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One of the major limitations to the efficacy of radar is the presence of responses from land, sea or rain-storms in the vicinity of a target, such responses being generally known as Â¿clutter.Â¿ If the clutter is of amplitude greater than the target-echo, detection of the echo is very difficult; in many cases, however, the clutter is of amplitude smaller than the target-echo, but the latter is lost owing to saturation of the receiver or display equipment by the clutter. It is shown, both theoretically and practically, that such loss can be prevented in the cases of sea and rain clutter by the use of a receiver, the amplification characteristic of which is such that the amplitude of an output signal is proportional to the logarithm of the amplitude of the corresponding input signal; in the case of land clutter, loss is not necessarily prevented, but the probability of such loss is reduced. A logarithmic receiver compresses input signals of any strength less than 100 db above the mean noise power, into output signals of voltage less than four times the output-noise deviation voltage, with negligible change in the detectability of small signals compared with the use of a linear receiver. In addition, with such a receiver, the difference in voltage of two output signals depends only on the ratio of the corresponding input signals, and not on their absolute amplitudes. These properties make a logarithmic receiver superior to a linear receiver for a number of subsidiary applications, some examples being given in the paper.
Electrical Engineers - Part III: Radio and Communication Engineering, Journal of the Institution of (Volume:95 , Issue: 38 )
Date of Publication: November 1948