TABLE OF CONTENTS For convenience, we have regarded the receiver gain as fixed, and have seen that P FA depends on the ratio k of threshold to noise plus clutter voltage at the amplifier output. Instead of adjusting threshold voltage, one could just as well change gain. P D and P FA can be controlled by adjustment of either gain or threshold voltage. In the following it is assumed that gain is varied because there is a control labelled gain, but as a matter of detail design this control might instead raise threshold voltage, or do a mixture of both - it is of no concern to the operator or to analysis of the system.
High gain is required at long range to detect feeble echoes, but at short range the inverse fourth power range term in the radar range equation makes all navigationally significant targets relatively strong. To prevent receiver overload and to minimise the need for the operator to reset gain as target range changes, all radar receivers include an inbuilt swept gain or sensitivity-time control (STC) function. This increases gain from an initial low value during the first 100 ?s or so after the transmitter fires, so reducing the necessary dynamic range. Figure 12.32 is a typical swept gain curve. For example, at range A, gain is -6 dB on maximum, halving noise voltage, doubling threshold/noise voltage ratio k
