400 REGULATORY CIRCUITS 



surfaces of the target. The frequency components of amplitude noise 

 causing angle tracking noise lie in two widely separated bands. 



A low-frequency region of noise extending from zero to approximately 10 

 cps causes a noise modulation within the closed-loop servo-target combina- 

 tion superimposed upon the tracking error caused by flight path input 

 information and associated system tracking errors. The low-frequency 

 band also influences angle noise as explained later. Removal of the effects 

 of low-frequency amplitude noise on angle tracking by suitable AGC design 

 is also discussed later. 



A high-frequency region of amplitude noise in the vicinity of the lobing 

 frequency (except in monopulse radars) contributes directly to angle 

 tracking noise. The angle tracking noise power arising from high-frequency 

 amplitude noise is proportional to the square of the beamwidth, the 

 fractional amplitude noise power modulation per cps of bandwidth, and 

 the angle tracking servo bandwidth.^ The principal sources of target- 

 generated high-frequency amplitude noise are propeller (power plant) 

 modulation and structural vibrations of the target surface elements. 



2. Angle noise is the variation in the apparent angle of arrival of the echo 

 from the target relative to the line of sight to the center of reflectivity of the 

 target. It is caused by variations in the phase front of the reradiated energy 

 from the multielement target. When low-frequency amplitude noise exists 

 incident to narrowband or slow AGC, the angle noise power (in suitable 

 units) equals one-half the square of the radius of gyration of the target 

 reflectivity distribution.^ When low-frequency amplitude noise is removed 

 by wideband or fast AGC, the angle noise power is approximately doubled 

 with practical AGC circuitry. 



3. Bright spot wander noise results from changes in the center of target 

 reflectivity principally caused by a redistribution of the significant target 

 reflecting surfaces; it does not depend upon the relative phases of the echoes 

 from the individual surface elements. The frequency components of bright 

 spot wander noise lie almost entirely in a low-frequency band since it is 

 associated with major aspect changes of the target. Because bright spot 

 wander noise is an uncorrelated component of target-generated angle 

 tracking noise, a complete elimination of angle noise (as defined above) does 

 not reduce angle tracking noise to zero. 



Examples of the spectral energy distribution of amplitude noise were 

 shown in Fig. 4-23.^ In the spectra illustrated, the analytical method 

 excluded low-frequency results below 30-40 cps. 



^Ibid., p. 3. 



■'B. L. Lewis, A. J. Stecca, and D. D. Howard, The Effect of an Automatic Gain Control on 

 the Tracking Performance of a Monopulse Radar, NRL Report 4796, 31 July 1956. 



^Source: D. D. Howard, from measurements made at the Naval Research Laboratory, 

 Washint^ton, D. C. 



