The Effect of AGC on Radar Tracking Noise

Radar angle tracking noise, such as that due to angular and amplitude scintillation of the target echo, is increased by the response of the receiver agc (automatic gain control) to the low frequency components of the fading of the echo envelope. An increase in angle tracking noise spectral density by a factor of two to three is representative of what can happen when the radar echo envelope is approximately Rayleigh distributed. This phenomenon has been investigated by analog simulation of the agc, both for an ordinary linear filter in the feedback path and for a nonlinear filter with quick attack and slow release in the loop. Since the increase in tracking noise decreases monotonically with increasing agc time constant, an analysis is presented to describe a particular basic problem which requires the agc time constant to be kept short, namely, the transient rise in average signal strength encountered by a radar when closing rapidly on a target. In fixing the agc time constant, a compromise must be reached between increase in tracking noise and the transient increase in mean output signal strength. Whatever considerations motivate a particular choice of agc time constant, the effect of the agc on angle noise spectral density can be determined from the curves presented. The results obtained show that the use of the nonlinear filter with quick attack and slow release does actually produce the desired result of reducing the transient rise in output signal strength while keeping the increase in noise spectral density constant.