Physics:Sensitivity time control
Sensitivity time control (STC) is used to attenuate the very strong signals returned from nearby ground clutter targets in the first few range gates of a Radar receiver. Without this signal attenuation, the receiver would routinely saturate due to the strong signals. This is used in air traffic control systems, and has an influence on the shape of the elevation pattern of the surveillance antenna. it is represented in terms of numerical value starting from zero. zero value of STC represents that the radar system is accepting all clutter returns.
https://en.wikipedia.org/wiki/Sensitivity time control was the original source. Read more.