflectivity. The emphasis here 

 is on the noise associated with the target motion and reflectivity variations 

 that lead to tracking errors. The various components of external radar 

 noise may be defined as follows: 



Range noise, with an rms value of o-^, is defined as deviation of the range 

 information content in the received echo with respect to some reference 

 point on the target. The reference point may be chosen as the long-time 

 average of the range information. Range noise is independent of the target 

 range since its source is pulse shape distortion caused by variations in the 

 vector summation of energy reflected from target surface elements. 



Amplitude noise , with an rms value of o-a^p, is defined as the pulse-to-pulse 

 variation in echo amplitude caused by the vector summation of the echoes 

 from the individual elements of the target. Amplitude noise, since it is 



iSee J. H. Dunn, D. D. Howard, and A. M. King, "Phenomena of Scintillation Noise in 

 Radar Tracking Systems," Proc. IRE, May 1959. 



